ST AN439 Application note

AN439

Application note

Snubberless™ and logic level TRIAC behavior at turn-off

Introduction

The use of TRIACs is limited by their switching behavior. Indeed, there is a risk of spurious triggering after conduction if the slope of the decreasing current is too high, and/or if the slope of the reapplied voltage is too high. The designer must then take some precautions: device over-rating, switching aid network (snubber), and junction temperature margin, and so on. This generally involves additional costs.

After a brief discussion of commutation when a TRIAC is turned off, this article will describe the behavior of the logic level and Snubberless TRIACs, which present high commutation capabilities.

Contents

1 TRIAC turn-off description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 (dI/dt)c versus (dV/dt)c characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Application requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1 TRIAC with resistive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.2 TRIAC with inductive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 TRIAC without snubber network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Logic level and Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 Operation in Q1-Q2-Q3 quadrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Performances and specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1 Logic level TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.2 Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3 Typical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3.1 Logic level TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3.2 Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

March 2008 Rev 3 1/16

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TRIAC turn-off description AN439

1 TRIAC turn-off description

1.1 Definition

The TRIAC can be compared to two thyristors mounted in back-to-back and coupled with a control area which allows the triggering of this Alternating Current Switch with only one gate (see Figure 1).

Looking at the TRIAC silicon structure (see Figure 2), it can be noted that the conduction areas, corresponding to these two thyristors, narrowly overlap each other on the control area.

Figure 1. Simplified equivalent

schematic of TRIAC circuit

Figure 2. Example of TRIAC silicon

structure

Gates

Ctrl

A1 G

Gates

Ctrl

During the conduction time, a certain quantity of charge is injected into the structure. The biggest part of this charge disappears by recombination during the current decrease, while another part is extracted after the turn-off by the reverse recovery current. Nonetheless, an excess charge remains, particularly in the neighboring regions of the gate, which can induce the triggering of the other conduction area when the mains voltage is reapplied across the TRIAC. This is the problem of commutation.

For a given structure at a determined junction temperature, the turn-off behavior depends on:

1. The quantity of charge which remains when the current drops to zero. The quantity of the charge is linked to the value of the current which was circulating in the TRIAC approximately 100 µs, about two or three times the minority carriers’ life time, before the turn-off. Thus, the parameter to consider is the slope of the decreasing current, called the turn-off dI/dt or dI/dt

. (see Figure 3)

OFF

2. The slope of the reapplied voltage during turn-off. This parameter is the commutation dV/dt, called the turn-off dV/dt or dV/dt current, proportional to the dV/dt

, flows into the structure, and therefore charges are

OFF

(see Figure 3). A capacitive

OFF

injected and added to those coming from the previous conduction.

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AN439 TRIAC turn-off description

Figure 3. dI/dt and dV/dt at turn-off

Mains

OUT

dI/dt

OFF

dV/dt

OFF

1.2 (dI/dt)c versus (dV/dt)c characterization

To characterize the turn-off TRIAC behavior, we consider a circuit in which the slope of the decreasing current can be adjusted. In addition, the slope of the reapplied voltage can be controlled by using, a circuit of resistors and capacitors connected across the TRIAC. For a determined dV/dt which induces the spontaneous triggering of the TRIAC. This is the critical dI/dt the (dI/dt)c in TRIAC datasheets. This is also the way to trace the curve of the TRIAC commutation behavior (see TRIAC datasheet curve “Relative variation of critical rate of decrease of main current (dI/dt)c versus reapplied (dV/dt)c”).

In TRIAC datasheets, the commutation behavior is specified in different way according to the TRIAC technologies. For standard TRIAC, a minimum (dV/dt)c is specified for a given (dI/dt)c. For logic level TRIACs, a minimum (dI/dt)c is specified for two given (dV/dt)c (0.1 V/µs and 10 V/µs). For Snubberless TRIACs, a minimum (dI/dt)c is specified without (dV/dt)c limitation.

Figure 4 represents the curve of the commutation behavior obtained with a standard 4 A

TRIAC. This TRIAC is available with different sensitivities:

● Z0402: I

● Z0405: I

● Z0409: I

● Z0410: I

For lower sensitive gate TRIACs (Z0409 and Z0410), the (dI/dt)c is slightly modified according to the (dV/dt)c. For sensitive gate TRIACs (Z0402 and Z0405), this parameter noticeably decreases when the slope of the reapplied voltage increases.

((dV/dt)c), we progressively increase the dI/dt

OFF

= 3 mA;

= 5 mA;

= 10 mA;

= 25 mA.

COM

until a certain level

OFF

, called

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TRIAC turn-off description AN439

Figure 4. Relative variation of (dI/dt)c versus (dV/dt)c for a 4 A standard TRIAC

(typical values)

Area of spurious firing at

commutation

Safe area

In practice, the current waveform, and thus the dI/dt we cannot change it.

So, in TRIAC applications, it is always necessary to know the dI/dt a TRIAC with a suitable (dI/dt)c. This is the most important parameter.

Suppose a circuit in which the dI/dt

OFF

4 A TRIACs, characterized by the curves in Figure 4, will be not suitable even if the dV/dt is equal to 0.1 V/µs.

1.3 Application requirements

1.3.1 TRIAC with resistive load

In this case, the TRIAC current and the mains voltage are in phase (see Figure 5). When the TRIAC switches off (i.e. when the current drops to zero), the mains voltage is equal to zero at this instant and will increase across the TRIAC according to the sinusoidal law:

Equation 1

)t·

sin(VV

·=

MaxMains

For the European mains, i.e. V

Equation 2

= 220 V at 50 Hz, the slope will be:

RMS

, is imposed by the load. Generally

OFF

of the load to choose

OFF

reaches 2 times the specified (dI/dt)c. The standard

OFF

≈

)Hz()V(RMS)s/V(OFF

s/V1.010f·22Vdt/dV

µπµ···

For 110 V, 60 Hz mains, the slope will be: dV/dt

These relatively low dV/dt dI/dt

only depends on the load rms current and the mains frequency. For resistive loads,

OFF

correspond to the left points on the curves in Figure 4. The

OFF

as for most other loads, we will have:

Equation 3

π=

4/16

≈ 0.06 V/µs.

OFF

≈

I5.010f22Idt/dI ·····

)Hz()A(RMS)ms/A(OFF

)A(RMS

AN439 TRIAC turn-off description

Figure 5. Current and voltage waveforms for resistive loads (phase control)

dI/dt

OFF

Mains

dV/dt

OFF

1.3.2 TRIAC with inductive load

An inductive load induces a phase lag between the TRIAC current and the mains voltage (see Figure 6).

When the current drops to zero, the TRIAC turns off and the voltage is abruptly applied across its terminals. To limit the speed of the reapplied voltage, a resistive / capacitive network mounted in parallel with the TRIAC is generally used (see Figure 13). This “snubber” is calculated to limit the dV/dt the (dI/dt)c specified in the datasheet. The dI/dt

at a value for which the dI/dt

OFF

is also determined in this case by the

OFF

is lower than

OFF

load impedance (Z) and the mains rms voltage. (see. AN437 for RC snubber circuit design)

Figure 6. Current and voltage waveforms for inductive loads (phase control)

dI/dt

OFF

Mains

dV/dt

OFF

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