Sunday, July 13, 2014

Pi Power - prototype testing results

Well, the prototypes are not perfect. They're good, but I'm going to revise them once and that will be what sells when (and if) someone buys one.

On the bench, the output voltage regulation stayed good everywhere from 6 volts to 14.75, where the bench supply itself dropped out (it couldn't quite make it to 15). This was on no load, .5A, 1A, 1.5A and (briefly) at 2A. The ripple stayed within 25 mV P-P everywhere above 9 volts. Lower than that and the ripple started to increase up to a maximum of around 45 mV P-P.

I spot-checked efficiency. I didn't have an ammeter on the load, so I could only estimate it with the resistance values in use - which may not have been fully accurate. Nevertheless, efficiencies were in the 80% range or so.

The only problem I encountered was that at high current draw, the switching MOSFET on the board got very, very hot. I didn't fully understand the thermal implications and assumed that a MOSFET rated at 700 mW dissipation could handle <~150 mW without special attention, but now that looks like a bad assumption. The next board will have a MOSFET that comes in a TSOP-6 case instead of an SOT-23-3. It has 4 pins bonded to the drain, and I've made a bigger-than-recommended heat spreader for it with chimneys leading to another heat spreader on the bottom. The new MOSFET is also rated at >3W dissipation. All in all, I expect similar performance, but without quite as much thermal drama.

Meanwhile, the item is for sale in the store. If you order one today, you'll be assured of getting the new version in about two weeks when the boards come back from OSHPark.

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