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Power electronics

  1. 1. Module 1Power Semiconductor Devices Version 2 EE IIT, Kharagpur 1
  2. 2. Lesson 1Power Electronics Version 2 EE IIT, Kharagpur 2
  3. 3. IntroductionThis lesson provides the reader the following: (i) Create an awareness of the general nature of Power electronic equipment; (ii) Brief idea about topics of study involved, (iii) The key features of the principal Power Electronic Devices; (iv) An idea about which device to choose for a particular application. (v) A few issues like base drive and protection of PE devices and equipment common to most varieties.Power Electronics is the art of converting electrical energy from one form to another in anefficient, clean, compact, and robust manner for convenient utilisation.A passenger lift in a modern building equipped with a Variable-Voltage-Variable-Speedinduction-machine drive offers a comfortable ride and stops exactly at the floor level. Behind thescene it consumes less power with reduced stresses on the motor and corruption of the utilitymains. Fig. 1.1 The block diagram of a typical Power Electronic converterPower Electronics involves the study of• Power semiconductor devices - their physics, characteristics, drive requirements and their protection for optimum utilisation of their capacities,• Power converter topologies involving them,• Control strategies of the converters,• Digital, analogue and microelectronics involved,• Capacitive and magnetic energy storage elements,• Rotating and static electrical devices,• Quality of waveforms generated,• Electro Magnetic and Radio Frequency Interference, Version 2 EE IIT, Kharagpur 3
  4. 4. • Thermal ManagementThe typical converter in Fig. 1.1 illustrates the multidisciplinary nature of this subject.How is Power electronics distinct from linear electronics? It is not primarily in their power handling capacities. While power management ICs in mobile sets working on Power Electronic principles aremeant to handle only a few milliwatts, large linear audio amplifiers are rated at a few thousandwatts. The utilisation of the Bipolar junction transistor, Fig. 1.2 in the two types of amplifiers bestsymbolises the difference. In Power Electronics all devices are operated in the switching mode -either FULLY-ON or FULLY-OFF states. The linear amplifier concentrates on fidelity insignal amplification, requiring transistors to operate strictly in the linear (active) zone, Fig 1.3.Saturation and cutoff zones in the VCE - IC plane are avoided. In a Power electronic switchingamplifier, only those areas in the VCE - IC plane which have been skirted above, are suitable. On-state dissipation is minimum if the device is in saturation (or quasi-saturation for optimisingother losses). In the off-state also, losses are minimum if the BJT is reverse biased. A BJT switchwill try to traverse the active zone as fast as possible to minimise switching losses. Fig. 1.2 Typical Bipolar transistor based (a) linear (common emitter) (voltage) amplifier stage and (b) switching (power) amplifier Version 2 EE IIT, Kharagpur 4
  5. 5. Fig 1.3 Operating zones for operating a Bipolar Junction Transistor as a linear and a switching amplifier Linear operation Switching operationActive zone selected: Active zone avoided :Good linearity between input/output High losses, encountered only during transientsSaturation & cut-off zones avoided: poor Saturation & cut-off (negative bias) zoneslinearity selected: low lossesTransistor biased to operate around No concept of quiescent pointquiescent pointCommon emitter, Common collector, Transistor driven directly at base - emittercommon base modes and load either on collector or emitterOutput transistor barely protected Switching-Aid-Network (SAN) and other protection to main transistorUtilisation of transistor rating of secondary Utilisation of transistor rating optimisedimportance Version 2 EE IIT, Kharagpur 5
  6. 6. An example illustrating the linear and switching solutions to a power supply specification willemphasise the difference. Spec: Input : 230 V, 50 Hz, Output: 12 V regulated DC, 20 W Ferrite core HF transfr: Light, efficient Series regulator - high losses 230 V 230 V Line freq transformer: (a) (b) heavy, lossy High-freq Duty-ratio (ON/OFF) control - low losses Fig. 1.4 (a) A Linear regulator and (b) a switching regulator solution of the specification above The linear solution, Fig. 1.4 (a), to this quite common specification would first step down thesupply voltage to 12-0-12 V through a power frequency transformer. The output would berectified using Power frequency diodes, electrolytic capacitor filter and then series regulatedusing a chip or a audio power transistor. The tantalum capacitor filter would follow. The balanceof the voltage between the output of the rectifier and the output drops across the regulator devicewhich also carries the full load current. The power loss is therefore considerable. Also, the step-down iron-core transformer is both heavy, and lossy. However, only twice-line-frequency ripplesappear at the output and material cost and technical know-how required is low. In the switching solution Fig. 1.4 (b) using a MOSFET driven flyback converter, first the linevoltage is rectified and then isolated, stepped-down and regulated. A ferrite-core high-frequency(HF) transformer is used. Losses are negligible compared to the first solution and the converter isextremely light. However significant high frequency (related to the switching frequency) noiseappear at the output which can only be minimised through the use of costly grass capacitors.Power Semiconductor device - history Power electronics and converters utilizing them made a head start when the first device theSilicon Controlled Rectifier was proposed by Bell Labs and commercially produced by GeneralElectric in the earlier fifties. The Mercury Arc Rectifiers were well in use by that time and therobust and compact SCR first started replacing it in the rectifiers and cycloconverters. Thenecessity arose of extending the application of the SCR beyond the line-commutated mode ofaction, which called for external measures to circumvent its turn-off incapability via its controlterminals. Various turn-off schemes were proposed and their classification was suggested but itbecame increasingly obvious that a device with turn-off capability was desirable, which wouldpermit it a wider application. The turn-off networks and aids were impractical at higher powers. The Bipolar transistor, which had by the sixties been developed to handle a few tens ofamperes and block a few hundred volts, arrived as the first competitor to the SCR. It is superiorto the SCR in its turn-off capability, which could be exercised via its control terminals. Thispermitted the replacement of the SCR in all forced-commutated inverters and choppers.However, the gain (power) of the SCR is a few decades superior to that of the Bipolar transistor Version 2 EE IIT, Kharagpur 6
  7. 7. and the high base currents required to switch the Bipolar spawned the Darlington. Three or morestage Darlingtons are available as a single chip complete with accessories for its convenientdrive. Higher operating frequencies were obtainable with a discrete Bipolars compared to thefast inverter-grade SCRs permitting reduction of filter components. But the Darlingtonsoperating frequency had to be reduced to permit a sequential turn-off of the drivers and the maintransistor. Further, the incapability of the Bipolar to block reverse voltages restricted its use. The Power MOSFET burst into the scene commercially near the end seventies. This devicealso represents the first successful marriage between modern integrated circuit and discretepower semiconductor manufacturing technologies. Its voltage drive capability – giving it again ahigher gain, the ease of its paralleling and most importantly the much higher operatingfrequencies reaching upto a few MHz saw it replacing the Bipolar also at the sub-10 KW rangemainly for SMPS type of applications. Extension of VLSI manufacturing facilities for theMOSFET reduced its price vis-à-vis the Bipolar also. However, being a majority carrier deviceits on-state voltage is dictated by the RDS(ON) of the device, which in turn is proportional to aboutVDSS2.3 rating of the MOSFET. Consequently, high-voltage MOSFETS are not commerciallyviable. Improvements were being tried out on the SCR regarding its turn-off capability mostly byreducing the turn-on gain. Different versions of the Gate-turn-off device, the Gate turn-offThyristor (GTO), were proposed by various manufacturers - each advocating their own symbolfor the device. The requirement for an extremely high turn-off control current via the gate andthe comparatively higher cost of the device restricted its application only to inverters rated abovea few hundred KVA. The lookout for a more efficient, cheap, fast and robust turn-off-able device proceeded indifferent directions with MOS drives for both the basic thysistor and the Bipolar. The InsulatedGate Bipolar Transistor (IGBT) – basically a MOSFET driven Bipolar from its terminalcharacteristics has been a successful proposition with devices being made available at about 4KV and 4 KA. Its switching frequency of about 25 KHz and ease of connection and drive sawit totally removing the Bipolar from practically all applications. Industrially, only the MOSFEThas been able to continue in the sub – 10 KVA range primarily because of its high switchingfrequency. The IGBT has also pushed up the GTO to applications above 2-5 MVA. Subsequent developments in converter topologies – especially the three-level inverterpermitted use of the IGBT in converters of 5 MVA range. However at ratings above that theGTO (6KV/6KA device of Mitsubishi) based converters had some space. Only SCR basedconverters are possible at the highest range where line-commutated or load-commutatedconverters were the only solution. The surge current, the peak repetition voltage and I2t ratingsare applicable only to the thyristors making them more robust, specially thermally, than thetransistors of all varieties. 1200V Version 3 ASIPMPresently there are few hybrid devices and Intelligent Power Modules (IPM) are marketed bysome manufacturers. The IPMs have already gathered wide acceptance. The 4500 V, 1200 A Version 2 EE IIT, Kharagpur 7
