Page 36 - Power Electronics Handbook

Bipolar transistors 29
Usually the working range of a transistor is limited by the maximum
voltage and current. However, when working into inductive loads the
device can be destroyed, even when below the maximum limits, giving
second breakdown. This is due to hot spots caused by current
concentration, resulting in local thermal runaway. Current concentrations
occur due to causes such as unstable lateral temperature distribution, base
region potential drops, uneven base widths, and uneven mounting of the
silicon chip onto the heat sink.
Figure 1.13(a) shows the second breakdown characteristic of a bipolar
power transistor. For a given base current the collector current (I,) will
increase as the collector-emitter voltage (V,) increases. After a point A
on the characteristic the device will go into saturation and the current will
remain substantially constant until point B, when avalanche breakdown, or
first breakdown, occurs. This causes a rapid rise in current until point C

Locus of second
breakdown
trigger points

,.

-
Collector - emitter voltage VCE
(a)
Bonding wire
and metallisation
IC f limited
/

Reducing
duty cycle

(b) VM "CE
Figure 1.13 Power bipolar transistor characteristic: (a) second breakdown characteristic;
(b) safe operating area