Philips Semiconductors Thyristors and Triacs free download

File name Power Control with Thyristors and Triacs.pdf

Thyristors and Triacs Power Semiconductor Applications Philips Semiconductors CHAPTER 6 Power Control with Thyristors and Triacs 6.1 Using Thyristors and Triacs 6.2 Thyristor and Triac Applications 6.3 Hi-Com Triacs 485 Thyristors and Triacs Power Semiconductor Applications Philips Semiconductors Using Thyristors and Triacs 487 Thyristors and Triacs Power Semiconductor Applications Philips Semiconductors 6.1.1 Introduction to Thyristors and Triacs Brief summary of the thyristor family The term thyristor is a generic name for a semiconductor switch having four or more layers and is, in essence, a p-n-p-n sandwich. Thyristors form a large family and it is helpful to consider the constituents which determine the type of any given thyristor. If an ohmic connection is made to the first p region and the last n region, and no other connection is made, the device is a diode thyristor. If an additional ohmic connection is made to the intermediate n region (n gate type) or the intermediate p region (p gate type), the device is a triode thyristor. If an ohmic connection is made to both intermediate regions, the device is a tetrode thyristor. All such devices have a forward characteristic of the general form shown in Fig. 1. There are three types of thyristor reverse characteristic: blocking (as in normal diodes), conducting (large reverse currents at low reverse voltages) and approximate mirror image of the forward characteristic (bidirectional thyristors). Reverse blocking devices usually have four layers or less whereas reverse conducting and mirror image devices usually have five layers. The simplest thyristor structure, and the most common, is the reverse blocking triode thyristor (usually simply referred to as the 'thyristor' or SCR 'silicon controlled rectifier'). Its circuit symbol and basic structure are shown in Fig. 2. The most complex common thyristor structure is the bidirectional triode thyristor, or triac. The triac (shown in Fig. 3) is able to pass current bidirectionally and is therefore an a.c. power control device. Its performance is that of a pair of thyristors in anti-parallel with a single gate terminal. The triac needs only one heatsink, but this must be large enough to remove the heat caused by bidirectional current flow. Triac gate triggering circuits must be designed with care to ensure that unwanted conduction, ie. loss of control, does not occur when triggering lasts too long. Thyristors and triacs are both bipolar devices. They have very low on-state voltages but, because the minority charge carriers in the devices must be removed before they can block an applied voltage, the switching times are comparatively long. This limits thyristor switching circuits to low frequency applications. Triacs are used almost exclusively at mains supply frequencies of 50 or 60Hz, while in some applications this extends up to the 400Hz supply frequency as used in aircraft. The voltage blocking capabilities of thyristors and triacs are q