AN3106 datasheet(5/34 Pages) STMICROELECTRONICS

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AN3106

Main characteristics and circuit description

Doc ID 16775 Rev 2

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Main characteristics and circuit description

The main features of the SMPS are listed here below:

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Extended input mains range: 85 to 305 VAC - frequency 45 to 55 Hz

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Output voltage: 48 V at 2.7 A

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Long-life, electrolytic capacitors are not used

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Mains harmonics: acc. to EN61000-3-2 Class-C

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Efficiency at full load: better than 90% at 115 VAC

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EMI: according to EN55022-Class-B, EN55015

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Safety: double insulation, according to EN60950, SELV

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Dimensions: 75 x 135 mm, 30 mm components maximum height

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PCB: single side, 35 µm, FR-4, mixed PTH/SMT

1.1

Power Factor corrector

The PFC stage, working in transition mode, acts as a preregulator and powers the resonant

stage with the output voltage of 450 V. The PFC power topology is a conventional boost

converter, connected to the output of the rectifier bridge D3. It is completed by the coil L1,

manufactured by MAGNETICA, the diode D2 and the capacitors C5, C6 and C7 in parallel.

The PFC output capacitors are film type, 5 µF - 800 V manufactured by EPCOS. Using film

capacitors to replace the typical electrolytic capacitors allows increasing considerably the

MTBF of the board.

The boost switch is represented by the power MOSFET Q2. The board is equipped with an

input EMI filter necessary to filter the commutation noise coming from the boost stage. The

PFC implements the controller L6562AT, a small and inexpensive controller that is

guaranteed for operation over a wide temperature range.

At startup, the L6562AT is supplied by the startup resistors R5, R8, R13 charging the

capacitor C13. Once the PFC begins switching, a charge pump connected to the auxiliary

winding of the PFC inductor L1 supplies both PFC and resonant controllers via a small

linear regulator implemented by Q1. Once both stages have been activated, the controllers

are supplied also by the auxiliary winding of the resonant transformer, assuring correct

supply voltage during operation of all load conditions. The L1 auxiliary winding is also

connected to the L6562AT pin #5 (ZCD) through the resistor R18. Its purpose is to provide

the information that L1 has demagnetized, needed by the internal logic for triggering a new

switching cycle. The PFC boost peak current is sensed by resistors R33 and R34 in series

to the MOSFET source. The signal is fed into pin #4 (CS) of the L6562AT, via the filter R27

and C16.

The divider R7, R12, R14 and R22 provides the L6562AT multiplier with the information of

the instantaneous mains voltage that is used to modulate the peak current of the boost.

The resistors R2, R6, R9 with R15 and R16 are dedicated to sense the output voltage and

feed to the L6562AT the feedback information necessary to maintain the output voltage

regulated. The components C11, R20 and C12 constitute the error amplifier compensation

network necessary to keep the required loop stability.

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