Page 80 - Rashid, Power Electronics Handbook

Simo
˜e
M.
Godoy
66 M. Godoy Simo˜es s
66
i B i C

V CE, sat
Increasing
base current

V BE V CE

V f Saturation Constant-current

region ( active) region
i= i b B
C
( a) ( b)

FIGURE 5.5 Family of current-voltage characteristic curves. (a) base-emitter input port; and (b) collector-emitter output port.

terms of preventing thermal runaway. Forward current ratings
o
are speci®ed at which the junction temperature does not V = 2 V ( 125 C)
) b CE
exceed a rated value so as to prevent leads and contacts V = 400 V ( 25 C)
o
from being evaporated. Power dissipated in a semiconductor CE
device produces a temperature rise and is related to thermal
resistance. A family of voltage-current characteristic curves is Current Gain (
o
shown in Fig. 5.5. Figure 5.5a shows the base current i plotted V = 2 V ( 25 C)
CE
B
as a function of the base-emitter voltage v BE and Fig. 5.5b
depicts the collector current i as a function of the collector-
C
emitter voltage V , with i as the controlling variable. log( )
i
CE
B
Figure 5.5 shows several curves distinguished from each C
other by the value of the base current. The active region is FIGURE 5.6 Current gain depends on temperature V CE and I C .
de®ned where ¯at, horizontal portions of voltage-current
curves show ‘‘constant'' i C current, because the collector Vertical-structure power transistors have an additional
current does not change signi®cantly with v CE for a given i . region of operation called quasi-saturation, indicated in the
B
Those portions are used only for small signal transistors
characteristics curve of Fig. 5.8. Such a feature is a conse-
operating as linear ampli®ers. On the other hand, switching
quence of the lightly doped collector drift region where the
power electronics systems require transistors to operate in collector base junction supports a low reverse bias. If the
either the saturation region where v CE is small or in the cutoff transistor enters the hard-saturation region the on-state power
region where the current is zero and the voltage is upheld by dissipation is minimized; there is, however, a tradeoff Ð
the device. A small base current drives the ¯ow of a much in quasi-saturation the stored charges are smaller. At high
larger current between collector and emitter; such gain (called
beta Ð Eq. (5.4)) depends upon temperature, V and I .
CE C
Figure 5.6 shows current gain increase with increased collector
voltage; gain falls off at both high and low current levels.
High voltage BJTs typically have low current gain, and hence
T 1
Darlington-connected devices, as indicated in Fig. 5.7 are
commonly used. Considering gains b and b for each one
1 2 T 2
of these transistors, the Darlington connection will have an
increased gain of b þ b þ b b and diode D1 speeds up the D 1
1 2 1 2
turn-off process by allowing the base driver to remove the
stored charge on the transistor bases. FIGURE 5.7 Darlington-connected BJTs.