  8. 8. IEGT (injection-enhanced gate transistor) of Toshiba or the 6000 V, 3500 A IGCT (IntegratedGate Commutated Thyristors) of ABB which are promising at the higher power ranges.However these new devices must prove themselves before they are accepted by the industry atlarge. Silicon carbide is a wide band gap semiconductor with an energy band gap wider than about2 eV that possesses extremely high thermal, chemical, and mechanical stability. Silicon carbideis the only wide band gap semiconductor among gallium nitride (GaN, EG = 3.4 eV), aluminumnitride (AlN, EG = 6.2 eV), and silicon carbide that possesses a high-quality native oxide suitablefor use as an MOS insulator in electronic devices The breakdown field in SiC is about 8 timeshigher than in silicon. This is important for high-voltage power switching transistors. Forexample, a device of a given size in SiC will have a blocking voltage 8 times higher than thesame device in silicon. More importantly, the on-resistance of the SiC device will be about twodecades lower than the silicon device. Consequently, the efficiency of the power converter ishigher. In addition, SiC-based semiconductor switches can operate at high temperatures(~600 C) without much change in their electrical properties. Thus the converter has a higherreliability. Reduced losses and allowable higher operating temperatures result in smaller heatsinksize. Moreover, the high frequency operating capability of SiC converters lowers the filteringrequirement and the filter size. As a result, they are compact, light, reliable, and efficient andhave a high power density. These qualities satisfy the requirements of power converters for mostapplications and they are expected to be the devices of the future. Ratings have been progressively increasing for all devices while the newer devices offersubstantially better performance. With the SCR and the pin-diodes, so called because of thesandwiched intrinsic ‘i’-layer between the ‘p’ and ‘n’ layers, having mostly line-commutatedconverter applications, emphasis was mostly on their static characteristics - forward and reversevoltage blocking, current carrying and over-current ratings, on-state forward voltage etc and alsoon issues like paralleling and series operation of the devices. As the operating speeds of thedevices increased, the dynamic (switching) characteristics of the devices assumed greaterimportance as most of the dissipation was during these transients. Attention turned to thedevelopment of efficient drive networks and protection techniques which were found to enhancethe performance of the devices and their peak power handling capacities. Issues related toparalleling were resolved by the system designer within the device itself like in MOSFETS,while the converter topology was required to take care of their series operation as in multi-levelconverters. The range of power devices thus developed over the last few decades can be represented as atree, Fig. 1.5, on the basis of their controllability and other dominant features. Version 2 EE IIT, Kharagpur 8
  9. 9. POWER SEMICONDUCTOR DEVICES UNCONTROLLED CONTROLLED RECTIFIERS ACCESSORIES REGENERATIVE NON-REGENERATIVE INTEGRATED POWER SILICON SCR BJT IGCT DIAC TRIAC MOSFET PIC DIODES Zenner GTO IGBT INTELLIGENT FREDS MOV POWER MODULES SCHOTTKY Fig. 1.5 Power semiconductor device varietyPower Diodes diF /dt t0 t1 t2 SNAPPY SOFT Q1 Q2 Δ to IRM VRM Fig. 1.6 Typical turn-off dynamics of a soft and a snappy diode Silicon Power diodes are the successors of Selenium rectifiers having significantly improvedforward characteristics and voltage ratings. They are classified mainly by their turn-off(dynamic) characteristics Fig. 1.6. The minority carriers in the diodes require finite time - trr(reverse recovery time) to recombine with opposite charges and neutralise. Large values of Qrr (=Q1 + Q2) - the charge to be dissipated as a negative current when the and diode turns off and trr(= t2 - t0) - the time it takes to regain its blocking features, impose strong current stresses on thecontrolled device in series. Also a snappy type of recovery of the diode effects high di/dtvoltages on all associated power device in the converter because of load or stray inductancespresent in the network. There are broadly three types of diodes used in Power electronicapplications:Line-frequency diodes: These PIN diodes with general-purpose rectifier type applications, areavailable at the highest voltage (~5kV) and current ratings (~5kA) and have excellent over-current (surge rating about six times average current rating) and surge-voltage withstandcapability. They have relatively large Qrr and trr specifications. Version 2 EE IIT, Kharagpur 9
  10. 10. Fast recovery diodes: Fast recovery diffused diodes and fast recovery epitaxial diodes, FREDs,have significantly lower Qrr and trr (~ 1.0 sec). They are available at high powers and aremainly used in association with fast controlled-devices as free-wheeling or DC-DC choppers andrectifier applications. Fast recovery diodes also find application in induction heating, UPS andtraction.Schottky rectifiers: These are the fastest rectifiers being majority carrier devices without anyQrr.. However, they are available with voltage ratings up to a hundred volts only though currentratings may be high. Their conduction voltages specifications are excellent (~0.2V). The freedomfrom minority carrier recovery permits reduced snubber requirements. Schottky diodes face nocompetition in low voltage SPMS applications and in instrumentation.Silicon Controlled Rectifier (SCR) The Silicon Controlled Rectifier is the most popular of the thyristor family of four layerregenerative devices. It is normally turned on by the application of a gate pulse when a forwardbias voltage is present at the main terminals. However, being regenerative or latching, it cannotbe turned off via the gate terminals specially at the extremely high amplification factor of thegate. There are two main types of SCRs. Converter grade or Phase Control thyristors These devices are the work horses of thePower Electronics. They are turned off by natural (line) commutation and are reverse biased atleast for a few milliseconds subsequent to a conduction period. No fast switching feature isdesired of these devices. They are available at voltage ratings in excess of 5 KV starting fromabout 50 V and current ratings of about 5 KA. The largest converters for HVDC transmission arebuilt with series-parallel combination of these devices. Conduction voltages are device voltagerating dependent and range between 1.5 V (600V) to about 3.0 V (+5 KV). These devices areunsuitable for any forced-commutated circuit requiring unwieldy large commutationcomponents. The dynamic di/dt and dv/dt capabilities of the SCR have vastly improved over the yearsborrowing emitter shorting and other techniques adopted for the faster variety. The requirementfor hard gate drives and di/dt limting inductors have been eliminated in the process. Inverter grade thyristors: Turn-off times of these thyristors range from about 5 to 50 μsecswhen hard switched. They are thus called fast or inverter grade SCRs. The SCRs are mainlyused in circuits that are operated on DC supplies and no alternating voltage is available to turnthem off. Commutation networks have to be added to the basic converter only to turn-off theSCRs. The efficiency, size and weight of these networks are directly related to the turn-off time,tq of the SCR. The commutation circuits utilised resonant networks or charged capacitors. Quitea few commutation networks were designed and some like the McMurray-Bedford becamewidely accepted. Asymmetrical, light-activated, reverse conducting SCRs Quite a few varieties of thebasic SCR have been proposed for specific applications. The Asymmetrical thyristor isconvenient when reactive powers are involved and the light activated SCR assists in parallelingor series operation. Version 2 EE IIT, Kharagpur 10
