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June 2004

AN871

APPLICATION NOTE

SCR’s AND TRIACS IN AUTOMOTIVE APPLICATIONS

REV. 2

INTRODUCTION

In power electronics the choice of the switch to be used is always very important for the designer: cost,

reliability, ruggedness and easiness to be driven must be permanently kept in mind. This is specially im-

portant in automotive where the designer has to optimize the numerous switching devices implemented

for a wide field of applications: ignition circuits, various electric motor controls, actuators in security sys-

tems, etc...

In the large family of electronic switches, the thyristor must be considered as a low cost and powerful de-

vice for many functions in a car. This paper reminds the reader of the main characteristics of the thyristors

and their advantages in certain automotive applications.

Some examples are presented to illustrate how SCR’s and triacs can bring innovative solutions in new

electronic designs.

THE THYRISTOR

In fact the term "thyristor" defines a silicon bipolar semi-conductor family including any switch whose

bistable action depends on PN-PN regenerative feed back. The main members of this family are the SCR

and the Triac (see appended table).

On the other hand, this structure is also widely used in the new product family integrating several "discrete"

components to build various power functions.

This new integrated circuits range is called ASD™ (Application Specific Discrete).

MAIN CHARACTERISTICS AND BENEFITS

Switching on

One of the major advantages of the thyristor is its ability to switch from off-state to on-state in response to

a low gate current.

According to the type of device used, a gate current of a few tens of microampere or a few milliampere is

sufficient to turn the switch on.

The triggering signal can be a short duration pulse in the range of 100 microseconds or less. As soon as

the latching current through the device is reached, the switch remains ON and the gate current can be

removed. This "memory" effect can be used in a broad area of applications, reducing the consumption of

drivers and saving auxiliary components.

An important point worth noting here is the four quadrants triggering capability of triacs. Whatever the po-

larity of the current through the device, it can be fired either with positive or negative gate current.

This characteristic allows a wide flexibility in the circuitry (see Figure 1).