ACST8-8C

4/8

CONTROL

UNIT

COM

OUT

G

TYPICAL APPLICATION DIAGRAM

ROBUSTNESS AGAINST FAST CAPACITOR DISCHARGE

When parasitic transients or controller mis-operation occur, the blocked switch may turn on by spurious

switch firing. Since the phase shift capacitor is charged, its energy is instantaneously dissipated through

the two ACSTs which can be destroyed. To prevent such a failure, a resistive inductive circuit R-L is added

in series with the phase shift capacitor.

tions), and on the inductance L:

dI

dt

V

L

=

max

The total switch turn on di/dt is the sum of the di/dt created by any RC noise suppressor discharge and the

dI/dt created by the motor capacitor discharge.

Since the maximal di/dt capability at turn-on of the ACST8 is 100A/µs, the motor capacitor di/dt is assumed

to be less than 50A/µs; therefore, the inductance should be 14µH.

The resistor R limits the surge current through the ACST8 during the capacitor discharge according to the

Ω is low enough to limit the resistor

dissipation (usually less than 1 W).

Finally both the 14µH inductance and the 1.2

Ω resistance provide a safety margin of two on the surge cur-

C

L

V

AC

T1

T2

ON

R

700V

M

Fast capacitor discharge when

one ACST switch turns on (T2)

and the motor runs (T1 ON).