Capacitive Discharge Ignition (CDI)

Mopeds or lawn movers ignition system design is based exclusively on CDI (1) in the world of small motor. CDI was in the past only used in the replacement module market in auto mobile. Today, the CDI system is becoming one of the most efficient choice available due to new standards of pollution control.

The same application circuit using both ICC01 and ICC03 is shown in figure a and figure b below. Its maximum value being calculated below in both conditioning stage limits the current through the gate.

R max = (VS min – VGT max) / 2 IGT max

with Vs = sensor voltage.

We suggest the use of the conditioning stage shown in figure c due to parasitic voltage occurring on the sensor signal. This gate drive circuit is using R1 between 1 and 10Kohm and C1 between 1 and 10 uF. The SCR run without problems of parasitic firing is allowed by this circuit. [Source: STMicroelectronics Application Note]