Adapter features

AN2432

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1

Adapter features

1.1

Main characteristics

The 60W AC-DC adapter board described in this application note has the following main

characteristics:

●

Input:

–

f: 45 ~ 66 Hz

●

Output:

–

●

No - Load:

–

Pin below 0.3W

●

Short circuit: protected with Auto-Restart feature

●

PCB type and size:

–FR4

–

Single side: 70 µm

–

120 x 75 mm

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Safety: according to EN60065

●

EMI: Compliance with EN55022 - Class B specifications

1.2

Circuit description

This circuit implements a flyback transformer which is a very popular topology for this kind of

application and power level, thanks to its simplicity and good trade-off between cost and

performance. To improve the converter's efficiency, the EVALSTSR30 demo board uses

synchronous rectification.

The converter works in both Continuous and Discontinuous conduction mode depending on

the input voltage (the circuit has a wide input voltage range) and the output load. The

68-kHz switching frequency provides a good compromise between the transformer size and

the harmonics of the switching frequency, optimizing input filter.

The input section includes protection elements (varistor, fuse and NTC for inrush current

limiting), a standard Pi-filter for EMC suppression, a bridge and an electrolytic bulk capacitor

as the front-end AC-DC converter. The transformer is a layer type, uses a standard ETD34

ferrite core and is designed to have a reflected voltage of 95V. The power MOS is a 700V -

1

Ω and a transil clamp network is used for leakage inductance demagnetization.

On the primary side, the ST L6668 PWM controller integrates all the functions needed in a

SMPS (switch mode power supply) and enables building a complete system with a low

amount of external components. It includes a high voltage start-up generator, an

overvoltage protection input, frequency foldback for better efficiency at light loads,

programmable burst mode operation and soft start circuit.