Page 81 - Power Electronics Handbook

Chapter 3
Power semiconductor control components

3.1 Introduction

This chapter examines components which are capable of handling
relatively low levels of power, but which are used to control high-power
semiconductors. These components often take the form of low-power
equivalents of the power components themselves, such as diodes,
transistors, and light-triggered thyristors, although in this chapter only two
types of components are considered; those which provide trigger pulses
and are often used in oscillator circuits, and those which are used to
isolate the drive circuits from the main power devices.
In addition to these components, which transmit the pulse to the power
semiconductors, logic devices are widely used as part of the overall control
electronic function, which includes semi-custom-integrated circuits and
microprocessors. These control devices are often packaged along with power
components to form smart power circuits (see Section 3.5).

3.2 Power semiconductor control requirements

Power semiconductors which have a control terminal and are therefore
capable of being controlled are transistors, thyristors, gate tum-off
switches and triacs. These devices vary in that a transistor needs a control
signal at its base during its entire conduction period, whilst the other
components are turned on by trigger pulses on their gate terminals. The
transistor, however, is similar to a gate turn-off switch in that it can be
turned on and off by a control signal, whilst thyristors and triacs depend on
a momentary break in the load current for turn-off.
Irrespective of the type of semiconductor device, they all require a
turn-on signal having a shape similar to that of Figure 3.1. A
high-amplitude pulse VI is essential to provide the overdrive needed during
the turn-on period, so that the turn-on time is reduced and the switching
losses in the power semiconductor minimised. The pulse must have a short
rise time since this also affects the turn-on delay time. The initial overdrive
is usually kept to a short period since otherwise the power dissipation in the
control section of the power semiconductor, and in the control circuitry,
would be excessive. After time tl this pulse is reduced to a lower value V,

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