I love it when a prototype first works

I've been playing around with a prototype 15 Amp, 200 Volt PWM power supply I built today trying to get it to work. It worked fine with only one MOSFET at low loads with a DC motor connected.

Once three parallel MOSFETS were hooked up it would immediately blow one MOSFET and not work with the motor but would work with a 100 lightbulb dummy load. Then it would blow with the motor despite having a suitable back EMF diode.

While scratching my head I noticed the FM radio was getting interference which is pretty hard to do with FM. I had to be getting some parasitic oscillations with strong harmonics up into the 10's of MHz. Anyway three ferrite beads on each gate terminal later and the whole thing worked fine with only slight faint interference on the AM band that a small filter cap fixed.

I love it when things work the way they were intended!.

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Mark Harriss
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