Page 164 - Power Electronics Handbook

156 Static switches
Figure 7.6, the odoff control terminal ensuring that the main thyristor is
fired for the start of the on-cycle and the auxiliary thyristor for the start of
the off-period. As before, protection signals, such as overtemperature and
overcurrent, can be fed in to turn the contactor off, and slow start can be
incorporated as in Figure 7.5(b) by gradually increasing the mark-to-space
ratio of the load voltage.
Smart power devices are available from several manufacturers for static
switching applications. A commercially available device, which uses a
MOSFET as the power device, is shown in Figure 7.7. It is available in a
TO220 package and is rated at vd, of 50 V and an Id, of 11 A. A signal on the
input terminal controls the on and off state of the power transistor TR,.
However the device is protected from excessive input voltage, including that
generated by ESD, by zener diode D,.
The device is also protected from over current and high temperatures. An
internal d.c. reference Vf is applied to the two comparators C1 and Cz, the
other input to the comparators coming from a current sense resistor and an
on-chip temperature sensor. An over current or high temperature will cause
the flip flop F, to latch. This will turn off transistor TR2 and turn on TR,, so
that the power MOSFET is rapidly turned off. The flip flop can be reset by
holding the input signal low for a specified minimum time.