AN2393 datasheet(4/35 Pages) STMICROELECTRONICS

4 / 35 page

Main characteristics and circuit description

AN2393

4/35

Main characteristics and circuit description

The main characteristics of the SMPS are listed below:

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Universal input mains range: 90 to 264 Vac and frequencies between 45 and 65 Hz

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Output voltages:

–

24 V@6 A continuous operation

–

12 V@ 5 A continuous operation

–

3.3 V@ 0.7 A continuous operation

–

5 V@ 1 A continuous operation

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Mains harmonics: Compliance with EN61000-3-2 specifications

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St-by mains consumption: Typical 0.5 W @230 Vac

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Overall efficiency: better than 88% at full load

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EMI: Compliance with EN55022-class B specifications

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Safety: Compliance with EN60950 specifications

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PCB single layer: 132x265 mm, mixed PTH/SMT technologies

The circuit consists of three stages. A front-end PFC pre-regulator implemented by the

controller L6563 (Figure 2), a half-bridge resonant DC/DC converter based on the resonant

controller L6599 (Figure 3) and a 7 W flyback converter intended for stand-by management

(Figure 4) utilizing the VIPer12A-E off-line primary switcher.

The PFC stage delivers a stable 400 VDC supply and provides for the reduction of the mains

harmonics, in order to meet the requirements of the European norm EN61000-3-2 and the

JEIDA-MITI norm for Japan.

The PFC controller is the L6563 (U1), working in FOT (fixed off-time) mode and integrating

all functions needed to operate the PFC and interface the downstream resonant converter.

Note:

The FOT control is implemented through components C15, C17, D5, Q3, R14, R17 and R29

(see AN1792 for a complete description of a FOT PFC pre-regulator).

The power stage of the PFC is a conventional boost converter, connected to the output of

the rectifier bridge through a differential mode filtering cell (C5, C6 and L3) for EMI

reduction. It includes a coil (L4), diode (D3) and two capacitors (C7 and C8).

The boost switch is represented by the Power MOSFET (Q2) which is directly driven by the

L6563 output drive thanks to the high current capability of the IC.

The divider (R30, R31 and R32) provides the L6563 (MULT Pin 3) with the information of the

instantaneous voltage that is used to modulate the boost current and to derive some further

information like the average value of the AC line used by the VFF (voltage feed-forward)

function. This function is used to keep the output voltage almost independent of the mains

one.

The first divider (R3, R6, R8, R10 and R11) is dedicated to detecting the output voltage

while the second divider (R5, R7, R9, R16 and R25) is used to protect the circuit in case of

voltage loop fail.

The second stage is an LLC resonant converter, with half bridge topology, working in ZVS

(zero voltage switching) mode.

The controller is the L6599 integrated circuit that incorporates the necessary functions to

drive properly the two half-bridge MOSFETs by a 50 percent fixed duty cycle with dead-time,

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