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12V 10A 120W industrial switching power supply

(Reverse Engineered Schematics)

Schematic of an industrial switching power supply module S-120W-12 in a metal housing. It's a flyback topology. The input is 100-240V AC mains and the output is 12V DC 10A 120W. It came from Ebay for only $8. In the listing it's called "switching power supply LED driver adapter", but it may have a lot of uses other than just powering LED strips. The SMPS is based on the old good UC3842 control chip, TL431 voltage reference and an 817 optocoupler. The switching transistor is 2SK2645 (K2645). It has an open loop protection added, made of discrete transistors. It also has a softstart, a proper fuse, NTC thermistor, safety capacitors, snubber networks, EMI filters, good isolation distance and a relatively safe-looking transformer. The only problem of this switching power supply is the way the power transistor and diode are mounted on the heatsink! Just the sharp corner of the plastic TO220 package actually touches the aluminium plate! There's also no heatsink compound. It would overheat very soon. The power supply is not so bad for its price, but you have to properly mount the two TO220 parts on the heatsink yourself. You can also ground/earth the negative terminal of the output (COM) for more safety.

Industrial switching power supply module S-120W-12 (12V 10A 120W)

Industrial switching power supply module S-120W-12 (12V 10A 120W) opened

Industrial switching power supply module S-120W-12 (12V 10A 120W) opened

PCB board from track side

A transistor in a TO220 package very poorly mounted on the heatsink

There's a gap between the transistor and the heatsink!

It's also mounted at quite an angle :).

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