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Bipolar Junction Transistors (BJT)

Bipolar Junction Transistors (BJT). NPN. PNP. BJT Cross-Sections. Emitter. Collector. NPN PNP. Common-Emitter NPN Transistor. Reverse bias the CBJ. Forward bias the BEJ. Input Characteristics. Plot I B as f(V BE , V CE )

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Bipolar Junction Transistors (BJT)

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  1. Bipolar Junction Transistors (BJT) NPN PNP ECE 442 Power Electronics

  2. BJT Cross-Sections Emitter Collector NPN PNP ECE 442 Power Electronics

  3. Common-Emitter NPN Transistor Reverse bias the CBJ Forward bias the BEJ ECE 442 Power Electronics

  4. Input Characteristics • Plot IB as f(VBE, VCE) • As VCE increases, more VBE required to turn the BE on so that IB>0. • Looks like a pn junction volt-ampere characteristic. ECE 442 Power Electronics

  5. Output Characteristics • Plot IC as f(VCE, IB) • Cutoff region (off) • both BE and BC reverse biased • Active region • BE Forward biased • BC Reverse biased • Saturation region (on) • both BE and BC forward biased ECE 442 Power Electronics

  6. Transfer Characteristics ECE 442 Power Electronics

  7. Large-Signal Model of a BJT KCL >> IE = IC + IB βF = hFE = IC/IB IC = βFIB + ICEO IE = IB(1 + βF) + ICEO IE = IB(1 + βF) IE = IC(1 + 1/βF) IE = IC(βF + 1)/βF ECE 442 Power Electronics

  8. ECE 442 Power Electronics

  9. Transistor Operating Point ECE 442 Power Electronics

  10. DC Load Line VCC/RC VCC ECE 442 Power Electronics

  11. BJT Transistor Switch ECE 442 Power Electronics

  12. BJT Transistor Switch (continued) ECE 442 Power Electronics

  13. BJT in Saturation ECE 442 Power Electronics

  14. Model with Current Gain ECE 442 Power Electronics

  15. Miller Effect iout vbe vce ECE 442 Power Electronics

  16. Miller Effect (continued) ECE 442 Power Electronics

  17. Miller Effect (continued) • Miller Capacitance, CMiller = Ccb(1 – A) • since A is usually negative (phase inversion), the Miller capacitance can be much greater than the capacitance Ccb • This capacitance must charge up to the base-emitter forward bias voltage, causing a delay time before any collector current flows. ECE 442 Power Electronics

  18. Saturating a BJT • Normally apply more base current than needed to saturate the transistor • This results in charges being stored in the base region • To calculate the extra charge (saturating charge), determine the emitter current ECE 442 Power Electronics

  19. The Saturating Charge • The saturating charge, Qs storage time constant of the transistor ECE 442 Power Electronics

  20. Transistor Switching Times ECE 442 Power Electronics

  21. Switching Times – turn on • Input voltage rises from 0 to V1 • Base current rises to IB1 • Collector current begins to rise after the delay time, td • Collector current rises to steady-state value ICS • This “rise time”, tr allows the Miller capacitance to charge to V1 • turn on time, ton = td + tr ECE 442 Power Electronics

  22. Switching Times – turn off • Input voltage changes from V1 to –V2 • Base current changes to –IB2 • Base current remains at –IB2 until the Miller capacitance discharges to zero, storage time, ts • Base current falls to zero as Miller capacitance charges to –V2, fall time, tf • turn off time, toff = ts + tf ECE 442 Power Electronics

  23. Charge Storage in Saturated BJTs Charge storage in the Base Charge Profile during turn-off ECE 442 Power Electronics

  24. Example 4.2 ECE 442 Power Electronics

  25. Waveforms for the Transistor Switch VCC = 250 V VBE(sat) = 3 V IB = 8 A VCS(sat) = 2 V ICS = 100 A td = 0.5 µs tr = 1 µs ts = 5 µs tf = 3 µs fs = 10 kHz duty cycle k = 50 % ICEO = 3 mA ECE 442 Power Electronics

  26. ECE 442 Power Electronics

  27. Power Loss due to IC for ton = td + tr • During the delay time, 0 ≤t ≤td • Instantaneous Power Loss • Average Power Loss ECE 442 Power Electronics

  28. During the rise time, 0 ≤t ≤tr ECE 442 Power Electronics

  29. ECE 442 Power Electronics

  30. Average Power during rise time ECE 442 Power Electronics

  31. Total Power Loss during turn-on ECE 442 Power Electronics

  32. ECE 442 Power Electronics

  33. Power Loss during the Conduction Period ECE 442 Power Electronics

  34. ECE 442 Power Electronics

  35. Power Loss during turn offStorage time ECE 442 Power Electronics

  36. ECE 442 Power Electronics

  37. Power Loss during Fall time ECE 442 Power Electronics

  38. Power Loss during Fall time (continued) ECE 442 Power Electronics

  39. ECE 442 Power Electronics

  40. Power Loss during the off time ECE 442 Power Electronics

  41. The total average power losses ECE 442 Power Electronics

  42. Instantaneous Power for Example 4.2 ECE 442 Power Electronics

  43. BJT Switch with an Inductive Load ECE 442 Power Electronics

  44. Load Lines ECE 442 Power Electronics

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