AN3339 datasheet(7/44 Pages) STMICROELECTRONICS

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AN3339

Main characteristics and circuit description

Doc ID 18449 Rev 2

7/44

Brownout protection

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

achieved by two separate circuits, one using the brownout pin of the VIPer® and a second

using an internal comparator of the L6564 dedicated to this function: this pin is internally

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

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

nominal limits.

If the input voltage is below ~80 Vac (typ.), circuit startup is not allowed.

Resonant power stage

The downstream converter featuring the ST L6599A, incorporates all the functions

necessary to properly drive the resonant converter with a 50 % fixed duty cycle and works

with variable frequency.

The transformer, using the integrated magnetic approach and incorporating the resonant

series inductance, delivers 3 output voltages without any post-regulator.

The transformer configuration chosen for the secondary winding delivering 12 V and 24 V

output is center-tap and makes use of power Schottky rectifiers. For the secondary winding

delivering 130 V, a full bridge configuration using four ultrafast diodes has been chosen. A

small LC filter has been added on each output, to filter the high frequency ripple.

Output voltage feedback loop

The resonant stage feedback loop is implemented by means of a typical circuit using a

TL431 modulating the current in the optocoupler diode. The three outputs are regulated by a

weighted feedback control.

On the primary side, R37 - connecting the RFMIN pin (#4) to the optocoupler's

phototransistor - closes the feedback loop and its value sets the maximum switching

frequency. R36, connecting the same pin to ground, sets the minimum switching frequency.

The RC series R22 and C21 sets both soft-start maximum frequency and duration.

L6599A overload and short-circuit protection

The current into the primary winding is sensed by the loss-less circuit R53, C36, D14, D12,

R55, and C38, and is fed into the ISEN pin (#6). In the case of overcurrent, the voltage on

the pin overpass the internal comparator threshold (0.8 V), triggering a protection sequence.

The capacitor (C37) connected to the DELAY pin (#2) is charged by an internal 150 µA

current generator and is slowly discharged by the resistor R54. This pin is connected to the

DIS pin and as soon as the voltage achieves 1.85 V, the IC stops switching latched. On/off

signal recycling is necessary to restart the resonant converter.

Overvoltage and open loop protection

Both PFC and resonant stages are equipped with their own overvoltage protection.

The PFC controller L6564 monitors its output voltage via the resistor divider connected to

the PFC_OK pin (#6) protecting the circuit in case of loop failures, disconnection, or

deviation from the nominal value of the feedback loop divider. When a fault condition is

detected, by monitoring both PFC_OK and INV pins, the PFC_OK circuitry latches the

L6564 operations. The PFC is kept latched until the mains voltage is recycled.

In the case of overvoltage by the resonant stage the Zener diodes D16, D17, and D29

detect the output voltages and conduct. This causes Q10 to turn on and, consequently, Q9

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