How to design snubber circuit for turn-off improvement

AN437

10/18

Figure 9.

Z0103 TRIAC turn-off on inductive load (26 W drain pump) without

snubber circuit and in transient operating

An RC snubber circuit must be used when there is a risk of TRIAC spurious triggering, i.e.

(dV/dt)c values.

In our application case and for the worst case load conditions (transient operating), the

These values are higher than the specified (dI/dt)c and (dV/dt)c values, see Section 1.1.2.

Thanks to an RC snubber circuit, rated in the next paragraph, the slope of the reapplied

voltage could be limited and thus spurious triggering at turn-off could be avoided.

2.2.2

Resistor and capacitor snubber circuit design

The snubber circuit design depends on the damping factor (

ξ).

When

ξ decreases, the snubber resistor and capacitor values decrease but the peak voltage

a time later than t = 0 when

ξ is lower than 0.5.

When

ξ increases, the snubber resistor and capacitor values increase and the voltage

higher than 0.5.

Low damping factors are recommended. Indeed, thanks to the high voltage capability of

TRIACs, the snubber circuit can be optimized to reduce the capacitor value and, in the same

way, reduce the snubber circuit cost.

To illustrate the RC snubber circuit design, a 26 W drain pump (L = 2.4 H and R = 190

Ω at

50 Hz) controlled by a Z0103 TRIAC is considered.

Two methods can be used to design the snubber circuit. The first method designs the RC

snubber circuit with load resistance consideration. The second method considers pure

inductive load.

The first method of RC snubber circuit design is divided in four steps:

1.

Snubber resistance choice

To limit the TRIAC turn-on stress and optimize the TRIAC immunity against fast voltage

transients, the snubber resistance is fixed to the minimum value.

For Z01 TRIAC, the dI/dt at turn-on is limited to 20 A/µs. The minimum snubber resistance

value is 620

Ω (refer to Section 1.4).

V

I

V

dV/dt

V

dI/dt

V

I

V

dV/dt

V

dI/dt