Service manual

Via a voltage divider, existing of (1 %) resistors R3507, R3510 and R3527, a voltage of
2.5 V (when VBAT = 141 V) is fed to the input of precision shunt regulator 7506. This
regulator will conduct, a current will flow through R3524, and TS7505 will be driven into
conductivity. The base of TS7502 will now be set at a certain positive voltage. As this
transistor switches the FET TS7504 on and off, this circuit can determine the duty-cycle.
E.g. when the load increases, VBAT will decrease. Consequently, the input-voltage of
regulator 7506 will decrease, resulting in a lower current. Through that, the emitter-base
voltage of TS7505 will diminish.
The current through R3504 will decline, changing the base-voltage of TS7502 and
through that the TON (will increase) of the FET. The output voltage VBAT will rise.
If the load continues to increase, the regulator will block at a certain moment, the
collector-current of TS7505 will now be zero. If no current flows through R3504, TON
will now be maximum (IMAX = 1.6 A). This is the point where VBAT will be below 141 V,
and at further increasing load will be switched off (The voltage across the co-coupled
coil (4, 5) will decrease due to the increasing load. Therefore, the voltage on the gate of
TS7504 comes below the threshold voltage. The supply switches off and an audible
hiccupping can be heard).
On the other hand when the load decreases, VBAT will rise. Consequently, the input-
voltage of 7506 will also rise resulting in a higher current. The current through R3504
will rise, changing the base-voltage of TS7502 and through that the TON (will decrease)
of the FET. The output voltage VBAT will be reduced.
If, for instance, VIN will decrease (e.g. UMAINS is 180 V i.s.o. 240 V), the slope of the
drain-current will be flattened, through which the FET will be longer into conductance,
keeping VOUT constant.