Lecture-7 : Semiconductor Power Switching Devices-4

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Semiconductor Power Switching Devices-4 (Lecture-7) R S Ananda Murthy Associate Professor and Head Department of Electrical & Electronics Engineering, Sri Jayachamarajendra College of Engineering, Mysore 570 006 R S Ananda Murthy Semiconductor Power Switching Devices-4

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This is Lecture-7 on power electronics. In this lecture we discuss power BJTs

Transcript of Lecture-7 : Semiconductor Power Switching Devices-4

  • 1. Semiconductor Power Switching Devices-4 (Lecture-7) R S Ananda Murthy Associate Professor and Head Department of Electrical & Electronics Engineering, Sri Jayachamarajendra College of Engineering, Mysore 570 006 R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 2. Structure of Power BJT IB IC vBE vCE (c)(a) C B E + + C B (b) C EB E n p n n+ n p n+ Structure shown in Figure (a) is preferred for low voltage ratings. Structure shown in Figure (b) is preferred for high voltage ratings. But this tends to increase the on-state voltage drop. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 3. BJT as a Power Switching Device CE V VBE RB VCB IE VCE 1 VCE 2 VBE + VBB IB IC IB RC VCC + + (b)(a) 0 B E + + > C CE conguration is always used as it offers high input impedance and higher current gain. When transistor is on IC = ICS = [VCC VCE(sat)]/RC. IBS = ICS/ is the base current that just turns on the BJT. For faster turn on usually we make IB > IBS. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 4. CE Output Characteristics of Power BJT VCCVCE(Sat) VCE Quasi saturation zone IB IC RC VCC IB4 IB3 IB2 IB1 IB increasing Linear zone Hard saturation line Cutoff zone Load Line = 0 b a d e c Off On Since hard saturation increases the turn-off time, power BJTs are always operated in cut-off and quasi-saturation regions. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 5. Darlington Connection for Higher Current Gain So overall current gain is and voltage drop is T1 T2 Darlington connection gives higher but it increases leakage current, on-state voltage drop, and reduces the switching frequency. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 6. Switching Times of Power BJT iB ICS ICS ICS td tr tn ts tf ton toff iC IB1 vCE B2I to VCC VCE(Sat) 0.9 0.1 t t t (c) (b) (a) R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 7. Preferred Base Current Waveform for BJT By reducing like this, BJT is prevented from entering into hard saturation 0 By making IB1 > IBS for a brief duration during turn on both td and tr and hence ton is reduced. To turn off BJT gradually apply a negative base current with a peak of IB2 which falls to zero after some time. This reduces ts and tf and hence toff . R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 8. Bakers Clamping Circuit to Reduce Storage Time Q D1 D2 D3 D4 C B E 0 X D2, D3, D4 are low voltage diodes. D1 should be a high voltage diode. D4 is needed to provide path for negative base current while turning off the BJT. When BJT turns on, VCE tends to drop causing D1 to turn on. This decreases IB preventing BJT going into hard saturation. When D1 is on, as all diode voltages are equal, by KVL we get VCE = VX VD1 = VBE +VD2 +VD3 VD1 = VBE +VD2 . R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 9. FSOA and RSOA of Power BJT FSOA shown in Fig (a) is applicable when the BJT is turned on by applying a positive base current. RSOA shown in Fig (b) is applicable when the BJT is turned off by applying a negative base current. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 10. Parallel Operation of BJT T1 T2 Let permissible Let permissible By KVL Let T1 and T2 be identical transistors. When they are on Then Suppose V and A then, If IC1 increases, the voltage across emitter resistor gives a negative feedback and reduces VBE1. Then, IB1 decreases causing a reduction in IC1. But emitter resistors cause power loss which reduces the efciency of the circuit. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 11. Base Drive Isolation using Opto-coupler Logic drive circuit Amplifier VCC B Q E COptocoupler Since BJT needs a continuous base current, isolation of base drive is possible only by using an opto-coupler. But this needs additional VCC and amplier which makes the base drive circuit bulkier if more power BJTs are to be used. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 12. Specications of Power BJT BU208D R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 13. Specications of Power BJT BU208D R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 14. Specications of Power BJT BU208D R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 15. Specications of Power BJT BU208D Safe Operating Area increases for pulsed operation. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 16. Specications of Power BJT BU208D R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 17. Demerits of Power BJT Though BJT is fully-controlled, it has the following demerits Since it is continuously triggered, it needs opto-coupler isolated bulky base drive circuit. It cannot block high reverse voltage. It has low current gain () which varies with collector current and temperature. Due to this, typically base current of the order of few amperes is needed for controlling the BJT. can be increased using Darlington connection. But this increases leakage current, on-state voltage drop, and reduces switching frequency. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 18. Demerits of Power BJT It is prone to fail due to Second Breakdown due to hot spots caused by concentration of current in a narrow area in the emitter junction when the device is turned ON and in the collector junction when it is turned OFF. Its switching frequency is typically within 20 kHz. Parallel operation of BJTs is difcult due to their negative temperature coefcient of resistance. BJT cannot be protected against overload using a fuse. It cannot withstand overload. So it should be turned off immediately when there is overload. This requires sensing of overload by some method. Due to the demerits mentioned above BJTs are not preferred in modern power converters. Instead MOSFETs and IGBTs are used. R S Ananda Murthy Semiconductor Power Switching Devices-4
  • 19. Next Lecture... In the next lecture we will discuss some more power semiconductor switching devices used in power electronics. Thank You. R S Ananda Murthy Semiconductor Power Switching Devices-4