AN2755

Main characteristics and circuit description

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Main characteristics and circuit description

The main characteristics of the SMPS are listed here below:

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Line voltage range:

90 to 265 Vac

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47 Hz

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Regulated output voltage:

400 V

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Rated output power:

400 W

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10 V pk-pk

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Hold-up time:

22 ms

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Maximum switching frequency:

85 kHz

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Minimum estimated efficiency:

90 %

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

voltage and provides for the reduction of the mains harmonics, which allows meeting the

European norm EN61000-3-2 or the Japanese norm JEIDA-MITI. This rail is the input for the

cascaded isolated DC-DC converter that provides the output rails 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 L6562A (U1), integrating all the functions needed to control the PFC

stage.

The L6562A 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 L6562A 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, it uses the line-modulated

FOT (LM-FOT) where the OFF-time of the power switch is not rigorously constant but is

modulated by the instantaneous mains voltage. Please 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 on the DC rail, in series to the output

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

Additionally, the splitting in two of output capacitors (C6 and C7) allows managing the AC

current mainly by the film capacitor C7 so that the electrolytic can be cheaper as it just has

to bear the DC part only.