Power Semiconductor Switches• Diodes (Uncontrolled switches)• Thyristors (Controllable at turn-on but uncontrolled at turn-off or commonly called as latched devices). Triac is under the same category.• BJT, MOSFET, IGBT, GTO, MCT etc. are fully controllable switches.
Power DiodesA A i AK i AK A P P N− v AK v AK N N KK K
Reverse Recovery Problems VFD S t rr I FDEd FD Io IS Io
Power diodesDiodes are classified as:- general purpose or line-frequency diodes- Fast recovery diodes- Schottky diodes
Schottky Diode• The schottky diode has a smaller voltage drop compared to conventional diodes (about 0.3 V).• The schottky diode has a smaller voltage breakdown than conventional diodes (less than 200 V).
Snubbers for Diodes and Thyristors• Maximum dv/dt across diodes or thyristors must be limited and can be done by using an RC snubber that is connected in parallel to the devices.• Maximum di/dt through diodes or thyristors must be limited and can be done by using an inductor that is connected in series to the devices.
Switching Characteristics Gate signal vT iT Io Transistor voltage & Ed IoEd iT current tdon tdoff vT t fv t fi t ri t son = tri + t fv trv t soff = t rv + t fi 1 1 Transistor Wson = Ed I ot son Wsoff = Ed I ot soff 2 2 power Pcd
Desired Switch Characteristics• Small leakage current in the off state• Small on-state voltage• Short turn-on and turn-off times• Large forward and reverse blocking voltage capabilities• High on-state current rating• Positive temperature coefficient of on-state resistance• Small control power• Wide Safe Operating Area• Large dv/dt and di/dt ratings
LossesSwitching losses : ( Ps = 1 E d I o f s t son + t soff 2 )fs is switching frequency.Conduction losses : TON Pcd = Von I o TsTs is switching period.
Bipolar Junction Transistor iC iB 5 iC iB 4 C iB 3 C iC iB 2 iB iB1 = 0 N B vCE vCEB P N E iB 5 > iB 4 > iB 3 > iB 2 > iB1 E
VI characteristics of BJT Hard - saturation Quasi - saturation Second breakdown IC I B5 Primary I B4 breakdown I B3 I B2 I B1 IB < 0 vCE BVSUS BVCB 0 I B0 = 0
Operating region• Hard-saturation provides low voltage-drop but a large storage time (turn-off time)• Quasi-saturation provides high voltage-drop but a small storage time.• Second breakdown must be avoided by using a snubber and proper base current control.• Negative base current results in higher voltage breakdown.
MOSFET iD vGS 5 iD vGS 4 D vGS 3 iD vGS 2 vGS1 = 0G vDS v DS S vGS 5 > vGS 4 > vGS 3 > vGS 2 > vGS1
MOSFET Features• Voltage controlled devices• Single carrier devices• High on-state voltage• Very high gain• No reverse blocking capability• No second breakdown problem• Can not be protected by fuse
Gate-Turn-Off (GTO) Thyristor iA Blocking condition v AK
GTO switching characteristic Anode voltage Anode IA current Vd Spike voltage Tail current 0 Time IGR (b)
GTO Features• Controllable at turn-on and turn-off• High-voltage capability• Can be designed with reverse blocking capabilty• Low gain at turn-off• Low on-state voltage• High turn-off losses
Insulated Gate Bipolar Transistors (IGBTs) iC vGE 5 C vGE 4 iC vGE 3 vGE 2 vGE1 = 0 G vCE E vGE5 > vGE 4 > vGE3 > vGE 2 > vGE1
IGBT Features• Combining the advantages of BJT and MOSFET• No reverse blocking capability• No second breakdown• High gain at turn on and turn off
Other Switching Devices• Static Induction Transistor and Static Induction Thyristor. The main problems are normally-on and high conduction loss. The advantage is that the speed is very high.• MOS Controlled Thyristor. Combining the advantages of MOSFET and Thyristor. Still under development.• IGCT (Integrated Gate Controlled Thyristor). This is further development of GTOs.
Switching Device Development ER 2000 POW 105 GTO : GATE TURN-OFF THYRISTOR E IV DR MCT : MOS CONTROLLED THYRISTOR THYRISTOR H Y SI Thy : STATIC INDUCTION THYRISTOR NC HIG E SY BPT : BIPOLAR POWER TRANSISTOR QU 104 EA GTO E P (kVA) IGBT : INSULATED GATE BIPOLAR TRANSISTOR FR G H HI 1990 MCT SI Thy CONTROLLABLE POWER 103 104 THYRISTOR IGBT 102 103 GTO 1980 BPT P (kVA) 101 102 IGBT MOS 104 THYRISTOR BPT 101 10-1 -1 103 10 100 101 102 104 105 106P (kVA) OPERATION FREQUENCY f (kHz) GTO MOS 102 -1 10 10-1 100 101 102 104 105 101 BPT f (kHz) 10-1 -1 10 100 101 102 104 f (kHz)
Properties and Rating of Semiconductor Power SwitchesSwitch Control Control Switching Voltage Maximum Maximum signal characteristic frequency drop voltage current rating ratingDiode medium 6.5 kV 5 kASCR current trigger low medium 6 kV 4 kATRIAC current trigger medium 1 kV 50 AGTO current trigger low medium 6.5 kV 4.5 kABJT current linear medium low 1.5 kV 1 kAMOSFET voltage linear Very high high 1 kV 200 AIGBT voltage linear high medium 3.5 kV 2 kA
Properties of New MaterialsProperty Si GaAs 3C-SiC 6H-SiC DiamondBandgap at 300 K 1.12 1.43 2.2 2.9 5.5(eV)Relative dielectric 11.8 12.8 9.7 10 5.5constantSaturated driftvelocity (cm/s) 1x107 2x107 2.5x107 2.5x107 2.7x107Thermal 1.5 0.5 5.0 5.0 20conductivity(W/cm/o CMaximum 400 460 873 1240 1100operatingtemperature (K)Melting 1415 1238 Sublime>1800 Sublime>1800 Phase changetemperature (C)Electron mobility 1400 8500 1000 600 2200at 300 K (cm2 /Vs)Breakdownelectric field 3x105 4x105 4x106 4x106 1x107(V/cm)
Applications• Thyristor is only used for very large power applications.• Forced commutated thyristors are no longer used.• Bipolar junction transistors are no longer used.• MOSFET is commonly used in low-power applications.• IGBT is used from low-power up to medium power applications.• GTO is used for large power applications.
Loss Considerations• Conduction losses• Switching losses• The loss will determine the junction temperature and the heatsink and cooler required.• In many cases, the switching frequency is limited by the temperature instead of device speed.
Snubbers• Turn-off losses can be reduced by using a turn-off snubber. This snubber is also useful to limit high dv/dt across the device.• Turn-on losses can be reduced by using a turn-on snubber. This snubber is also useful to limit high di/dt through the device.• Snubbers are useful to reduce the switching losses on the switching devices. The total switching losses, however, may still the same or even increase.
Reducing Switching Losses• Switching losses can be reduced by using lossless snubbers. These snubbers, however, may make the converter circuit became complicated.• IGBTs may operate without snubbers.• GTOs and IGCTs usually need a turn-off snubber because of high tail current.• Switching losses can be reduced or even eliminated by using soft-switching techniques. These methods, however, may increase the required voltage and/or current ratings.