Main characteristics and circuit description

AN2838

4/23

DocID15099 Rev 2

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Main characteristics and circuit description

The main characteristics of the SMPS evaluation board are:

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Regulated output voltage: 48 V

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Rated output power: 35 W

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Power factor (load = 50 %): 0.9 minimum

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Minimum efficiency: 85 % at full load

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Maximum ambient temperature: 50 °C

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Conducted EMI: In acc. with EN55022 Class-B

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Surge rejection: surge test 2.5 kV

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Primary to secondary insulation: 4 kV

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PCB type and size: double-sided, 35 µm, FR-4, 120 x 82 mm

The main feature of this converter is that the input current is almost in phase with the mains

voltage, therefore the power factor is close to unity. This is achieved by the L6562A

controller, which shapes the input current as a sinewave in phase with the mains voltage.

The power supply utilizes a typical flyback converter topology, using a transformer to

provide the required insulation between the primary and secondary side. The converter is

connected after the mains rectifier and the capacitor filter, which in this case is quite small to

avoid damage to the shape of the input current. The flyback switch is represented by the

power MOSFET Q1, and driven by the L6562A.

and R4, that powers the L6562A during normal operation. R3 is also connected to the

auxiliary winding to provide the transformer demagnetization signal to the L6562A ZCD pin,

turning on the MOSFET at any switching cycle. The MOSFET used is the STP5NK80ZFP, a

standard, low-cost 800 V device housed in a TO-220FP package, and needing only a small

heat sink. The transformer is layer type, using a standard ferrite size ETD-29 and is

manufactured by Magnetica. The flyback reflected voltage is ~190 V, providing enough

room for the leakage inductance voltage spike still within the reliability margin of the

MOSFET. The rectifier D2 and the Transil D3 clamp the peak of the leakage inductance

voltage spike at MOSFET turn-off.

The resistors R14 and R15 sense the current flowing into the transformer primary side.

Once the signal at the current sense pin has reached the level programmed by the internal

multiplier of the L6562A, the MOSFET turns off.

The divider R7, R8, R9 and R6 provides to the L6562A multiplier pin with instantaneous

voltage information which is used to modulate the current flowing into the transformer

primary side.

The divider R20 and R21 is dedicated to sensing the output voltage, and capacitor C16 and

diodes D7 and D8 provide a soft-start at turn-on. Output regulation is done by means of an