Page 28 - Power Electronics Handbook

Power recrifier operation 2 I

described as the surge current through the device junction capacitance,
although this is not strictly true. In addition to this external current flow,
holes and electrons disappear in the vicinity of the reverse-biased region,
due to recombination. The current decays to zero, as the reverse
voltage-blocking capability of the diode increases, as shown in Figures 1.8(e)
and 1.8(f).
The reverse recovery time of the rectifier is several orders of magnitude
greater than its forward turn-on time, so the power dissipation during
switch-off is much greater. Clearly the reverse recovery time tm must be
kept as short as possible, in order to limit the device dissipation. However,
the decay of current, over time tr, must not be too abrupt as this will give
rise to large voltage spikes in any associated inductive circuits. The ratio of
t,hm should be as large as possible and devices which have a relatively high
value of this ratio are said to be ‘soft’.
In circuit applications the reverse recovery current, which for power
devices can be high due to the larger silicon area involved, is often
overlooked, with disastrous effects. For instance, consider the chopper
circuit shown is Figure 1.9(a). The semiconductor switch is opened and
closed at a relatively high frequency and varying duty cycle, in order to
control the magnitude of the mean load voltage and current. Suppose the
load is inductive. This requires a diode D, often called a free-wheeling
diode, to be connected across it, to prevent excessive surges in the switch
when it opens.
Ignoring the reverse recovery current in the diode, the system would
operate as follows. With the switch closed D is non-conducting and load
current is supplied via the d.c. source V,. When the switch opens the load
current free-wheels in D and, provided the loss in this path is not large, the
current will be substantially unchanged when the switch is again closed. If
the reverse recovery current through D is not ignored then, with the load
current free-wheeling through it, the operation is basically as before.
However, when the semiconductor switch closes this current transfers to
the supply. In addition there is now a reverse recovery path as shown, and
since this is of a very low impedance, a surge of current can pass, which
could destroy the switch.
Chopper circuits are not exclusively prone to reverse current effects.
Figure 1.9(b) shows a commonly used bridge rectifier Circuit. Suppose line
A is positive to B so that load current is supplied via rectifiers 1 and 4.
When the voltage reverses and line B becomes positive, the current in
rectifiers 1 and 4 will decay to zero, whilst that in 3 and 2 will increase to
support the load. As soon as 1 and 4 turn off a reverse current path exists
through them, as illustrated. This provides a short circuit across the supply
lines, which could destroy rectifiers 2 and 3. In all such applications great
care must be taken to ensure that there is sufficient line impedance, which
would limit this short-circuit current until the reverse-biased device
recovers its full blocking capability.
Several design techniques exist to enhance power rectifier characteris-
tics. A p+-p-n-n+ (or n+-n-p-p+) structure can be used in which the
base consists of high-resistivity n- (or p-) type material, chosen to
withstand the required reverse voltage. One face has ap (or n) diffusion
made into it and the opposite face has an n+ (orp+) diffusion layer. Due to