General circuit description

STEVAL-ISA001V1

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General circuit description

The circuit operates from 85Vac to 264Vac input voltage with an output power peak of 6W

through two output voltages of 5Vdc and 12Vdc. It contains an input fuse (F2), EMI filtering (C1,

L1, and C2), and the secondary regulation is provided by U2 and U3, the optocoupler and

TL431 respectively. For output filtering, the components used are the output capacitors C8 and

C10. These two output capacitors are sized according to the ripple current and ripple voltage

specifications. The two specifications determine the ESR and RMS current capabilities of the

capacitors used. An additional LC (PI) filter is added to the 5Vdc output, C9-L2 configuration,

for further effective ripple and noise rejection. The AC input is rectified and filtered by the bridge

BR1 and the bulk capacitor C1 to generate the high voltage DC bus applied to the primary

winding of the transformer, T1. The 5Vdc output is sensed with an optocoupler. It is possible to

modify the output voltages by changing the transformer turns ratio and modifying the resistance

values of R7 and R8 in the feedback loop. R2 and C4 form the snubber circuit needed to

reduce the voltage ringing on the drain pin and the leakage spikes.