DC Converter 12V to 1.5V at 4A by 78S40

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The 78S40 is a very old, but still useful power conversion chip. It contains a 1.25V reference, comparator, gating, output transistor, flyback diode and an uncommitted operational amplifier, which is not used here, and draws very little power. As the internal transistor and diode are good only for 1A maximum, I had to use external parts instead. Even so, the circuit is simple enough! A P-channel MOSFET was used as power switch, and an unknown toroid from my junk box was pressed into service for the filter choke. The circuit works very well, with an efficiency of better than 60%, which is superb for such a low voltage output!
I assembled the entire circuit, complete with power switch, in a small aluminum box measuring 4×4x6cm. The MOSFET and the diode were bolted to this box, for heat dissipation. The diode will dissipate around 2W at full output, so it would get too hot when free running. The MOSFET would probably have no trouble, as it dissipates much less power.

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Thank you.

This entry was posted on Wednesday, January 30th, 2008 at 10:59 pm and is filed under Converter Circuit, Power supply, switching power supply. You can follow any responses to this entry through the RSS 2.0 feed.