Main characteristics and circuit description

AN3172

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Doc ID 17230 Rev 1

1.1

Startup sequence

As previously indicated, the PFC acts as master and the resonant stage can operate only if

the PFC output is delivering its nominal output voltage. Therefore the circuit is designed so

that at startup the PFC starts first, then the downstream converter turns-on. At the

beginning, the L6563H is supplied by the integrated high voltage startup circuit; once the

PFC starts switching, a charging pump connected to the PFC inductor supplies both PFC

and resonant controllers. Once both stages have been activated, the controllers are also

supplied by the auxiliary winding of the resonant transformer, assuring correct supply

voltage, even during standby operation.

Because the L6563H integrated HV startup circuit is turned off, and therefore not dissipative

during normal operation, it makes a significant contribution to power consumption reduction

once the power supply operates at light-load, in agreement with standby worldwide

standards currently required.

1.2

Brownout protection

Brownout protection prevents the circuit from working with abnormal mains levels. It is easily

achieved using the RUN pin (#12) of the L6563H; this pin is connected through a resistor

divider to the VFF pin (#5) providing the information of the mains voltage peak value. An

internal comparator allows IC operations if the mains level is correct, within the nominal

limits. At startup, if the input voltage is below 90 Vac (typ.)

, circuit operations are inhibited.

The L6599A has similar protection on the LINE pin (#7). It is used to prevent the resonant

converter from working with a too low input voltage which can cause incorrect capacitive

mode operation. If the bulk voltage (PFC output) is below 380 V, the resonant stage startup

is not allowed. The L6599A LINE pin internal comparator has a hysteresis allowing the turn-

on and turn-off voltage to be set independently. The turn-off threshold has been set to 300 V

in order to avoid capacitive mode operation but allow the resonant stage to operate even in

the case of mains sag and a consequent PFC output dip.