  11. 11. MOSFET The Power MOSFET technology has mostly reached maturity and is the most popular devicefor SMPS, lighting ballast type of application where high switching frequencies are desired butoperating voltages are low. Being a voltage fed, majority carrier device (resistive behaviour)with a typically rectangular Safe Operating Area, it can be conveniently utilized. Utilising sharedmanufacturing processes, comparative costs of MOSFETs are attractive. For low frequencyapplications, where the currents drawn by the equivalent capacitances across its terminals aresmall, it can also be driven directly by integrated circuits. These capacitances are the mainhindrance to operating the MOSFETS at speeds of several MHz. The resistive characteristics ofits main terminals permit easy paralleling externally also. At high current low voltageapplications the MOSFET offers best conduction voltage specifications as the RDS(ON)specification is current rating dependent. However, the inferior features of the inherent anti-parallel diode and its higher conduction losses at power frequencies and voltage levels restrict itswider application.The IGBT It is a voltage controlled four-layer device with the advantages of the MOSFET driverand the Bipolar Main terminal. IGBTs can be classified as punch-through (PT) and non-punch-through (NPT) structures. In the punch-through IGBT, a better trade-off between the forwardvoltage drop and turn-off time can be achieved. Punch-through IGBTs are available up to about1200 V. NPT IGBTs of up to about 4 KV have been reported in literature and they are morerobust than PT IGBTs particularly under short circuit conditions. However they have a higherforward voltage drop than the PT IGBTs. Its switching times can be controlled by suitablyshaping the drive signal. This gives the IGBT a number of advantages: it does not requireprotective circuits, it can be connected in parallel without difficulty, and series connection ispossible without dv/dt snubbers. The IGBT is presently one of the most popular device in viewof its wide ratings, switching speed of about 100 KHz a easy voltage drive and a square SafeOperating Area devoid of a Second Breakdown region.The GTOThe GTO is a power switching device that can be turned on by a short pulse of gate current andturned off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anodecurrent to be turned off. Hence there is no need for an external commutation circuit to turn it off.Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time isshort, thus giving it more capability for highfrequency operation than thyristors. The GTOsymbol and turn-off characteristics are shown in Fig. 30.3. GTOs have the I2t withstandcapability and hence can be protected by semiconductor fuses. For reliable operation of GTOs,the critical aspects are proper design of the gate turn-off circuit and the snubber circuit.Power Converter TopologiesA Power Electronic Converter processes the available form to another having a differentfrequency and/or voltage magnitude. There can be four basic types of converters depending uponthe function performed: Version 2 EE IIT, Kharagpur 11
  12. 12. CONVERSION FROM/TO NAME FUNCTION SYMBOL DC to DC Chopper Constant to variable DC or variable to constant DC DC to AC Inverter DC to AC of desired voltage and frequency ~ AC to DC Rectifier AC to unipolar (DC) current ~ Cycloconverter, AC to AC AC-PAC, AC of desired frequency and/or ~ Matrix converter magnitude from generally line AC ~Base / gate drive circuitAll discrete controlled devices, regenerative or otherwise have three terminals. Two of these arethe Main Terminals. One of the Main Terminals and the third form the Control Terminal. Theamplification factor of all the devices (barring the now practically obsolete BJT) are quite high,though turn-on gain is not equal to turn-off gain. The drive circuit is required to satisfy thecontrol terminal characteristics to efficiently tun-on each of the devices of the converter, turnthem off, if possible, again optimally and also to protect the device against faults, mostly over-currents. Being driven by a common controller, the drives must also be isolated from each otheras the potentials of the Main Terminal which doubles as a Control terminal are different atvarious locations of the converter. Gate-turn-off-able devices require precise gate drivewaveform for optimal switching. This necessitates a wave-shaping amplifier. This amplifier islocated after the isolation stage. Thus separate isolated power supplies are also required for each Power device in theconverter (the ones having a common Control Terminal - say the Emitter in an IGBT - mayrequire a few less). There are functionally two types of isolators: the pulse transformer whichcan transmit after isolation, in a multi-device converter, both the un-shaped signal and power andoptical isolators which transmit only the signal. The former is sufficient for a SCR withoutisolated power supplies at the secondary. The latter is a must for practically all other devices.Fig. 1.7 illustrates to typical drive circuits for an IGBT and an SCR. Version 2 EE IIT, Kharagpur 12
  13. 13. IGBT Vref COMPARATOR TIMER Fig. 1.7 Simple gate-drive and protection circuit for a stand-alone IGBT and a SCRProtection of Power devices and converters Power electronic converters often operate from the utility mains and are exposed to thedisturbances associated with it. Even otherwise, the transients associated with switching circuitsand faults that occur at the load point stress converters and devices. Consequently, severalprotection schemes must be incorporated in a converter. It is necessary to protect both the MainTerminals and the control terminals. Some of these techniques are common for all devices andconverters. However, differences in essential features of devices call for special protectionschemes particular for those devices. The IGBT must be protected against latching, and similarlythe GTOs turn-off drive is to be disabled if the Anode current exceeds the maximum permissibleturn-off-able current specification. Power semiconductor devices are commonly protectedagainst: 1. Over-current; 2. di/dt; 3. Voltage spike or over-voltage; 4. dv/dt ; 5. Gate-under voltage; 6. Over voltage at gate; 7. Excessive temperature rise; 8. Electro-static discharge; Semiconductor devices of all types exhibit similar responses to most of the stresses, howeverthere are marked differences. The SCR is the most robust device on practically all counts. That ithas an I2t rating is proof that its internal thermal capacities are excellent. A HRC fuse, suitablyselected, and in co-ordination with fast circuit breakers would mostly protect it. This sometimesbecomes a curse when the cost of the fuse becomes exorbitant. All transistors, specially the BJTand the IGBT is actively protected (without any operating cost!) by sensing the Main Terminalvoltage, as shown in Fig. 1.7. This voltage is related to the current carried by the device. Further,the transistors permit designed gate current waveforms to minimise voltage spikes as aconsequence of sharply rising Main terminal currents. Gate resistances have significant effect onturn-on and turn-off times of these devices - permitting optimisation of switching times for thereduction of switching losses and voltage spikes. Version 2 EE IIT, Kharagpur 13
  14. 14. Protection schemes for over-voltages - the prolonged ones and those of short duration - areguided by the energy content of the surges. Metal Oxide Varistors (MOVs), capacitive dynamicvoltage-clamps and crow-bar circuits are some of the strategies commonly used. For high dv/dtstresses, which again have similar effect on all devices, R-C or R-C-D clamps are useddepending on the speed of the device. These snubbers or switching-aid-networks, additionallyminimise switching losses of the device - thus reducing its temperature rise. Gates of all devices are required to be protected against over-voltages (typically + 20 V)specially for the voltage driven ones. This is achieved with the help of Zener clamps - the zenerbeing also a very fast-acting device. Protection against issues like excessive case temperatures and ESD follow well-set practices.Forced-cooling techniques are very important for the higher rated converters and wholeenvironments are air-cooled to lower the ambient.Objective type questionsQs#1 Which is the Power semiconductor device having a) Highest switching speed; b) Highest voltage / current ratings; c) Easy drive features; d) Can be most effectively paralleled; e) Can be protected against over-currents with a fuse; f) Gate-turn off capability with regenerative features; g) Easy drive and High power handling capabilityAns: a) MOSFET; b) SCR; c) MOSFET; d) MOSFET; e) SCR ; (f) GTO; (g) IGBTQs#2 An SCR requires 50 mA gate current to switch it on. It has a resistive load and is suppliedfrom a 100 V DC supply. Specify the Pulse transformer details and the circuit following it, if thedriver circuit supply voltage is 10 V and the gate-cathode drop is about 1 V.Ans: The most important ratings of the Pulse transformer are its volt-secs rating, the isolationvoltage and the turns ratio.The volt-secs is decided by the product of the primary pulse-voltage multiplied by the period forwhich the pulse is applied to the windingIf the primary pulse voltage = (Supply voltage – drive transistor drop)The turn-on time of he SCR may be in the range 50 μsecs for an SCR of this rating.Consequently the volt secs may be in the range of 9 x 50 = 450 μvolt-secsThe Pulse transformer may be chosen as: 1:1, 450 μVs, Visol = 2.5 KV, IM = 150 mAThe circuit shown in Fig. 1.7 may be used. Diodes 1N4002Series resistance= (Supply voltage – drive transistor drop – gate-cathode drop)/100mA= (10 –1 –1) / 100 E-3= 80 Ohm= 49 or 57 Ohm (nearest available lower value) Version 2 EE IIT, Kharagpur 14
  15. 15. Module 1Power Semiconductor Devices Version 2 EE IIT, Kharagpur 1
  16. 16. Lesson 2Constructional Features, Operating Principle, Characteristics and Specification of Power Semiconductor Diode Version 2 EE IIT, Kharagpur 2
  17. 17. Instructional ObjectiveOn Completion the student will be able to 1. Draw the spatial distribution of charge density, electric field and electric potential in a step junction p-n diode. 2. Calculate the voltage drop across a forward biased diode for a given forward current and vice-verse. 3. Identify the constructional features that distinguish a power diode from a signal level diode. 4. Differentiate between different reverse voltage ratings found in a Power Diode speciation sheet. 5. Identify the difference between the forward characteristic of a power diode and a signal level diode and explain it. 6. Evaluate the forward current specifications of a diode for a given application. 7. Draw the “Turn On” and “Turn Off” characteristics of a power diode. 8. Define “Forward recovery voltage”, “Reverse recovery current” “Reverse Recovery charge” as applicable to a power diode. Version 2 EE IIT, Kharagpur 3
