ECE 442 Power Electronics1 Bipolar Junction Transistors (BJT) NPNPNP

ECE 442 Power Electronics2 BJT Cross-Sections NPN PNP Emitter Collector

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

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9 Transistor Operating Point

ECE 442 Power Electronics10 DC Load Line V CC V CC /R C

ECE 442 Power Electronics11 BJT Transistor Switch

ECE 442 Power Electronics12 BJT Transistor Switch (continued)

ECE 442 Power Electronics13 BJT in Saturation

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 Electronics24 Example 4.2

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

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ECE 442 Power Electronics27 Power Loss due to I C for t on = t d + t r During the delay time, 0 ≤t ≤t d Instantaneous Power Loss Average Power Loss

ECE 442 Power Electronics28 During the rise time, 0 ≤t ≤t r

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ECE 442 Power Electronics30 Average Power during rise time

ECE 442 Power Electronics31 Total Power Loss during turn-on

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ECE 442 Power Electronics33 Power Loss during the Conduction Period

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ECE 442 Power Electronics35 Power Loss during turn off Storage time

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ECE 442 Power Electronics37 Power Loss during Fall time

ECE 442 Power Electronics38 Power Loss during Fall time (continued)

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ECE 442 Power Electronics40 Power Loss during the off time

ECE 442 Power Electronics41 The total average power losses

ECE 442 Power Electronics42 Instantaneous Power for Example 4.2

ECE 442 Power Electronics43 BJT Switch with an Inductive Load

ECE 442 Power Electronics44 Load Lines