ST AN308 Application note

AN308

Application note

TRIAC analog control circuits for inductive loads

Introduction

The TRIACs of today are well suited to the requirements of switching inductive loads.

TRIAC control circuits must be particularly well tuned to be both economical and applicable to inductive loads.

The purpose of this document is to present different methods of TRIAC control with their applications and to analyze their relative advantages and disadvantages.

A simple circuit offering all the guarantees of reliability is proposed for inductive loads.

September 2008 Rev 4 1/16

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Triggering methods AN308

1 Triggering methods

1.1 Triggering with synchronization on the TRIAC voltage

The triggering circuit with synchronization across the TRIAC (See Figure 1 and Figure 2) turns on the component at an angle β after the current drops to zero, such that

β = ω · Tr.

Time Tr is defined by the time constant (P + Rt)C.

ω = 2 · π · f with f = mains frequency.

Figure 1. Typical circuit - synchronization across the TRIAC

Diac

AC Mains

C PRt

Figure 2. Synchronization across the TRIAC - waveforms (general case)

Mains voltage

ϕ : Current lag (full angle) β : Blocking of the component α : Conduction angle

Gate pulse

TRIAC voltage

TRIAC current

ϕ full angle

This is the simplest possible circuit but in certain cases it can have an important drawback.

For example, consider a highly inductive load (L ω / R > 4) where the TRIAC is turned on with a considerable delay β, perhaps 100° after the mains voltage zero as inFigure 3.

If the TRIAC is turned on at point A, the conduction (α) lasts up to about 150°. The TRIAC turns off at point B at α + β = 250° after the zero voltage point. At that instant a negative voltage is applied to the triggering circuit which turns on the TRIAC at point C after an angle β of 100°, that is, 350° from the starting point.

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AN308 Triggering methods

The second turn-on occurs at a very low voltage and the angle α’ is much smaller than α. The following period begins under similar conditions and the unbalance persists. This type of asymmetrical operation is not only unacceptable but can be dangerous (high current due to load magnetic saturation due to the dc content of the waveform).

The unbalance is illustrated for a particular case, starting from zero of the mains voltage. Other causes also produce this behavior

● variation of the load impedance, for example, with motors, due to torque variation

● modification of the control turn-on angle

This phenomenon is due to the fact that the circuit does not take its time reference from the mains zero voltage. Rather, the synchronization is taken from the voltage across the TRIAC, which is dependent on the load current, that is, on the load phase shift.

Figure 3. Synchronization across the TRIAC - waveforms (delayed turn on)

Mains voltage

Gate pulse

TRIAC voltage

α'

TRIAC current

full angle

To sum up, this first very simple triggering circuit, synchronized by the voltage across the TRIAC, has the following characteristics:

● Advantages:

– Simple design and low cost

– Connection by two wires, without polarity issue

– Absence of a separate power supply

– Little power dissipated in P and Rt

● A serious disadvantage:

– Because of its principle, this circuit cannot be used for highly inductive loads with a

narrow conduction angle because it can result in unacceptable asymmetrical operation.

This very simple triggering circuit should be reserved for low-cost applications with the following characteristics:

● Resistive or slightly inductive loads

● No stringent requirements concerning the accuracy of regulation

● Highly inductive loads where the power varies between 85 and 100% of the maximum

power

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Triggering methods AN308

1.2 Triggering with synchronization by the mains voltage

This triggering circuit of Figure 4 is synchronized by the mains voltage. The pulses are always shifted by 180° with respect to each other, whatever the type of load.

Figure 4. Typical circuit - synchronization by mains voltage

AC Mains

Figure 5. Synchronization by the mains voltage - waveforms

Mains voltage

Gate pulse

TRIAC voltage

TRIAC current

ϕ : Current lag (full angle) β : Blocking of the component α : Angle of conduction θ : Triggering delay angle

full angle

Angle θ is the delay between the mains zero voltage and the triggering pulse. It can be adjusted by means of potentiometer, P, from 0 to 180° to vary the load voltage. The current in an inductive load (L.R) lags the voltage by an angle ϕ: (tan ϕ = ω · L/ R).

For triggering angles, θ, higher than ϕ, operation is perfectly symmetrical and stable.

This simple circuit can still present the risk of a fault in the case where θ is smaller than ϕ, as shown in Figure 6.

As an example, take the case of a highly inductive load and an angle θ = 60°. The TRIAC is turned on at point A (60°). It conducts over more than 180°, up around 230°. It is blocked at point B: (290°).

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AN308 Triggering methods

The second triggering pulse occurs at point C when θ + α = 240°. It has no action on the TRIAC which is still conducting. The TRIAC is not turned on for the next half-wave. As in the previous case, the operation is totally asymmetrical, and thus unacceptable.

Figure 6. Synchronization by the mains voltage - asymmetrical operation θ < ϕ

Mains voltage

Gate pulse

TRIAC voltage

TRIAC current

full angle

To prevent this fault, it is necessary to limit the turn-on angle to maintain θ > ϕ. This is possible for loads whose L and R parameters remain strictly constant.

Experience shows that for the majority of inductive loads used in industrial applications (like motor controls and transformers) the values of L and R are not constant and vary a great deal during operation. For these types of applications it is not possible to limit the turn-on angle without considerably reducing the voltage excursion.

To sum up, this simple triggering circuit, synchronized by the mains voltage, is more developed than the previous one. It has the following charactersitics:

● Advantages:

– Simple design

– More accurate control than the previous circuit

– No auxiliary power supply or transformer required

● Disadvantages:

– Connection of the circuit by 3 marked wires, instead of 2 without polarity in the

previous circuits

– Higher power dissipated in passive components P and Rt (since the mains voltage

is continuously applied across them)

– Operation becomes completely asymmetrical if the control angle θ is less than ϕ.

This triggering circuit can be used only for applications in which the phase shift of the load remains constant (air inductor) or if operation is restricted to values of θ much higher than ϕ, that is, for low load voltage operations.

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