AN3329 datasheet(7/47 Pages) STMICROELECTRONICS

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AN3329

Main characteristics and circuit description

Doc ID 18376 Rev 1

7/47

Resonant power stage

The downstream converter features the ST L6599A, which embeds all the functions needed

to drive properly the resonant converter with 50 % fixed duty cycle and variable frequency.

The converter makes use of a transformer designed with the integrated magnetic approach,

using the primary leakage as the resonant series inductance and the magnetizing

inductance as the resonant shunt inductance. The transformer secondary-side is center-

tapped and power Schottky diodes are used as output rectifiers. Additional LC filter stages

have been added on each output to minimize high-frequency ripple.

Output voltage feedback loop

The regulation feedback loop is implemented through a typical circuit using a TL431, which

modulates the current through the optocoupler diode. In order to improve the cross

regulation, the two resonant stage output voltages are regulated by a weighted feedback

control, that is using a single rail to regulate multiple outputs. The feedback loop is closed to

the primary side by R37, which connects the RFMIN (#4) pin of the resonant controller

L6599A to the optocoupler phototransistor and sets the maximum switching frequency at

around 130 kHz. This value has been chosen to limit the switching losses at light load

operation. On the same pin, R36 connected to ground, sets the minimum switching

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

duration.

L6599A overload and short-circuit protection

Half bridge primary-side current is sensed by the lossless circuit consisting of R53, C36,

D14, D12, R55, and C38 and is fed into the ISEN pin (#6). During an overcurrent event, the

pin voltage rises to the internal comparator threshold (0.8 V), triggering the following

protection sequence: the soft-start capacitor (C10) connected to the DELAY pin (#2) is

charged by an internal 150 µA current generator and is slowly discharged by the resistor

R12. This pin is connected to the DIS (#8) pin and, if the voltage reaches 1.85 V, the IC

stops switching, being latched off. Once latched, an on/off signal recycle is needed to restart

the converter.

Overvoltage and open loop protection

Both PFC and resonant stages are provided with their own overvoltage protections. The

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

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

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

L6564 is shut down and latched off by an internal circuit monitoring the voltage on the

PFC_OK and INV pins, until the mains voltage is recycled. Upon the occurrence of an

overvoltage condition, of either the 24 V or 12 V output of the resonant stage, the Zener

diodes D16 and D17 conduct, respectively, forcing Q10 to be turned on by the resulting base

current, which causes Q9 to conduct. These two transistors form a pnp-npn SCR (silicon

controlled rectifier) structure that shorts to ground the anode of the U5 optocoupler in such a

way that the IC supply voltage Vcc cannot be delivered to controllers by Q7, forcing them to

be latched off until the mains voltage is recycled.

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