AN2994 datasheet(5/38 Pages) STMICROELECTRONICS

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AN2994

Main characteristics and circuit description

Doc ID 15796 Rev 2

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Main characteristics and circuit description

The EVL6563S-400W demonstration board has the following characteristics.

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Line voltage range: 90 to 265 Vac

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Minimum line frequency (fL): 47 Hz

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Regulated output voltage: 400 V

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Rated output power: 400 W

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Maximum 2fL output voltage ripple: 10 V pk-pk

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Hold-up time: 22 ms (VDROP after hold-up time: 300 V)

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Maximum switching frequency: 85 kHz (Vin = 90 Vac, Pout = 400 W)

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Minimum estimated efficiency: 90% (Vin = 90 Vac, Pout = 400 W)

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Maximum ambient temperature: 50 °C

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EMI: in accordance with EN55022 Class-B

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PCB type and size: single side, 70 µm, CEM-1, 148.5 x 132 mm

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Low profile design: 35 mm component maximum height

The demonstration board implements a power factor correction (PFC) pre-regulator

delivering 400 W of continuous power on a regulated 400 V rail from a wide-range mains

voltage, and providing for the reduction of the mains harmonics, thus complying with the

European norm EN61000-3-2 and the Japanese norm JEITA-MITI. This rail will be the input

for the cascaded isolated DC-DC converter that will provide the output voltages required by

the load.

The board is equipped with enough heat sinking to allow full-load operation in still air. With

an appropriate airflow and without any change in the circuit, the demonstration board can

easily deliver up to 450 W.

The controller is the L6563S (U1), integrating all the functions needed to control the PFC

stage and to interface with the downstream converter. The L6563S controller chip is

designed for transition-mode (TM) operation, where the boost inductor works next to the

boundary between continuous (CCM) and discontinuous conduction mode (DCM). However,

with a slightly different usage, the chip can operate so that the boost inductor works in CCM,

hence surpassing the limitations of TM operation in terms of power handling capability. The

gate-drive capability of the L6563S is also adequate to drive the MOSFETs used at higher

power levels. This approach, which couples the simplicity and cost-effectiveness of TM

operation with the high-current capability of CCM operation, is the fixed off time (FOT)

control. The control modulates the ON time of the power switch, while its OFF time is kept

constant. More precisely, the line-modulated FOT (LM FOT) is used, where the OFF time of

the power switch is not rigorously constant but is modulated by the instantaneous mains

voltage. Refer to [2] for a detailed description of this technique.

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

the rectifier bridge D2. It includes the coil L4, the diode D3 and the capacitors C6 and C7.

The boost switch is represented by the power MOSFETs Q1 and Q2. The NTC R2 limits the

inrush current at switch on. It has been connected to the DC rail, in series to the output

electrolytic capacitor, in order to improve the efficiency during low line operation.

Additionally, the splitting into two of the output capacitors (C6 and C7) means that the AC

current is mainly managed by the film capacitor C7, making the electrolytic cheaper

because it only has to bear the DC part.

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