In message <008b01c67c53$681d0560$0201a8c0@dad>
          olin_piclist@embedinc.com (Olin Lathrop) wrote:

> Only if you run in discontinuous mode.  The 1N4001 has a rather slow reverse
> recovery time.

Guess I need to go looking for a faster diode then. AFAICT the problem is
most diode datasheets don't spec the Trr (reverse recovery time) and I can't
see any easy way to calculate it. Even OnSemi and Fairchild only seem to spec
Trr for ultrafast diodes. That and I don't really know what other specs
(besides forward/reverse voltage and current) I need to pay attention to...

At the moment, I'm looking at the 1N5817 series and the MBRD835L. I might
have a half-finished project or two kicking around with a thievable 5817 on
board, but I'd have to order the 835...

Specs are (at the moment):
  Vin: 12V
  Vout: 6V
  Topology: Buck (step-down)
  L: 47uH (actually measured at 43uH)
  F: 32kHz
  Filter cap: 47uF jellybean electrolytic (also tried a 1000uF low-ESR).
    
Even with a 4001 fitted, the SMPSU does seem to be handling itself quite
well. I can see the duty cycle varying as I change the load on the supply,
and it seems the PWM is turning off completely if the output voltage goes too
high. The pass transistor is running pretty cool as well - certainly beats a
7805. Transient response is pretty dire though. I'm surprised it handled
no-load without overshooting (too) badly though. There's still some nasty
overshoot on powerup, but I suspect that's mostly to do with running it with
an unloaded output.

I guess the next test is to get it to do basic current regulation, then try
and charge a nicad...

Thanks.
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