Power supplies & regulators


Published on

Its the class notes meant for M.Sc electronics and ECE students

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Power supplies & regulators

  1. 1. POWER SUPPLIES AND REGULATORS yayavaram@yahoo.comIntroduction: It is a known fact that all the electronic circuits work well with DC supply . TheDC voltage supply required by these electronic circuits is normally derived by transforming andrectifying the standard or industrial AC supply. The voltage obtained by this method is notsufficiently stable and contains unwanted ripple.So,the DC voltage regulator circuits areemployed to stabilize the voltage and attenuate the ripple.All the voltage regulators use a zener diode as astable reference voltage source.A zener diodewith a series resistor can be used as a simple low current voltage regulator.By adding a seriestransistor a large load current can be supplied.the performance of the regulator can be furtherimproved when an error amplifier is added.Types of Power supplies :There are two major types of power supplies : (i) Regulated power supplies and (ii)Unregulatedpower supplies. Regulated power supplies are further devided into linearly regulated powersupplies and switch mode power supplies.The classification is shown below.Unregulated power supplies are primarily used for simple electromechanical applications that donot require exact output voltages.
  2. 2. The efficiency of linearly regulated power supplies is only approx. 50 % due to the high lossesinside the power transistor. The remaining energy is emitted in the form of heat. Due to this,sufficient ventilation is required to cool the power supply. Compared with unregulated powersupplies, linearly regulated power supplies have a very small residual ripple of the output voltage(in the dimension of millivolts).Linearly regulated power supplies are used for all applications that require a very exact outputvoltage, e.g. for highly precise medical devices.Bipolar and Two- value Unregulated Power Supplies.Certain electronic devices require both positive and negative supply voltages.These voltages aremeasured w.r.t a common terminal (ground) .These positive and negative voltages are obtainedusing a centre tap transformer.So, the Bipolar unregulated power supply consists of a a centrertapped transformer , a rectifier, and a filter as shown in the circuit below.The step-down transformer gives a voltage of 12 V w.r.t centre tap and both the no load voltagesare 1.41 x 12V rms = 17V.The bridge rectifier circuit converts the AC into suitable DC.This DCis filtered by the capacitors C1 and C2 and the voltage across the load resistors(RL) is measuredw.r.t common terminal(CT).At the output we can get two voltages .Hence it is called a two valueunregulated power supply.For this circuit, regulation is very poor. As the load varies, the output-voltage must remainconstant, for good regulation. But this is not true for unregulated power supply circuit. The
  3. 3. limitations of unregulated power supply are: Poor regulation ,DC output varies directly as theAC input varies, In simple rectifiers and filter circuits, the DC output voltage varies withtemperature also, if semiconductor devices are used.BIPOLAR UNREGULATED SUPPLIESUsing the unregulated bridge rectifier circuit we can also get Bi-polar voltages also.In suchcircuits w.r.t the common terminal one end will give positive voltage and the other end will givenegative voltage as shown in the circuitSince this circuit gives both positive and negative voltages at the output w.r.t common terminalit is called bipolar supply. 2.Two value positive supply
  4. 4. . 3.Two Value negative supply.The circuits in figures 2 and 3 shows the two value positive and two value negative supplies.Need for voltage Regulation :Ideally, the output of a power supply should always be constant because if the supply voltagechanges, the operation of the circuit is adversely affected. But unfortunately, the output of mostof the power supplies is not constant .This is due to the fact that , the ac line voltage is notconstant and it varies between 180 to 250 volts. The ac line voltage alone can be responsible fornearly a 20 percent change in the dc output voltage. The second factor that can change the dcoutput voltage is a change in the load resistance. In complex electronic equipment, the load canchange as circuits are switched in and out. In a television receiver, the load on a particular powersupply may depend on the brightness of the screen, the control settings, or even the channelselected. These variations in load resistance tend to change the applied dc voltage because thepower supply has a fixed internal resistance. If the load resistance decreases, the internalresistance of the power supply drops more voltage. This causes a decrease in the voltage acrossthe load.Hence to overcome the problems due to the changes in the output voltage we need a voltageregulator which takes unregulated supply and gives a stable DC output voltage .i.e the output of aregulator must be reasonably constant regardless of variations in the input voltage and withvarying load currents.Voltage Regulators : Voltage Regulator Circuits are electronic circuits which give constant DCoutput voltage, irrespective of variations in Input Voltage Vi, current drawn by the load IL fromoutput terminals, and Temperature T. The term voltage regulator is used when the output
