AN3203 datasheet(7/49 Pages) STMICROELECTRONICS

7 / 49 page

AN3203

Main characteristics and circuit description

Doc ID 17402 Rev 2

7/49

The PFC_STOP pin is the interface for the PFC controller, it is connected to the L6563S

RUN pin through R104 and it stops the PFC operation (not latched) in case of overload,

output short-circuit or transformer saturation detection.

The +5 V and +3.3 V are obtained from the +12 VA bus (AHB output) thanks to two DC-DC

converters mounted on two daughter boards. These stages are driven by the L6727, single

phase PWM controller. The topology is a standard step down. For more information please

refer to the L6727; Single phase PWM controller datasheet.

The last stage is the auxiliary power supply that provides the +5 V_SB output (2A capability)

and the VCC supply for the L6563S and L6591. It is realized with a standard flyback

topology operating in CCM/DCM with fixed frequency using the VIPer27H. This stage takes

the PFC output voltage as input and is always working when the mains is plugged in. The

VIPer27H has all the protection needed to safely drive the standby stage. It protects the

circuitry in case of overload, output short-circuit, or output overvoltage.

All the other stages (and therefore the outputs +12 V, +5 V and +3.3 V) can be turned on /

off using the signal PS_ON. If it is disconnected or connected to GND, the OPTO2 current is

zero, Q601 is open and the VCC of the L6563S and L6591 is zero. If PS_ON is connected to

+5 V_SB, the OPTO2 current turns Q601 on. This BJT, together with the Zener diode

ZD601, acts as a linear regulator and provides the supply to the PFC and AHB controllers.

The same optocoupler is used to turn off the PFC and AHB stages in case of an overvoltage

on one of the three main outputs. Such protection is realized with three Zener diodes (one

for each output) that set the OVP thresholds. If one of the three output voltages goes over its

threshold, the Zener diode conducts and turns on the latch realized with Q604 and Q605.

The current in OPTO2 is reduced to zero (overriding the PS_ON information) and the

L6563S and L6591 are turned off.

Only the +5 V_SB stays on and continues to keep the protection latched.

AN3203 Datasheet(PDF) 7 Page - STMicroelectronics