  18. 18. Power Semiconductor Diodes2.1 IntroductionPower semiconductor diode is the “power level” counter part of the “low power signal diodes”with which most of us have some degree of familiarity. These power devices, however, arerequired to carry up to several KA of current under forward bias condition and block up toseveral KV under reverse biased condition. These extreme requirements call for importantstructural changes in a power diode which significantly affect their operating characteristics.These structural modifications are generic in the sense that the same basic modifications areapplied to all other low power semiconductor devices (all of which have one or more p-njunctions) to scale up their power capabilities. It is, therefore, important to understand the natureand implication of these modifications in relation to the simplest of the power devices, i.e., apower semiconductor diode.2.2 Review of Basic p-n Diode CharacteristicsA p-n junction diode is formed by placing p and n type semiconductor materials in intimatecontact on an atomic scale. This may be achieved by diffusing acceptor impurities in to an n typesilicon crystal or by the opposite sequence.In an open circuit p-n junction diode, majority carriers from either side will defuse across thejunction to the opposite side where they are in minority. These diffusing carriers will leavebehind a region of ionized atoms at the immediate vicinity of the metallurgical junction. Thisregion of immobile ionized atoms is called the space charge region. This process continues tillthe resultant electric field (created by the space charge density) and the potential barrier at thejunction builds up to sufficient level to prevent any further migration of carriers. At this point thep-n junction is said to be in thermal equilibrium condition. Variation of the space charge density,the electric field and the potential along the device is shown in Fig 2.1 (a). Version 2 EE IIT, Kharagpur 4
  19. 19. (a) (b) (c)Fig 2.1: Space change density the electric field and the electric potential in side a p-n junction under (a) thermal equilibrium condition, (b) reverse biased condition, (c) forward biased condition.When an external voltage is applied with p side move negative then the n side the junction issaid to be under reverse bias condition. This reverse bias adds to the height of the potentialbarrier. The electric field strength at the junction and the width of the space change region (alsocalled “the depletion region” because of the absence of free carriers) also increases. On the otherhand, free minority carrier densities (np in the p side and pn in the n side) will be zero at the edgeof the depletion region on either side (Fig 2.1 (b)). This gradient in minority carrier densitycauses a small flux of minority carriers to defuse towards the deletion layer where they are sweptimmediately by the large electric field into the electrical neutral region of the opposite side. Thiswill constitute a small leakage current across the junction from the n side to the p side. Therewill also be a contribution to the leakage current by the electron hole pairs generated in the spacechange layer by the thermal ionization process. These two components of current together iscalled the “reverse saturation current Is” of the diode. Value of Is is independent of the reversevoltage magnitude (up to a certain level) but extremely sensitive to temperature variation.When the applied reverse voltage exceeds some threshold value (for a given diode) the reversecurrent increases rapidly. The diode is said to have undergone “reverse break down”.Reverse break down is caused by "impact ionization" as explained below. Electrons acceleratedby the large depletion layer electric field due to the applied reverse voltage may attain sufficientknick energy to liberate another electron from the covalent bonds when it strikes a silicon atom.The liberated electron in turn may repeat the process. This cascading effect (avalanche) mayproduce a large number of free electrons very quickly resulting in a large reverse current. Thepower dissipated in the device increases manifold and may cause its destruction. Therefore,operation of a diode in the reverse breakdown region must be avoided. Version 2 EE IIT, Kharagpur 5
  20. 20. When the diode is forward biased (i.e., p side more positive than n side) the potential barrier islowered and a very large number of minority carriers are injected to both sides of the junction.The injected minority carriers eventually recombines with the majority carries as they defusefurther into the electrically neutral drift region. The excess free carrier density in both p and nside follows exponential decay characteristics. The characteristic decay length is called the"minority carrier diffusion length"Carrier density gradients on either side of the junction are supported by a forward current IF(flowing from p side to n side) which can be expressed as IF = IS ( exp ( qv/kT ) ) -1 (2.1)Where Is = Reverse saturation current ( Amps) v = Applied forward voltage across the device (volts) q = Change of an electron k = Boltzman’s constant T = Temperature in KelvinFrom the foregoing discussion the i-v characteristics of a p-n junction diode can be drawn asshown in Fig 2.2. While drawing this characteristics the ohmic drop in the bulk of thesemiconductor body has been neglected. Fig 2.2: Volt-Ampere ( i-v ) characteristics of a p-n junction diodeExercise 2.1(1) Fill in the blanks with the appropriate word(s). (i) The width of the space charge region increases as the applied ______________ voltage increases. (ii) The maximum electric field strength at the center of the depletion layer increases with _______________ in the reverse voltage. (iii) Reverse saturation current in a power diode is extremely sensitive to ___________ variation. Version 2 EE IIT, Kharagpur 6
  21. 21. (iv) Donor atoms are _____________________ carrier providers in the p type and _________________ carrier providers in the n type semiconductor materials. (v) Forward current density in a diode is __________________________ proportional to the life time of carriers.Answer: (i) Reverse, (ii) increase, (iii) temperature, (iv) Minority Majority, (v) inversely(2) A p-n junction diode has a reverse saturation current rating of 50 nA at 32°C. Whatshould be the value of the forward current for a forward voltage drop of 0.5V. Assume VT =KT/q at 32°C = 26 mv.Answer ⎛ V ⎞ I F = I s ⎜ e VT - 1 ⎟ , Is = 5×10-8 A, VT = 26×10-3 V V = 0.5V ⎝ ⎠∴ I F = 11.24 Am ps. di(3) For the diode of Problem-2 calculate the dynamic ac resistance ra c = F d v F at 32°C and aforward voltage drop of 0.5V.Answer: ⎛ VF VT ⎞ diF Is VF iF = Is ⎜ e -1⎟ ∴ = e VT ⎝ ⎠ dVF VT N ow I s = 5 × 10 -8 A , V F = 0.5V , -3 VT = 26 ×10 V at 32o C V dVF V - F ∴ = ra c = T e V T = 2 .3 1 3 m Ω diF Is2.3 Construction and Characteristics of Power DiodesAs mention in the introduction Power Diodes of largest power rating are required to conductseveral kilo amps of current in the forward direction with very little power loss while blockingseveral kilo volts in the reverse direction. Large blocking voltage requires wide depletion layer inorder to restrict the maximum electric field strength below the “impact ionization” level. Spacecharge density in the depletion layer should also be low in order to yield a wide depletion layerfor a given maximum Electric fields strength. These two requirements will be satisfied in alightly doped p-n junction diode of sufficient width to accommodate the required depletion layer.Such a construction, however, will result in a device with high resistively in the forwarddirection. Consequently, the power loss at the required rated current will be unacceptably high.On the other hand if forward resistance (and hence power loss) is reduced by increasing thedoping level, reverse break down voltage will reduce. This apparent contradiction in therequirements of a power diode is resolved by introducing a lightly doped “drift layer” of required Version 2 EE IIT, Kharagpur 7
  22. 22. thickness between two heavily doped p and n layers as shown in Fig 2.3(c). Fig 2.3 (a) and (b)shows the circuit symbol and the photograph of a typical power diode respectively. (b)Fig. 2.3: Diagram of a power; (a) circuit symbol (b) photograph; (c) schematic cross section.To arrive at the structure shown in Fig 2.3 (c) a lightly doped n- epitaxial layer of specified width(depending on the required break down voltage) and donor atom density (NdD) is grown on aheavily doped n+ substrate (NdK donor atoms.Cm -3) which acts as the cathode. Finally the p-njunction is formed by defusing a heavily doped (NaA acceptor atoms.Cm-3) p+ region into theepitaxial layer. This p type region acts as the anode.Impurity atom densities in the heavily doped cathode (Ndk .Cm -3) and anode (NaA.Cm -3) areapproximately of the same order of magnitude (10 19 Cm -3) while that of the epitaxial layer (alsocalled the drift region) is lower by several orders of magnitude (NdD ≈ 10 14 Cm-3). In a lowpower diode this drift region is absent. The Implication of introducing this drift region in a powerdiode is explained next.2.3.1 Power Diode under Reverse Bias Conditions BackAs in the case of a low power diode the applied reverse voltage is supported by the depletionlayer formed at the p+ n- metallurgical junction. Overall neutrality of the space change regiondictates that the number of ionized atoms in the p+ region should be same as that in the n- region.However, since NdD << NaA, the space charge region almost exclusively extends into the n- drift Version 2 EE IIT, Kharagpur 8