  5. 5. delivered is DC voltage. The input can be DC which is not constant and fluctuating also. If theinput is AC, it is converted to DC by Rectifier and Filter circuits and given to Voltage Regulatorcircuit, to get constant DC output voltage.Note: The term Voltage Stabilizer is used, if the output voltage is AC and not DC. The circuitsused for voltage stabilizers are different from voltage regulators.Two types of voltage regulations are defined for a circuit. They are line regulation and loadregulation. The purpose of line regulation is to maintain a nearly constant output voltage whenthe input voltage varies. The purpose of load regulation is to maintain a nearly constant outputvoltage when load variesLine Regulation : Line regulation is defined as the percentage change in the output voltage for agiven change in the input (line) voltage. When taken over a range of input voltage values, lineregulation is expressed as a percentage. = (Δ /Δ )100 %Load Regulation : Load regulation is defined as the percentage change in output voltage for agiven change in load current. It can be expressed as a percentage change in output voltage fromno-load (NL) to full load (FL). oad =( − / ) 100%Alternately, the load regulation can be expressed as a percentage change in output voltage foreach mA change in load current. For example, a load regulation of 0.01 %/mA means that theoutput voltage changes 0.01 percent when the load current increases or decreases 1 mA.Simple Op-Amp Regulator :This circuit consists of a zener diode ,an op-amp and a power transistor connected in series.Hence this transistor is also called series transistor or control element.Here the input to thecircuit is the unregulated voltage.The op-amp is used as a comparator . Resistors R2 and R3asthe voltage divider network which is used to sense any change at the output. The change sensedby the divider network is applied at the inverting input of the comparator.The zener diode
  6. 6. provides the reference voltage which is applied at the non-inverting input of the comparator.Theoutput of the comparator is applied at the base of the series transistor .When the output voltage decreases because of the decrease in input voltage , the voltage dropacross R2,R3 decreases and so this error voltage (-ve) is sensed by the divider network and isapplied at the inverting terminal of the comparator. The comparator compares the referenceconstant voltage and the –ve error voltage and provide the difference (+ve) of the two voltages atthe output ,which in turn is applied to the base of the the series transistor . The transistor conductaccording to the forward bias voltage at the base and so, the output voltage remain constant.When the output voltage tries to increase, the voltage sensed by the divider network is increasedand this is applied at the inverting input of the comparator. The comparator provide thedifference (-ve) of the two voltage at the base of the transistor and so, the conduction oftransistor decreases to the constant (required) voltage level.So,the output voltage remains constant irrespective of the changes in the input voltage..Hencethis circuit is called asimple op-amp based voltage regulator.Linear IC Voltage Regulators :A linear regulator is a system used to maintain a steady voltage. The resistance of the regulatorvaries in accordance with the load resulting in a constant output voltage. The regulating device is
  7. 7. made to act like a variable resistor, continuously adjusting a voltage divider network to maintaina constant output voltage, and continually dissipating the difference between the input andregulated voltages as waste heat.There are five basic linear regulator types; positive, negative,fixed output, tracking and floating regulators. Each has its own particular characteristics and bestuses, and selection depends on the designer’s needs and trade–offs in performance and cost.Linear IC voltage regulators use an active pass element to reduce the input voltage to theregulated output voltage. Some devices use bipolar transistors and others use metal-oxidesemiconductor field effect transistors (MOSFETS).Linear IC voltage regulators are classified by four characteristics : (i).Polarity : negative , positive or dual tracking (ii).Terminal count: three terminal or multiterminal (iii).Fixed or adjustable output : Standard fixed voltages are ±5, ±12 and ± 15 V.Adjustablerange is typically 1.2 to 37 V or -1.2 to – 37 V. (iv) Output current : Typical output current capabilities are 0.1,0.2,0.25,0.5,1.5 and 3 A andalso 5 and 10 A.The instantaneous voltage ,that is to be applied at the input of an IC regulator must be alwayshigher than the output voltage of the regulator by an amount equal to 0.5 to 3 volts.This voltageis known as dropout voltage or simply headroom.The IC regulators have the feature called safe area protection which protects the IC from highoverload currents.The IC regulators have the ripple rejection feature which is defined as the ratio of peak to peakinput voltage to the peak to peak output ripple voltage.This is typically 60dB,which is areduction of ripple voltage of atleast 1000:1.Three– terminal fixed voltage Regulators : The most common linear regulators are three-terminal integrated circuits in the TO220 package. The 78xx series regulate positive voltageswhile the79xx series regulate negative voltages. For example 7805 is a +5 volts regulator and
