AN2941 datasheet(9/37 Pages) STMICROELECTRONICS

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AN2941

Main characteristics and circuit description

Doc ID 15447 Rev 4

9/37

The output rectifiers are two dual center tap Schottky diodes (D7 and D5) in parallel. They

have been selected according to the maximum reverse voltage, forward voltage drop and

power dissipation. The snubber made up of R14, R66 and C8 damps the oscillation

produced by the diode capacitance and the leakage inductance. A small LC filter has been

added on the output, filtering the high frequency ripple.

D17, R75÷R78, Q10 and Q15 implement an output voltage “fast discharge” circuit,

discharging quickly the output capacitors when the converter is turned off. It has been

implemented to quickly decrease the residual output voltage after the converter is turned off

at no load.

Startup sequence

The circuit is designed so that at startup the flyback starts first, then it turns on the PFC

stage controlling the L6563S via the Vcc_PFC pin. Therefore, the flyback stage is designed

to manage at startup the full output power over the entire input voltage range because it

must guarantee the regulation of the output voltage even during load transitions when the

load is increasing but the PFC is still not yet delivering the nominal output voltage. Of course

this condition can be maintained only for a short time, typically tens of milliseconds, because

the flyback is not designed to sustain this condition from a thermal point of view. The flyback

controller L6566A pin #1 (HV) is directly connected to the bulk capacitor. At startup, an

internal high voltage current source charges C32 and C33 until the L6566A turn-on voltage

threshold is reached, then the high voltage current source is automatically switched off. As

the IC starts switching it is initially supplied by C32 and C33, then the transformer auxiliary

winding (pins 8-9) provide the voltage to power the IC. Afterwards, according to the load

level monitored by the COMP pin, the L6566A activates the L6563S powering it via the

Vcc_PFC pin.

Because the L6566A integrated HV startup circuit is turned off and therefore not dissipative

during normal operation, it contributes significantly to reducing power consumption when

the power supply operates at light load which in turn contributes significantly in meeting

worldwide standby standards currently required.

Brownout protection

Brownout protection prevents the circuit from working with abnormal mains levels. It can be

easily achieved using pin #16 (AC_OK) of the L6566A. D6, Q3, C23, R7, R12, R26, R62

and R64 implement a circuit sensing the mains voltage peak value and feed it into L6566A

pin #16. An internal comparator then enables the IC operations if the mains level is correct,

within the nominal limits. If the input voltage is below 90 Vac the startup of the circuit is

inhibited, while the turn-off voltage has been set at the voltage reached by the bulk capacitor

after the hold-up time. The internal comparator has in fact a hysteresis allowing the L6566A

turn-on and turn-off voltage to be set independently. Sensing the mains voltage before the

input rectifier is a less dissipative solution with respect to sensing the bulk voltage. In

addition it allows faster restart because there is no need to wait for the bulk capacitor to

discharge.

The L6563S has a similar protection on the RUN pin (#10) but in this schematic it is not used

because in this architecture it acts as slave, therefore the main controls are managed by the

flyback stage.

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