ST AN307 Application note

mains voltage

mains voltage

mains voltage

mains voltage

AN307

APPLICATION NOTE

USE OF TRIACS ON INDUCTIVE LOADS

INTRODUCTION

Although triac circuits are now well known by designers. The use of these components for inductive loads requires certain precautions which should not be neglected of optimum use is to be made of them. That is the purpose of this article which reviews the various triac control modes and recalls the principles which guarantee its correct operation.

Phenomena Occurring when the Circuit is Closed

The triac is known as a component which is essential in controlling power from an AC source (mains). In most cases, the circuit has an inductive component: either because of the nature of the load itself: motors, transformers, ballast inductance; or because of the source impedance: utilization of the secondary of a transformer, length of the supply line, etc. On inductive loads, the operating conditions vary considerably, when closing the circuit, depending on the control mode (gate current, polarity and width) and synchronization of the firing. In order to build an optimal control circuit it is indispensable to analyse the various possibilities.

FIRING

Control Signal

The triac is fired by a gate current Ig > Igt whose duration should enable the main current to reach the triac holding current value (IL). The width of the control signal is determined by the rate of increase of the main current (di/dt), limited by the load inductance and by the choice of the firing quadrant. The loading current, IL, is highest in the second quadrant (A2 positive with respect to A1, Ig negative): (see Figure 1-a).

The rate of rise of the main current, di/dt, is proportional to the amplitude of the power supply voltage at the moment of firing (di/dt = V/L). The width of the firing signal required is less when firing occurs near the peak of the mains voltage than when its occurs around zero of that voltage (see Figure 1-b).

To fire the triac and to ensure conduction in continuous operation, we can compare various types of control circuits.

Gate Current Control by Single Pulse

To ensure correct operation, the gate pulse should be synchronized with the triac current zero point and should be long enough to enable the main current to reach the latching current IL level (see Figure 2-a).

In case the pulse occurs before the triac current reaches its zero point (incorrect synchronization) or if its duration is too short to allow the main current to exceed the latching current IL, the triac conducts only during alternate half-cycles. The high DC component thus introduced in the load can produce considerable overloads due to saturation of magnetic materials.

	REV. 2
March 2004	1/9

AN307 APPLICATION NOTE

Figure 1. Width of Control Signal Required as a Function of the Firing Quadrant (a); width of Control Signal Required as a Function of the Moment of Firing (b)

triac current

mains voltage

quadrant II

quadrants I,

III and IV

quadrants I, III and IV

triac current

quadrant III

firing at zero voltage

firing at peak

voltage

Quadrants

III

Polarity

–

with respect to A1

Figure 2. Gate Control by a Single Pulse Synchronized with Zero Current (a); in Case of a Single Pulse whose Duration is too Short, the Triac only Conducts during Alternate Half-cycles (b)

I	triac current	I	triac current
			IL
0	t	0	t
	t		t
Ig		Ig
0	t	0	t
	t		t

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AN307 APPLICATION NOTE

Gate Control by Pulse Train

The control by gate pulse train eliminates problems of synchronization on the current. A recurrence frequency of several kilohertz guarantees correct operation of this type of control (see Figure 3).

This procedure, whose results are satisfactory, is often used for controlling triacs in inductive circuits. A variant of this principle consists in making use of a circuit which monitors firing and which delivers pulses to the gate as long as the voltage across the triac is higher than a threshold, usually fixed at about 10 volts (see Figure 4). This type of circuit enables delivering just the amount of gate current required for firing.

Gate Control by DC Current

Gate control by DC guarantees ideal firing but has the disadvantage of high consumption, specially when the control power supply is provided by the mains. In this case, it is preferable to use a negative current for the gate control (quadrants II and III).

Figure 3. Gate Control by Pulse Train

V mains voltage

I triac current

TRANSIENT PHENOMENA DURING TRIGGERING

Principles

During continuous operation, the magnetic field H, proportional to the current in the coil, varies with respect to the induction B, with a delay as shown by the hysteresis cycle in Figure 5. In transient operation, the induction can follow a different path and reach the saturation value BS for which the magnetic field H (according to the coil current) increases very rapidly (see Figure 8).

In the circuits controlled by a triac, opening occurs when the current is at zero. The induction thus has a remanent value Br, corresponding to H = 0 (see Figure 5). When the triac begins to conduct, the transients depend on the instant of synchronization of the control signal with respect to the mains voltage.

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