  23. 23. region. Now the physical width of the drift region (WD) can be either larger or smaller than thedepletion layer width at the break down voltage. Consequently two type of diodes exist, (i) nonpunch through type, (ii) punch through type. In “non-punch through” diodes the depletion layerboundary doesn’t reach the end of the drift layer. On the other hand in “punch through” diodesthe depletion layer spans the entire drift region and is in contact with the n+ cathode. However,due to very large doping density of the cathode, penetration of drift region inside cathode isnegligible. Electric field strength inside the drift region of both these type of diodes at breakdown voltage is shown in Fig 2.4.Fig 2.4: Electric field strength in reverse biased power Diodes; (a) Non-punch through type; (b) punch through type.In non-punch through type diodes the electric field strength is maximum at the p+ n- junction anddecrease to zero at the end of the depletion region. Where as, in the punch through constructionthe field strength is more uniform. In fact, by choosing a very lightly doped n- drift region,Electric field strength in this region can be mode almost constant. Under the assumption ofuniform electric field strength it can be shown that for the same break down voltage, the “punchthrough” construction will require approximately half the drift region width of a comparable “non - punch through” construction.Lower drift region doping in a “punch through” diode does not carry the penalty of higherconduction lasses due to “conductivity modulation” to be discussed shortly. In fact, reducedwidth of the drift region in these diodes lowers the on-state voltage drop for the same forwardcurrent density compared to a non-punch through diode.Under reverse bias condition only a small leakage current (less than 100mA for a rated forwardcurrent in excess of 1000A) flows in the reverse direction (i.e from cathode to anode). Thisreverse current is independent of the applied reverse voltage but highly sensitive to junctiontemperature variation. When the applied reverse voltage reaches the break down voltage, reversecurrent increases very rapidly due to impact ionization and consequent avalanche multiplicationprocess. Voltage across the device dose not increase any further while the reverse current islimited by the external circuit. Excessive power loss and consequent increase in the junctiontemperature due to continued operation in the reverse brake down region quickly destroies thediode. Therefore, continued operation in the reverse break down region should be avoided. Atypical I-V characteristic of a power diode under reverse bias condition is shown in Fig 2.5. Version 2 EE IIT, Kharagpur 9
  24. 24. Fig 2.5: Reverse bias i-v characteristics of a power Diode.A few other important specifications of a power Diode under reverse bias condition usuallyfound in manufacturer’s data sheet are explained below.DC Blocking Voltage (VRDC): Maximum direct voltage that can be applied in the reversedirection (i.e cathode positive with respect to anode) across the device for indefinite period oftime. It is useful for selecting free-wheeling diodes in DC-DC Choppers and DC-AC voltagesource inverter circuits.RMS Reverse Voltage (VRMS): It is the RMS value of the power frequency (50/60 HZ) sincewave voltage that can be directly applied across the device. Useful for selecting diodes forcontrolled / uncontrolled power frequency line commutated AC to DC rectifiers. It is given bythe manufacturer under the assumption that the supply voltage may rise by 10% at the most. Thisrating is different for resistive and capacitive loads.Peak Repetitive Reverse Voltage (VRRM): This is the maximum permissible value of theinstantiations reverse voltage appearing periodically across the device. The time period betweentwo consecutive appearances is assumed to be equal to half the power cycle (i.e 10ms for 50 HZsupply). This type of period reverse voltage may appear due to “commutation” in a converter.Peak Non-Repetitive Reverse Voltage (VRSM): It is the maximum allowable value of theinstantaneous reverse voltage across the device that must not recur. Such transient reversevoltage can be generated by power line switching (i.e circuit Breaker opening / closing) orlightning surges.Fig. 2.6 shows the relationship among these different reverse voltage specifications. Version 2 EE IIT, Kharagpur 10
  25. 25. Fig. 2.6: Reverse Voltage ratings of a power diode; (a) Supply voltage wave form; (b) Reverse i-v characteristics2.3.2 Power Diode under Forward Bias ConditionIn the previous section it was shown how the introduction of a lightly doped drift region in the p-n structure of a diode boosts its blocking voltage capacity. It may appear that this lightly dopeddrift region will offer high resistance during forward conduction. However, the effectiveresistance of this region in the ON state is much less than the apparent ohmic resistancecalculated on the basis of the geometric size and the thermal equilibrium carrier densities. This isdue to substantial injection of excess carriers from both the p+ and the n+ regions in the driftregion as explained next.As the metallurgical p+ n- junction becomes forward biased there will be injection of excess ptype carrier into the n- side. At low level of injections (i.e δp << nno) all excess p type carriersrecombine with n type carriers in the n- drift region. However at high level of injection (i.e largeforward current density) the excess p type carrier density distribution reaches the n- n+ junctionand attracts electron from the n+ cathode. This leads to electron injection into the drift regionacross the n- n+ junction with carrier densities δn = δp. This mechanism is called “doubleinjection”Excess p and n type carriers defuse and recombine inside the drift region. If the width of the driftregion is less than the diffusion length of carries the spatial distribution of excess carrier densityin the drift region will be fairly flat and several orders of magnitude higher than the thermalequilibrium carrier density of this region. Conductivity of the drift region will be greatlyenhanced as a consequence (also called conductivity modulation).The voltage dropt across a forward conducting power diode has two components i.e Vak = Vj + VRD (2.2)Where Vj is the drop across the p+n- junction and can be calculated from equation (2.1) for agiven forward current jF. The component VRD is due to ohmic drop mostly in the drift region.Detailed calculation shows VRD ∞ JF WD (2.3)Where JF is the forword current density in the diode and WD is the width of the drift region.Therefore Vak = Vj + RON IF (2.3)The ohmic drop makes the forward i-v characteristic of a power diode more linear. Version 2 EE IIT, Kharagpur 11
  26. 26. Fig 2.7: Characteristics of a forward biased power Diode; (a) Excess free carrier density distribution; (b) i-v characteristics.Both Vj and VAK have negative temperature coefficient as shown in the figure.Few other important specifications related to forward bias operation of power diode as found inmanufacturer’s data sheet are explained next.Maximum RMS Forward current (IFRMS): Due to predominantly resistive nature of theforward voltage drop across a forward biased power diode, RMS value of the forward currentdetermines the conduction power loss. The specification gives the maximum allowable RMSvalue of the forward current of a given wave shape (usually a half cycle sine wave of powerfrequency) and at a specified case temperature. However, this specification can be used as aguideline for almost all wave shapes of the forward current.Maximum Average Forward Current (IFAVM): Diodes are often used in rectifier circuitssupplying a DC (average) current to be load. In such cases the average load current and the diodeforward current usually have a simple relationship. Therefore, it will be of interest to know the Version 2 EE IIT, Kharagpur 12
  27. 27. maximum average current a diode can conduct in the forward direction. This specification givesthe maximum average value of power frequency half cycle sine wave current allowed to flowthrough the diode in the forward direction. Average current rating of a diode decreases withreduction in conduction angle due to increase in current “form factor”.Both IFRMS and IFAVM ratings are given at a specified case temperature. If the case temperatureincreases beyond this limit these ratings has to be reduced correspondingly. “Derating curves”provide by the manufacturers give the relationship between IFAVM (IFRMS) with allowable casetemperature as shown in Fig. 2.8. Fig 2.8: Derating curves for the forward current of a Power Diode.Average Forward Power loss (PAVF): Almost all power loss in a diode occurs during forwardconduction state. The forward power loss is therefore an important parameter in designing thecooling arrangement. Average forward power loss over a full cycle is specified by themanufacturers as a function of the average forward current (IAVF) for different conduction anglesas shown in Fig 2.9. Fig 2.9: Average forward power loss vs. average forward current of a power Diode. Version 2 EE IIT, Kharagpur 13