  8. 8. 7812 is a +12 volts regulator and 7824 is a +24 volts regulator. Similarly 7905 is a -5 voltsregulator and 7912 is a -12 volts regulator and 7924 is a -24 volts regulator.By adding somecomponents these ICs can be used to build reasonably good power supplies. LM 309 is a fixed5V voltage regulator from National semiconductor Inc. It is similar to 7805 of Fairchild. 78xxseries ICs have built-in protection against a circuit drawing too much power. They haveprotection against overheating and short-circuits, making them quite suitable in mostapplications. In some cases, the current-limiting features of the 78xx devices can provideprotection not only for the 78xx itself, but also for other parts of the circuit.These three terminal regulators consists of three pins as shown in the figure below.The pin 1 is the input and the pin2 is the output and the pin3 is the ground. C1 and C2 arerequired if regulator is located far from power supply filter and load, or oscillation may inducedon the loop.The unregulated supply is connected between the pin1 and pin3 of the chip and the output ismeasured between the load and the common terminal(pin 3).The circuit below shows the LM7805 based 5 volts regulated power supply.The input can bebetween 7 to 35 volts and the output will be 5 volts regulated voltage.The output is measured across the load resistor .This circuit is provided can handle maximum 1ampere current and the noise is also low.
  9. 9. Similarly the LM7905 can be used to build a -5 volts power supply as shown below.This is alsoan example for three terminal voltage regulator.The 7905 gives a negative regulated DC voltageof 5 volts.Power Supply for Logic circuits : The TTL ICs and certain digital chips like Microprocessorsrequire a power supply whose output is +5 V DC.For these logic circuits a power supply isconstructed using the IC regulator LM340-05 .It is a 3-terminal regulator whose output voltage is5 vand the current rating is up to 1.5 amp. The circuit diagram is shown below.This IC has internal current limit at 2 A for pulse operation. It also has safe area protectionandthermal shutdown protection at a junction temperature of 150oC to prevent burnout.The shuntdiode protects the regulator from short circuits occurring at its input terminals.For 5 volts output the input must be atleast 8 volts and the diode must have 25 V PIV rating witha current rating higher than 1 A.
  10. 10. ±15 V Power Supplies for Linear Application: Some electronic devices need bipolar supplyfor their operation.In such cases the IC regulators LM340K-15 and LM320K-15 can be used tobuild the bipolar power supply.The first Ic gives an output of +15 volts and the second IC gives aregulated voltage of -15V .The circuit diagram is shown below.The two capacitors provide ripplefree DC output voltage and the resistor R1 is needed to ensure that the positive regulator starts upwhen the negative regulator has heavy load..R2 offsets the effect on ± 15 V regulation caused byadding R1. The diodes must have high current rating higher than 1A.The input voltage of +18 and -18V from any stepdown centre tap transformer can be applied tothe circuit.The LM340 K-15 is a +15V regulator with load current capability of up to 1.5A andthe LM320 K-15 is a -15V regulator with current capability of up to 1.5 A. Both these regulatorshave current limit,safe area and thermal shut down protection.Proper heat sinking is required.
  11. 11. Adjustable Three- Terminal Positive Voltage regulator (LM 317 HV) : For laboratoryapplications adjustable regulated load voltages are required.There are certain IC regulatorsavailable for this purporse. LM 317, LM 338, and LM 350, are adjustable regulators. They havemaximum load currents from 0.10 to 1.5 A. For example, the LM317 is a three terminal positivevoltage regulator, which can be operated with the output voltage regulated at any setting over therange of 1.2 V to 57 V. The following circuit explains how the regulated output voltage of anLM317 can be set.The LM317 require a minimum dropout voltage of 3V across its input and output terminals .Inthe circuit the capacitor C1 minimizes problems caused by the long leads between the rectifierand LM317. The capacitor C2 improves the transient response.Any ripple voltage from therectifier will be reduced by a factor of about 1000 if R2 is bypassed by a 1 uF tantalumcapacitor or 10uF aluminum electrolytic capacitor. PIN DIAGRAMResistors R1 and R2 set the output to any desired voltage over the adjustment range (1.2 to 57V). The output voltage desired can be determined using equation.
