UM0575

Operation principle

7/20

When the PTC conduction time has been sufficient to start the compressor, its voltage

suddenly rises. This voltage rise is sensed by the R3/R4 divisor bridge.

If the level is higher than the DZ1 clamping level, the thyristor, consisting of Q1 and Q2, is

latched. M1 is then turned on.

As M1 is ON, there is no more gate current to trigger T2, so T1 also stays OFF. The PTC is

then turned off.

No more current circulates through the PTC, thus no power is dissipated in it.

Figure 7 shows the typical behavior of this circuit. The C1 capacitor voltage gives an image

of the PTC voltage. When its value reaches the DZ1 clamping level (15 V), the MOS gate is

latched to 15 V. The PTC is then turned off (refer to the IA ACST6 current waveform).

Figure 6.

Voltage spikes at zero current

Figure 7.

PTC turnoff

AM00902v1

AM00903v1