ECE 442 Power Electronics3 Common-Emitter NPN Transistor Forward bias the BEJ Reverse bias the CBJ
ECE 442 Power Electronics4 Input Characteristics Plot I B as f(V BE, V CE ) As V CE increases, more V BE required to turn the BE on so that I B >0. Looks like a pn junction volt-ampere characteristic.
ECE 442 Power Electronics5 Output Characteristics Plot I C as f(V CE, I B ) 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 Electronics6 Transfer Characteristics
ECE 442 Power Electronics7 Large-Signal Model of a BJT KCL >> I E = I C + I B β F = h FE = I C /I B I C = β F I B + I CEO I E = I B (1 + β F ) + I CEO I E = I B (1 + β F ) I E = I C (1 + 1/β F ) I E = I C (β F + 1)/β F
ECE 442 Power Electronics14 Model with Current Gain
ECE 442 Power Electronics15 Miller Effect v be v ce i out
ECE 442 Power Electronics16 Miller Effect (continued)
ECE 442 Power Electronics17 Miller Effect (continued) Miller Capacitance, C Miller = C cb (1 – A) –since A is usually negative (phase inversion), the Miller capacitance can be much greater than the capacitance C cb This capacitance must charge up to the base-emitter forward bias voltage, causing a delay time before any collector current flows.
ECE 442 Power Electronics18 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 Electronics19 The Saturating Charge The saturating charge, Q s storage time constant of the transistor
ECE 442 Power Electronics20 Transistor Switching Times
ECE 442 Power Electronics21 Switching Times – turn on Input voltage rises from 0 to V 1 Base current rises to I B1 Collector current begins to rise after the delay time, t d Collector current rises to steady-state value I CS This “rise time”, t r allows the Miller capacitance to charge to V 1 turn on time, t on = t d + t r
ECE 442 Power Electronics22 Switching Times – turn off Input voltage changes from V 1 to –V 2 Base current changes to –I B2 Base current remains at –I B2 until the Miller capacitance discharges to zero, storage time, t s Base current falls to zero as Miller capacitance charges to –V 2, fall time, t f turn off time, t off = t s + t f
ECE 442 Power Electronics23 Charge Storage in Saturated BJTs Charge storage in the Base Charge Profile during turn-off
ECE 442 Power Electronics25 Waveforms for the Transistor Switch V CC = 250 V V BE(sat) = 3 V I B = 8 A V CS(sat) = 2 V I CS = 100 A t d = 0.5 µs t r = 1 µs t s = 5 µs t f = 3 µs f s = 10 kHz duty cycle k = 50 % I CEO = 3 mA