  12. 12. Vo = VREF [1+ R2/R1] + IADJ .R2where VREF = 1.25 V, the reference voltage between the output and adjustment terminals.However, the current IADJ is very small (100 micro Amperes) and constant. So the voltage dropacross R2 due to IADJ is also very small and can be neglected so that equation can be written as Vout = 1.25 [1+ R2/R1]The current set resistor Rx is usually 240 Ohms, and to achieve good load regulation it should betied directly to the output of the regulator rather than near the load.The load regulation is 0.1 percent while the line regulation is 0.01 percent per volt. This meansthat the output voltage varies only 0.01 percent for each volt of input voltage. The ripplerejection is 80 db, equivalent to 10,000.The LM317 IC protects itself against overheating and it shutdowns by itself during hightemperatures and resumes itself once the overload condition is removed.External Protection :To protect the regulator from the input short circuit and high currents normally diodes areused.If these diodes are not used the load capacitance can pump back the current into its outputand can destroy the regulator. The regulator circuit with external protection is shown below.
  13. 13. In the above circuit the diode1 protects the regulator from input short circuits and the diode2protects the regulator from output short circuits.Adjustable Three- Terminal Negative voltage Regulator ( LM 337 HV) : The LM337 is anadjustable 3−terminal negative voltage regulator capable of supplying in excess of 1.5 A over anoutput voltage range of −1.2 V to −37 V. This IC regulator has internal Thermal OverloadProtection , Internal Short Circuit Current Limiting Constant with Temperature , OutputTransistor Safe−Area Compensation and Floating Operation for High Voltage Applications.Thecircuit diagram is shown below. In the circuit the capacitor C1 minimizes problems caused bythe long leads between the rectifier and LM317. The capacitor C2 improves the transientresponse.This negative voltage regulator has line regulation of 0.01%/V , Load regulation of typically0.3% and has excellent thermal regulation, 0.002%/W. The negative regulator operates on thesame principle of positive regulator except that R1 is a 120 Ω resistor and the maximum inputvoltage is only 50 V.The output of this negative adjustable supply varies between -1.2 V to -47V.The output voltage is given by Vo = 1.25V + (10mA)R2Switching Power Supply : The basic advantages of linear regulators are they are known to berelatively noise-free and reasonably reliable. They are generally easy to design and fairlyinexpensive to manufacture .But there are certain limitations in series voltage regulators. Thebulk size of step down transformers, heat dissipation in series pass transistors and requirement oflarge heat sinks to dissipate energy loss and the need of large valued capacitors to suppress theripple are the major drawbacks with the linear voltage regulators. Hence Switched regulators
  14. 14. have become popular now a days. The switching regulator is increasing in popularity because itoffers the advantages of higher power conversion efficiency and increased design flexibility,i.emultiple output voltages of different polarities can be generated from a single input voltage.Principle of working : Switching regulation is the technique by which an unregulated power isefficiently converted to regulated load power through the use of controlled switching devices.Switching voltage regulators are used where it is required to supply a stable output voltage froma varying input voltage with a minimum power loss even under varying load conditions.A simple switching regulator consists of a contro switch (transistor), a low pass filter andsampling network etc…The circuit diagram is shown below.In the circuit S is a transistor used as a control switch. It is switched between saturation and cut-off. During ‘ON’ mode the switch is in saturation mode with negligible voltage drop across thecollector and emitter terminals of the switch where as in ‘OFF’ mode the switch is in cut-offmode with negligible current through the collector and emitter terminals. On the contrary thevoltage-regulating switch, in a linear regulator circuit, always remains in the active region.Due tothis current is not passed continuously through the transistor and hence the power dissipation isreduced to minimum. The transistor is switched ON and OFF by an electronic pulse widthmodulator. The filter circuit smoothens the output wave form and the sampling network (R1-R2)controls the duty cycle such that the output voltage remains constant.
  15. 15. There are three types of basic switching regulator configurations. They are(i).Step down or Buck switching regulator(ii) Step up or Boost regulator(iii) Inverting or Fly back regulator. ---------------xxx--------------