  28. 28. Surge and Fault Current: In some rectifier applications a diode may be required to conductforward currents far in excess of its RMS or average forward current rating for some duration(several cycles of the power frequency). This is called the repetitive surge forward current of adiode. A diode is expected to operate normally after the surge duration is over.On the other hand, fault current arising due to some abnormality in the power circuit may have ahigher peak valve but exists for shorter duration (usually less than an half cycle of the powerfrequency). A diode circuit is expected to be disconnected from the power line following a fault.Therefore, a fault current is a non repetitive surge current. Power diodes are capable ofwithstanding both types of surge currents and this capability is expressed in terms of two surgecurrent ratings as discussed next.Peak Repetitive surge current rating (IFRM): This is the peak valve of the repetitive surgecurrent that can be allowed to flow through the diode for a specific duration and for specifiedconditions before and after the surge. The surge current waveform is assumed to be halfsinusoidal of power frequency with current pulses separated by “OFF” periods of equal duration.The case temperature is usually specified at its maximum allowable valve before the surge. Thediode should be capable of withstanding maximum repetitive peak reverse voltage (VRRM) andMaximum allowable average forward current (IFAVM) following the surge. The surge currentspecification is usually given as a function of the surge duration in number of cycles of thepower frequency as shown in figure 2.10 Fig 2.10: Peak Repetitive surge current VS time curve of a power diode.In case the surge current is specified only for a fixed number of cycles ‘m’then the surge current specification applicable to some other cycle number ‘n’ can be found fromthe approximate formula. m I FRM n = I (2.4) n FRM mPeak Non-Repetitive surge current (IFRM): This specification is similar to the previous oneexcept that the current pulse duration is assumed to be within one half cycle of the power Version 2 EE IIT, Kharagpur 14
  29. 29. frequency. This specification is given as a function of the current pulse duration as shown in Fig2.11.Maximum surge current Integral (∫i2dt): This is a surge current related specification and givesa measure of the heat energy generated inside the device during a non-repetitive surge. It isuseful for selecting the protective fuse to be connected in series with the diode. This specificationis also given as a function of the current pulse duration as shown Fig 2.11Fig. 2.11: Non-repetitive surge current and surge current integral vs. current pulse width characteristics of a power Diode.Exercise 2.2 (1) Fill in the blanks with the appropriate word(s). i. The ____________ region in a power diode increases its reverse voltage blocking capacity. ii. The maximum DC voltage rating (VRDC) of a power diode is useful for selecting ________________ diodes in a DC-DC chopper. iii. The reverse breakdown voltage of a Power Diode must be greater than ________________ . iv. The i-v characteristics of a power diode for large forward current is __________ . v. The average current rating of a power diode _______________ with reduction in the conduction angle due to increase in the current ___________________ . vi. The derating curves of a Power diode provides relationship between the ______________ and the _________________ . ∫ i dt rating of a power diode is useful for selecting the ________________ . 2vii.Answer: (i) drift, (ii) free wheeling, (iii) VRSM, (iv) linear, (v) decrease, form factor, (vi)IFAVM/IFRM, case temperature, (vii) protective fuse. Version 2 EE IIT, Kharagpur 15
  30. 30. (2). (a) For the single phase half wove rectifier shown find out the VRRM rating of D. (b) Will the required VRRM rating change if a inductor is placed between the diode and n capacitor. (c) What will be the required VRRM rating if the capacitor is removed. Assume a resistive load. (d) The source of the single phase rectifier circuit has an internal resistance of 2 Ω. Find out the required Non repetitive peak surge current rating of the diode. Also find the i2t rating of the protective fuse to be connected in series with the diode.Answer: (a) During every positive half cycle of the supply the capacitor charges to the peakvalue of the supply voltage. If the load disconnected the capacitor voltage will not change whenthe supply goes through its negative peak as shown in the associated waveform. Therefore thediode will be subjected to a reverse voltage equal to the peak to peak supply voltage in eachcycle. Hence, the required VRRM rating will be VRRM = 2 × 2 × 230V = 650V(b) When an inductor is connected between the diode and the capacitor the inductor currentwill have some positive value at t = t1. If the load is disconnected the stored energy in theinductor will charge the capacitor beyond the peak supply voltage. Since there is no dischargepath for the capacitor this voltage across the capacitor will be maintained when the supplyvoltage goes through negative peak. Therefore, the diode will be subjected to a reverse voltagegreater than the peak to peak supply voltage. The required VRRM rating will increase. Version 2 EE IIT, Kharagpur 16
  31. 31. (c) If the capacitor is removed and the load is resistive the voltage VKN during negative halfcycle of the supply will be zero since the load current will be zero. Therefore the reverse voltageacross the diode will be equal to the peak supply voltage. So the required VRRM rating will be VRRM = 2 × 230V = 325 Volts(d) Peak surge current will flow through the circuit when the load is accidentally short circuited.The peak surge current rating will be 2 × 230 I FSM = A = 162.64 A 2 The peak non repetitive surge current should not recur. Therefore, the protective fuse (tobe connected in series with the diode) must blow during the negative half cycle following thefault. Therefore the maximum i2t rating of the fuse is π ∫i 2 dt = ∫ I 2 F S M S in 2 w td w t = π I 2 F S m = 4 1 .5 5 × 1 0 3 A 2 s e c M ax o 22.3.3 Switching Characteristics of Power DiodesPower Diodes take finite time to make transition from reverse bias to forward bias condition(switch ON) and vice versa (switch OFF).Behavior of the diode current and voltage during these switching periods are important due to thefollowing reasons. • Severe over voltage / over current may be caused by a diode switching at different points in the circuit using the diode. • Voltage and current exist simultaneously during switching operation of a diode. Therefore, every switching of the diode is associated with some energy loss. At high switching frequency this may contribute significantly to the overall power loss in the diode.Observed Turn ON behavior of a power Diode: Diodes are often used in circuits with di/dtlimiting inductors. The rate of rise of the forward current through the diode during Turn ON hassignificant effect on the forward voltage drop characteristics. A typical turn on transient is shownin Fig. 2.12. Version 2 EE IIT, Kharagpur 17
  32. 32. Fig. 2.12: Forward current and voltage waveforms of a power diode during Turn On operation.It is observed that the forward diode voltage during turn ON may transiently reach a significantlyhigher value Vfr compared to the steady slate voltage drop at the steady current IF.In some power converter circuits (e.g voltage source inverter) where a free wheeling diode isused across an asymmetrical blocking power switch (i.e GTO) this transient over voltage may behigh enough to destroy the main power switch.Vfr (called forward recovery voltage) is given as a function of the forward di/dt in themanufacturer’s data sheet. Typical values lie within the range of 10-30V. Forward recovery time(tfr) is typically within 10 us.Observed Turn OFF behavior of a Power Diode: Figure 2.13 shows a typical turn offbehavior of a power diode assuming controlled rate of decrease of the forward current. Version 2 EE IIT, Kharagpur 18
  33. 33. Fig. 2.13: Reverse Recovery characteristics of a power diodeSalient features of this characteristics are: • The diode current does not stop at zero, instead it grows in the negative direction to Irr called “peak reverse recovery current” which can be comparable to IF. In many power electronic circuits (e.g. choppers, inverters) this reverse current flows through the main power switch in addition to the load current. Therefore, this reverse recovery current has to be accounted for while selecting the main switch. • Voltage drop across the diode does not change appreciably from its steady state value till the diode current reaches reverse recovery level. In many power electric circuits (choppers, inverters) this may create an effective short circuit across the supply, current being limited only by the stray wiring inductance. Also in high frequency switching circuits (e.g, SMPS) if the time period t4 is comparable to switching cycle qualitative modification to the circuit behavior is possible. • Towards the end of the reverse recovery period if the reverse current falls too sharply, (low value of S), stray circuit inductance may cause dangerous over voltage (Vrr) across the device. It may be required to protect the diode using an RC snubber.During the period t5 large current and voltage exist simultaneously in the device. At highswitching frequency this may result in considerable increase in the total power loss.Important parameters defining the turn off characteristics are, peak reverse recovery current (Irr),reverse recovery time (trr), reverse recovery charge (Qrr) and the snappiness factor S.Of these parameters, the snappiness factor S depends mainly on the construction of the diode(e.g. drift region width, doping lever, carrier life time etc.). Other parameters are interrelated andalso depend on S. Manufacturers usually specify these parameters as functions of diF/dt fordifferent values of IF. Both Irr and Qrr increases with IF and diF/dt while trr increases with IF anddecreases with diF/dt. Version 2 EE IIT, Kharagpur 19
