AN437 datasheet(9/18 Pages) STMICROELECTRONICS

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AN437

How to design snubber circuit for turn-off improvement

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2.2

RC snubber circuit design

2.2.1

Is the snubber circuit required?

Without snubber circuit, the slope of reapplied voltage is limited by the TRIAC capacitance

between anode and cathode junction. The oscillating circuit is constituted by the load, L and

R, and the internal capacitance, CT, of the TRIAC.

For example, the typical internal capacitances of 1 A, 12 A and 24 A TRIACs are

respectively 12 pF, 90 pF and 180 pF (without direct voltage junction polarisation, worst

case). Without snubber circuit and for most part of inductive loads, the damping factor (

ξ) is

generally lower than 1.

For an underdamped oscillating circuit (0

≤ ξ < 1), the voltage variation across the TRIAC

(VT(t)) is:

Equation 7

With damped natural resonance:

Equation 8

For example, in the case of a 26 W drain pump (L = 2.4 H and R = 190

Ω at 50 Hz)

controlled by a Z0103 TRIAC (CT = 12 pF), the damping factor is close to zero (ξ = 2.1 x 10

4).

For low damping factor, the normalized voltage rise slope K is equal to 1 (refer to Figure 8,

lower graph). The maximum slope of reapplied voltage across the TRIAC is then:

Equation 9

According to this formula, the estimated dV/dtOFF is equal to 59 V/µs without snubber circuit.

As shown in Figure 9, the measured dV/dtOFF is in fact about 10 V/µs.

The error between the Equation 9 result and real value is due to the fact that we didn’t take

into account the load inductance saturation (real value is higher at low current), the parallel

parasitic capacitor of the load, the recovery current and the load resistance increase with

frequency. Moreover, the turn-off measurement is done with a voltage probe which adds a

12 pF capacitor across the TRIAC. So, the measured dV/dtOFF is always lower than the

theoretical value, given by Equation 9.

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