• minority carrier device • on-state: base-emitter and collector-base junctions are both forward-biased
nn
• on-state: substantial minority charge in p and n- regions, conductivity modulation
Collector Fundamentals of Power Electronics
• npn device is shown
56
Chapter 4: Switch realization
BJT switching times vs(t)
Vs2
–Vs1
VCC
vBE(t) 0.7V
RL iC(t) iB(t)
vs(t)
+ –
RB + vBE(t) –
–Vs1
+
iB(t) IB1
vCE(t) 0
–
–IB2 vCE(t) VCC IConRon iC(t)
ICon
0 (1) (2) (3)
Fundamentals of Power Electronics
57
(4)
(5)
(6)
(7)
(8)
(9)
t
Chapter 4: Switch realization
Ideal base current waveform
iB(t)
IB1 IBon
0 t –IB2
Fundamentals of Power Electronics
58
Chapter 4: Switch realization
Current crowding due to excessive IB2 Base
Emitter
–IB2
p
– –
n –
+
+
– –
n-
–
p
can lead to formation of hot spots and device failure
n
Collector
Fundamentals of Power Electronics
59
Chapter 4: Switch realization
BJT characteristics IC
n
egio r e v
acti
10A
tion
a satur i s a u
q
CE
V CE = 5V
saturation region
slope =β
5A
V CE = 200V V = 20V
VCE = 0.5V
• Off state: IB = 0 • On state: IB > IC /β • Current gain β decreases rapidly at high current. Device should not be operated at instantaneous currents exceeding the rated value
VCE = 0.2V
cutoff 0A 0V
5V
10V
15V
IB
Fundamentals of Power Electronics
60
Chapter 4: Switch realization
Darlington-connected BJT
• Increased current gain, for high-voltage applications
Q1 Q2
D1
Fundamentals of Power Electronics
• In a monolithic Darlington device, transistors Q1 and Q2 are integrated on the same silicon wafer • Diode D1 speeds up the turn-off process, by allowing the base driver to actively remove the stored charge of both Q1 and Q2 during the turn-off transition