  34. 34. The reverse recovery characteristics shown in Fig. 2.13 is typical of a particular type of diodescalled “normal recovery” or “soft recovery” diode (S>1). The total recovery time (trr) in this caseis a few tens of microseconds. While this is acceptable for line frequency rectifiers (these diodesare also called rectifier grade diodes) high frequency circuits (e.g PWM inverters, SMPS)demand faster diode recovery. Diode reverse recovery time can be reduce by increasing the rateof decrease of the forward current (i.e, by reducing stray circuit inductance) and by using“snappy” recovery (S<<1) diode. The problems with this approach are: i) Increase of diF/dt also increases the magnitude of Irr ii) Large recovery current coupled with ”snappy” recovery may give rise to current and voltage oscillation in the diode due to the resonant circuit formed by the stray circuit inductance and the diode depletion layer capacitance. A typical recovery characteristics of a “snappy” recovery diode is shown in Fig 2.14 (a).Fig. 2.14: Diode overvoltage protection circuit; (a) “Snappy recovery characteristics; (b) Capacitive snubber circuit; (c) snubber characteristics.Large reverse recovery current may lead to reverse voltage peak (Vrr) in excess of VRSM anddestroy the device. A capacitive protection circuit (also called a “snubber circuit) as shown inFig. 2.14 (b) may to used to restrict Vrr. Here the current flowing through Ll at the time of diodecurrent “snapping” is bypassed to Cs. Ll,Rs & Cs forms a damped resonance circuit and the initialenergy stored in Ll is partially dissipated in Rs, thereby, restricting Vrr . Normalized values of Vrras a function of the damping factor ξ with normalized Irr as a parameter is shown in Fig. 2.14(c).However, it is difficult to correctly estimate the value of Ll and hence design a proper snubbercircuit. Also snubber circuits increase the overall power loss in the circuit since the energy storedin the snubber capacitor is dissipated in the snubber resistance during turning ON of the diode.Therefore, in high frequency circuits other types of fast recovery diodes (Inverter grade) arepreferred. Fast recovery diodes offer significant reduction in both Irr and trr (10% - 20% of arectifier grade diode of comparable rating). This improvement in turn OFF performance,however, comes at the expense of the steady state performance. It can be shown that the forwardvoltage drop in a diode is directly proportion to the width of the drift region and inverselyproportional to the carrier life time in the drift region. On the other hand both Irr and trr increaseswith increase in carrier life time and drift region width. Therefore if Irr and trr are reduced byreducing the carrier life time, forward voltage drop increases. On the other hand, if the drift Version 2 EE IIT, Kharagpur 20
  35. 35. region width is reduced the reverse break down voltage of the diode reduces. The performance ofa fast recovery diode is therefore, a compromise between the steady state performance and theswitching performance. In high voltage high frequency circuits switching loss is the dominantcomponent of the overall power loss. Therefore, some increase in the forward voltage drop in thediode (and hence conduction power lass) can be tolerated since the Turn OFF loss associatedwith reverse recovery is greatly reduced.In some very high frequency applications (fsw >100KHZ), improvement in the reverse recoveryperformance offered by normal fast recovery diode is not sufficient. If the required reverseblocking voltage is less (<100v) schottky diodes are preferred over fast recovery diodes.Compared to p-n junction diodes schottky diodes have very little Turn OFF transient and almostno Turn ON transient. On state voltage drop is also less compared to a p-n junction diode forequal forward current densities. However, reverse breakdown voltage of these diodes are less(below 200V) Power schottky diodes with forward current rating in excess of 100A areavailable.Exerciser 2.3 1. Fill in the blanks with appropriate word(s) i. Forward recovery voltage appears due to higher ohmic drop in the ______________ region of a power diode in the beginning of the Turn On process. ii. The magnitude of the forward recovery voltage is typically of the order of few ______________ of volts. iii. The magnitude of the forward recovery voltage also depends on the _______________ of the diode forward current. iv. The reverse recovery charge of a power diode increases with the _______________ of the diode forward current. v. For a given forward current the reverse recovery current of a Power Diode ______________ with the rate of decrease of the forward current. vi. For a given forward current the reverse recovery time of a Power diode ______________ with the rate of decrease of the forward current. vii. A “snappy” recovery diode is subjected to _________________ voltage over shoot on recovery.viii. A fast recovery diode has _______________________ reverse recovery current and time compared to a __________________ recovery diode. ix. A Schottky diode has _______________ forward voltage drop and ______________ reverse voltage blocking capacity. x. Schottky diodes have no __________________ transient and very little _________________ transient.Answer: (i) drift, (ii) tens, (iii) rate of rise, (iv) magnitude, (v) increases, (vi) decreases, (vii) large, (viii) lower, (ix) low, law, (x) Turn On, Turn Off. 2. In the buck converter shown the diode D has a lead inductance of 0.2μH and a reverse recovery change of 10μC at iF =10A. Find peak current through Q. Version 2 EE IIT, Kharagpur 21
  36. 36. Answer: Assuming iL=10A (constant) the above waveforms can be drawnAs soon as Q is turned ON. a reverse voltage is applied across D and its lead inductance. diF 20 ∴ = A S ec = 10 7 A S ec dt .2 × 1 0 -6Assuming a snappy recovery diode (s ≈ o) 1 ⎛ diF ⎞ 2 Q rr = 1 I rr t rr = ⎜ ⎟ t rr 2 2 ⎝ dt ⎠ = 1 0 × 1 0 -6 C∴ t rr = 1 . 4 1 4 μ s diF∴ I rr = t rr = 1 4 . 1 4 A dt∴i =I +I = 24.14 A Q peak L rr Version 2 EE IIT, Kharagpur 22
  37. 37. References 1. Ned Mohan, Tore M. Undeland, William P. Robbins, “Power Electronics, Converters, Application and Design” John Wiley & Sons(Asia), Publishers. Third Edition 2003. 2. P. C. Sen, “Power Electronics” Tata McGraw Hill Publishing Company Limited, New Delhi, 1987. 3. Jacob Millman, Christos C. Halkias, “Integrated Electronics, Analog and Digital Circuits and Systems”, Tata McGraw-Hill Publishing Company Limited, New Delhi, 1991. Version 2 EE IIT, Kharagpur 23
  38. 38. Module Summary • A p-n junction diode is a minority carrier, unidirectional, uncontrolled switching device. • A power diode incorporates a lightly doped drift region between two heavily doped p type and n type regions. • Maximum reverse voltage withstanding capability of a power diode depends on the width and the doping level of the drift region. • A power diode should never be subjected to a reverse voltage greater than the reverse break down voltage. • The i-v characteristics of a forward biased power diode is comparatively more linear due to the voltage drop in the drift region. • The forward voltage drop across a conducting power diode depends on the width of the drift region but not affected significantly by its doping density. • For continuous forward biased operation the RMS value of the diode forward current should always be less than its rated RMS current at a given case temperature. • Surge forward current through a diode should be less than the applicable surge current rating. • During “Turn On” the instantaneous forward voltage drop across a diode may reach a level considerably higher than its steady state voltage drop for the given forward current. This is called forward recovery voltage. • During “Turn Off” the diode current goes negative first before reducing to zero. This is called reverse recovery of a diode. • The peak negative current flowing through a diode during Turn Off is called the “reverse recovery current” of the diode. • The total time for which the diode current remains negative during Turn Off is called “the reverse recovery time” of the diode. • A diode can not block reverse voltage till the reverse current through the diode reaches its peak value. • Both the “reverse recovery current” and the “reverse recovery time” of a diode depends on the forward current during Turn Off, rate of decrease of the forward current and the type of the diode. • Normal or slow recovery diodes have smaller reverse recovery current but longer reverse recovery time. They are suitable for line frequency rectifier operation. • Fast recovery diodes have faster switching times but comparatively lower break down voltages. They are suitable for high frequency rectifier or inverter free- wheeling operation. • Fast recovery diodes need to be protected against voltage transients during Turn Off” using R-C snubber circuit. Version 2 EE IIT, Kharagpur 24
  39. 39. • Schottky diodes have lower forward voltage drop and faster switching times but comparatively lower break down voltage. They are suitable for low voltage very high frequency switching power supply applications. Version 2 EE IIT, Kharagpur 25
