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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
Bipolar Junction Transistors (BJT)

NPN

PNP

ECE 442 Power Electronics

bjt cross sections
BJT Cross-Sections

Emitter

Collector

NPN PNP

ECE 442 Power Electronics

common emitter npn transistor
Common-Emitter NPN Transistor

Reverse bias the CBJ

Forward bias the BEJ

ECE 442 Power Electronics

input characteristics
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

output characteristics
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

transfer characteristics
Transfer Characteristics

ECE 442 Power Electronics

large signal model of a bjt
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

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transistor operating point
Transistor Operating Point

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dc load line
DC Load Line

VCC/RC

VCC

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bjt transistor switch
BJT Transistor Switch

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bjt transistor switch continued
BJT Transistor Switch (continued)

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bjt in saturation
BJT in Saturation

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model with current gain
Model with Current Gain

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miller effect
Miller Effect

iout

vbe

vce

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miller effect continued
Miller Effect (continued)

ECE 442 Power Electronics

miller effect continued17
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

saturating a bjt
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

the saturating charge
The Saturating Charge
  • The saturating charge, Qs

storage time constant of the transistor

ECE 442 Power Electronics

transistor switching times
Transistor Switching Times

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switching times turn on
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

switching times turn off
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

charge storage in saturated bjts
Charge Storage in Saturated BJTs

Charge storage in the Base Charge Profile during turn-off

ECE 442 Power Electronics

example 4 2
Example 4.2

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waveforms for the transistor switch
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

power loss due to i c for t on t d t r
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

during the rise time 0 t t r
During the rise time, 0 ≤t ≤tr

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average power during rise time
Average Power during rise time

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total power loss during turn on
Total Power Loss during turn-on

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power loss during fall time
Power Loss during Fall time

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power loss during the off time
Power Loss during the off time

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the total average power losses
The total average power losses

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instantaneous power for example 4 2
Instantaneous Power for Example 4.2

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bjt switch with an inductive load
BJT Switch with an Inductive Load

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load lines
Load Lines

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