Page 27 - Power Electronics Handbook

20 Power semiconductor devices

during its lifetime. This third current is usually used to blow
protection fuses associated with the rectifier. The working current of
a rectifier may be specified as a direct current or as an average value.
(iii) The peak power dissipation which the device can withstand. Power
dissipation is linked to the device junction temperature and its
cooling characteristics.
(iv) The maximum junction temperature at which the rectifier can be
operated. Also specified is the maximum storage temperature of the
device, and this is given as the highest and lowest temperatures, since
the semiconductor can also be damaged by prolonged exposure to
sub-zero temperatures.

1.4.3 Rectifler characteristics
The characteristics of a semiconductor component can often be divided
into static and dynamic. The following are some of the static
characteristics:

(i) Forward voltage drop. This varies with forward current, as shown in
Figure 1.8(b), so it is normally specified at a given current or by a
graph. The average forward voltage drop is usually specified as an
average over a full cycle, at a stated frequency, average forward
current and case temperature.
(ii) Reverse leakage current. This is specified at a defined reverse voltage
and temperature, or as an average value over one cycle at a given
temperature.
(iii) The forward power dissipation. This is the product of the forward
voltage drop and current and is given as a curve.
(iv) The reverse power dissipation. This is equal to the reverse leakage
current times the reverse voltage, at a defined point in the device
curve.
(v) The junction to case thermal resistance. This characteristic, specified
in units of degrees Centigrade per watt, indicates the difference in
temperature between junction and case, when the rectifier is
dissipating power. It is required for designing rectifier cooling
systems, as described in Chapter 2.
The dynamic characteristics of a rectifier relate to its switching periods,
both on and off. When a rectifier is switched into conduction a finite time is
required for the minority carriers to flow across the junction and to prime it
to carry the full load current. This can result in an initial peak voltage
across the device, as seen in Figure 1.8(d). The load current rises rapidly,
being limited by the circuit inductances, but the rectifier is in its fully-on
mode after a delay time of fd. This time is very short, but it does represent
a period of large power dissipation, which for very high-frequency
operation can start to become important. Data sheets normally specify this
forward delay time
When a diode is switched from the forward-conducting to the
reverse-biased mode two changes occur. First, the minority charge is swept
away as reverse current, in the external supply. This current can be large,
often being limited only by the external circuit impedances. It could be