  40. 40. Practice Problems and Answers Version 2 EE IIT, Kharagpur 26
  41. 41. Practice Problems (Module-2) 1. If a number of p-n junction diodes with identical i-v characteristics are connected in parallel will they share current equally? Justify your answer. 2. A power diode have a reverse saturation current of 15μA at 32°C which doubles for every 10° rise in temperature. The dc resistance of the diode is 2.5 mΩ. Find the forward voltage drop and power loss for a forward current of 200 Amps. Assume that the maximum junction temperature is restricted to 102°C. VT = k T = 26 m v at 32 o C q 3. In the voltage commutated chopper T & TA are turned ON alternately at 400 HZ. C is initially charged to 200 V with polarity as shown. Find the IFRMS and VRRM ratings of DI & DF. 4. In the voltage commutated chopper of Problem 5 the voltage on C reduces by 1% due to reverse recovery of DI. Find out Irr & trr for DI. (Assume S = 1 for DI). 5. What precaution must be taken regarding the forward recovery voltage of the free wheeling diodes in a PWM voltage source inverter employing Bipolar Junction Transistors of the n-p-n type? Version 2 EE IIT, Kharagpur 27
  42. 42. Answers to Practice Problems1. The reverse saturation current of a p-n junction diode increases rapidly with temperature. If follows then (from Eqn. 2.1) the voltage drop across a diode for a given forward current decreases with increase in temperature. In other words if the volt ampere characteristics of a diode is modeled as a non linear (current dependent) resistant it will have a negative temperature coefficient. Let us now consider the situation where a number of diodes are connected in parallel. If due to some transient disturbance the current in a diode increases momentarily the junction temperature of that diode will increase due increased power dissipation. The voltage drop across that particular diode will decrease as a result and more current will be diverted towards that diode. This “positive feedback mechanism” will continue to increase its current share till parasitic lead resistance drop becomes large enough to prevent farther voltage drop across that diode. Therefore, it can be concluded that a number of p-n junction diodes conned in parallel will not, in general, share current equally even if it is assumed that they have identical i-v characteristics. However, equal current sharing can be forced by connecting suitable resistances in series with the diodes so that the total resistance of each branch has positive temperature coefficient.2. Since the reverse saturation current double with every 10°C rise in junction temperature. 1 0 2 -3 2 Is 102o C = 2 10 × Is 32o C = 1 .9 2 m A KT Vt = = 26mv at 32 o C ∴ Vt at 102 o = 31.97mv q ∴ V j fo r i F = 2 0 0 A is iF V j = Vt o 102 C × ln = 0 .3 7 V Is 102o C Voltage drop across drift region VR = iF ×RD = 0.5V Therefore, the total voltage drop across the diode is VD = VR + V j = 0.87V Version 2 EE IIT, Kharagpur 28
  43. 43. 3. Important wave forms of the system are shown in the figure. As soon as T is turned ON the capacitor voltage starts reversing due to the L-C resenant circuit formed by C-T-L & DI. Neglecting all the capacitor voltage reaches a -200V. The current idi is given by i D I = I D IP S in ω n 0 ≤ ωn ≤ 7 w here I D IP = 200 C = 89.44 A L 1 & ωn = = 22.36×103 LC Version 2 EE IIT, Kharagpur 29
  44. 44. ∴ Capacitor voltage reversal time Tn 1 π = = = = 140μs. 2 2 fn ωnCapacitor voltage remains at -200 V till TA is turned ON when it is charged linearly towards+200 V. Time taken for charging is 2 × 200 × C TC = = 400μs ILAt the end of charging DF turns ON and remains on till T is turned on again. I DIP 140 ∴ I FRMS For D I is = 10.58 Amps 2 5000 2100 I FRMS For D F is 20 = 12.96 Amps 5000 From figure VRRM for D I is 200 V VRRM for D F is 400 V 4. Since the Capacitor voltage reduces by 1% Q rr = 0.01× C × 200 = 40μc d i dI w ith S = 1 Q rr = I rr t rr = t rr 2 dt Now id I = I DIP Sin ω n t di dI ∴ = ω n I D IP C osω n t dt di dI 1 C at ω n t = π, = ω n I DIP = , 200 = 2A dt LC L μs ∴ t rr 2 = 20 ×10-12 sec 2 or t rr = 4.472 μs ∴ I rr = 8.94 Am ps5. Figure shows one leg of a PWM VSI using n-p-n transistor and freewheeling diode. Version 2 EE IIT, Kharagpur 30
  45. 45. Consider turning off operation of Q1. As the current through Q1 reduces D1 turns On. Theforward recovery voltage of D1 appears as a reverse voltage across the n-p-n transistor whosebase emitter junction must with stand this reverse voltage. Therefore, the forward recoveryvoltage of the free wheel diodes must be less them the reverse break down voltage of the base-emitter junction of the n-p-n transistors for safe operation of the inverter. Version 2 EE IIT, Kharagpur 31
  46. 46. Module 1Power Semiconductor Devices Version 2 EE IIT, Kharagpur 1
  47. 47. Lesson 3Power Bipolar Junction Transistor (BJT) Version 2 EE IIT, Kharagpur 2
  48. 48. Constructional Features, Operating Principles, Characteristics and specifications of PowerBipolar Junction transistors.Objective: On completion the student will be able to 1. Distinguish between, cut off, active, and saturation region operation of a Bipolar Junction Transistor. 2. Draw the input and output characteristics of a junction transistor and explain their nature. 3. List the salient constructional features of a power BJT and explain their importance. 4. Draw the output characteristics of a Power BJT and explain the applicable operating limits under Forward and Reverse bias conditions. 5. Interpret manufacturer’s data sheet ratings for a Power BJT. 6. Differentiate between the characteristics of an ideal switch and a BJT. 7. Draw and explain the Turn On characteristics of a BJT. 8. Draw and explain the Turn Off characteristics of a BJT. 9. Calculate switching and conduction losses of a Power BJT. 10. Design a BJT base drive circuit. Version 2 EE IIT, Kharagpur 3
  49. 49. 3.1 IntroductionPower Bipolar Junction Transistor (BJT) is the first semiconductor device to allow full controlover its Turn on and Turn off operations. It simplified the design of a large number of PowerElectronic circuits that used forced commutated thyristors at that time and also helped realize anumber of new circuits. Subsequently, many other devices that can broadly be classified as“Transistors” have been developed. Many of them have superior performance compared to theBJT in some respects. They have, by now, almost completely replaced BJTs. However, it shouldbe emphasized that the BJT was the first semiconductor device to closely approximate an idealfully controlled Power switch. Other “transistors” have characteristics that are qualitativelysimilar to those of the BJT (although the physics of operation may differ). Hence, it will beworthwhile studying the characteristics and operation a BJT in some depth. From the point ofview of construction and operation BJT is a bipolar (i.e. minority carrier) current controlleddevice. It has been used at signal level power for a long time. However, the construction andoperating characteristics of a Power BJT differs significantly from its signal level counterpartdue to the requirement for a large blocking voltage in the “OFF” state and a high current carryingcapacity in the “ON” state. In this module, the construction, operating principle andcharacteristics of a Power BJT will be explored.3.2 Basic Operating Principle of a Bipolar Junction TransistorA junction transistor consists of a semiconductor crystal in which a p type region is sandwichedbetween two n type regions. This is called an n-p-n transistor. Alternatively an n type regionmay be placed in between two p type regions to give a p-n-p transistor. Fig 3.1 shows the circuitsymbols and schematic representations of an n-p-n and a p-n-p transistor. The terminals of atransistor are called Emitter (E), Base (B) & Collector (C) as shown in the figure. VCE VCE E (n) C (n) E (p) C (p) - iE VBE + i iC C iE VBE iB B (p) B (n) RC RC VBB RB VBB iB VCC VCC Version 2 EE IIT, Kharagpur 4
  50. 50. (Emitter) (Base) (Collector) (Emitter) (Base) (Collector) n p n n p n (E) (B) (C) (E) (B) (C) S WBE 0 A φCB A φBE A 0 φCB S S φCB φ φBE S φBE φCB BE S x A φCB 0 φ 0 x WS CB S WBE φBE BE S φCB A WCB WCB A WBE A WBE 0 WCB 0 0 0 WCB WBE A WBE WCB JBE JCB JBE JCB nS pB pS nB pS pS nC nS nS pC nE pE n poB p noB pA nE p noE p noC nA pE n poE n poC A n A pB p nC p A nB nA pC x x (a) (b) Fig. 3.1: Bipolar junction transistor under different biasing condition. (a) n – p – n transistor ; (b) p – n – p transistor.If no external biasing voltages are applied (i.e.; VBB and VCC are open circuited) all transistorcurrents must be zero. The transistor will be in thermal equilibrium condition with potentialbarriers φο and φCB at the base emitter and the base collector functions respectively. ΒΕ o O OCorresponding depletion layer widths will be WBE and WCB . It is clear from the diagram that ptype carriers in the base region of an n-p-n transistor are trapped in a “potential well” and cannotescape. Similarly, in a p-n-p transistor p type carriers in the emitter and collector regions areseparated by a “potential hill”.When biasing voltages are applied as shown in the figure, the base emitter junction (JBE)becomes forward biased where as the base collector junction is reverse biased. Potential barrier Αand depletion layer width at JBE reduces to φΒΕ and WBE respectively. Both these quantities Aincrease at JCB ( φA , WCB ) . As the potential barrier at JBE is reduced a large number of minority CB Acarriers are introduced in to Base and the Emitter regions as shown in Fig. 3.1 ( PnE , n A for n-p-n A pB Version 2 EE IIT, Kharagpur 5