Main characteristics and circuit description

AN2485

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Main characteristics and circuit description

The main characteristics of the SMPS are:

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Line voltage range: 90 to 265 Vac

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Regulated output voltage: 400 V

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Rated output power: 400 W

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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 acc. with EN55022 Class-B

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PCB type and size: Single side, 70 um, CEM-1, 148.5 x 132 mm

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Low profile design: 35 mm component maximum height

The evaluation 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. The

board provides for the reduction of the mains harmonics which allows meeting the standards

of 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 evaluation board can easily

deliver up to 450 W.

The controller is the L6563 (U1), integrating all the functions needed to control the PFC

stage and to interface with the downstream converter. The L6563 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,

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

drive capability of the L6563 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), where the OFF-time of the power

switch is not rigorously constant but is modulated by the instantaneous mains voltage, will

be used. Please refer to AN1792 (“Design of Fixed-Off-Time-Controlled PFC Pre-regulators

with the L6562”) for a detailed description of this technique as indicated in Section 9:

References (point 2).

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 efficiency during low line operation. Additionally,

the splitting in two of output capacitors (C6 and C7) provides for managing the AC current