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Old 21st Jun 2018, 1:11 pm   #9
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 4,895
Default Re: Switched Mode Power Supplies

Yes, I've designed and built, in the 'day job'

Pitfalls? Make a prototype that is easy to repair. With 24V input, things die relatively quietly, but switching devices can still take out their driver stage. If you have a transformer to drive the switching devices, this is less likely. Expect to have to replace switching devices many times.

You'll learn that an inch of wire or PCB track in the wrong place can have enough inductance to completely upset operation.

If you use bipolar transistors to do the switching, you'll learn about storage time and how to turn off with reverse base current. If you use MOSFETs then you'll miss out on this fun.

SG3524 and 3854 are good IC's. They are popular for a reason. Read the application notes, they'll get you going. Trying to design with logic IC's, comparators, and op-amps is only for the masochistic! And doing entirely with discretes, again it can be done, but you will need hours and hours of debugging.

The test gear you will need is a couple of bench power supplies, some load resistors, a twin-beam 'scope, and a transient load switch. One supply is for the 'housekeeping' control circuitry, which you leave on so drive to the switching devices is present and robust; one is for the power-stage input, which you wind up from zero while watching waveforms. With care you can see problems before the voltages/currents are high enough to blow the devices up.

When you get the circuit running and stable, at some point you'll want to optimise the
regulation circuitry. This is where the transient load switch comes in. Mine is a 555 oscillator running at 10Hz - 1kHz (switchable) and driving a power MOSFET. You use it to switch in an additional load in parallel with a steady load, on and off continually, while monitoring with your 'scope the output voltage, to see how the circuit copes with load changes. Expect to see all sorts of behaviour, from continuous oscillation, through damped ringing, before you achieve the ideal of an brief excursion of output voltage followed by a rapid return to the 'correct' value without overshoot or undershoot, each time the load is switched in our out by the transient load switch.

You'll have lots of fun, and learn loads!
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