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AN3089

Main characteristics and circuit description

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Main characteristics and circuit description

The main features of the SMPS are listed below:



Universal input mains range: 90



Output voltage: 19 V at 3.42 A continuous operation



Mains harmonics: acc. to EN61000-3-2 Class-D or JEITA-MITI Class-D



Standby mains consumption: < 100 mW at 230 Vac



Average efficiency: better than 89% without synchronous rectification



EMI: according to EN55022-Class-B



Safety: according to EN60950



Dimensions: 58 x 121 mm, 25 mm maximum component height



PCB: single side, 35 µm, CEM-1, mixed PTH/SMT.

1.1

Power stage

The flyback converter implements the new ST dedicated current mode L6566B (U2)

controller operating in quasi-resonant mode and detecting the transformer demagnetization

through the ZCD (#11) pin.

R23 on the OSC (#13) pin sets the maximum switching frequency at about 165 kHz.

Because the maximum switching frequency is imposed, the converter operates in

discontinuous conduction mode during light-load operation. The L6566B valley skipping

function is capable of turning-on the MOSFET in valley switching even in DCM, therefore

reducing switching losses.

The MOSFET is a standard 800 V, STF7NM80, housed in a TO-220FP package, needing

just a small heatsink. The transformer is a layer type, using a standard ferrite size EER28L,

designed according to EN60950 and manufactured by MAGNETICA.

The flyback reflected voltage is ~150 V, providing enough room for the leakage inductance

voltage spike with a still margin for reliability of the MOSFET. The D5 rectifier and the D4

Transil™ clamp the peak of the leakage inductance voltage spike at MOSFET turn-off.

A small capacitance in parallel with D4 smooth leakage inductance spikes, reducing EMI

and Transil dissipation.

The output rectifiers are two STPS20H100CFP dual center tap Schottky diodes (D2 and D3)

in parallel, housed in the TO-220FP. They have been selected according to the maximum

reverse voltage, forward voltage drop, and power dissipation. The snubber, made up of R5,

R7 and C12, dampens the oscillation produced by the diode capacitance and the leakage

inductance.

A small LC filter has been added on the output, filtering the high frequency ripple and

spikes.

D6, R4, R5, R8, R9, Q2 and Q3 implement an output voltage “fast discharge” circuit, quickly

discharging the output capacitors when the converter is turned off. It has been implemented

to quickly decrease the residual output voltage after the converter is turned off at no load.