TRIAC turn-off description

AN439

6/16

1.4

TRIAC without snubber network

junction of the TRIAC. When the current drops to zero, the TRIAC is considered as a switch

which turns off. The dampened oscillating circuit is constituted by the loads, L and R, and

peak mains voltage and the phase difference (

φ) between voltage and current.

Figure 7.

TRIAC commutation on an inductive load without a snubber network

For a second order linear differential equation with a step function input, the voltage

Equation 4

With damping factor:

Equation 5

Undamped natural resonance:

Equation 6

Final voltage value:

Equation 7

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, and for most part of inductive loads, the damping factor (

ξ) is

generally lower than 1.

V

T

I

G

G

I

T

V

Mains

Load

LR

I

T

V

T

E

Load

LR

C

T

V

E

t

dV/dt

OFF

V

T

I

G

G

I

T

V

Mains

Load

LR

I

T

V

T

≈ E

Load

LR

C

T

V

E

t

dV/dt

OFF

E

)

t

(

V

dt

)

t

(

dV

2

dt

)

t

(

V

d

1

T

T

0

2

T

2

2

0

=

+

+·

.

·

ω

ξ

ω

)

H

(

)

F

(

T

)

(

L

C

2

R

·

Ω

=

ξ

)

F

(

T

)

H

(

)

s

/

rad

(

0

C

L

1

·

=

ω

)

sin(

2

V

E

RMS

φ

·

·

=