Thursday, July 17, 2014

Pi Power's upgraded MOSFET

I couldn't wait the full two weeks. Last night before bed, I considered what it would take to kludge the TSOP-6 MOSFET onto the SOT-23-3 footprint of the prototype. The result wouldn't have the full thermal improvements of the new design, but at least electrically I'd get a preview of any particularly nasty surprises lurking.

Fortunately, there appear to be none.

The TSOP-6 device has pins 1, 2, 5 and 6 bonded to the drain; pin 3 is the gate; and pin 4 is the source. They intend you to put a heat spreader under and around the device that's electrically the drain signal.

The SOT-23-3 device is in a G-D-S triangle (with the drain on the side by itself). It turns out that the width of the SOT-23-3 package gate and source pins actually lines up with the TSOP-6 pins pretty well! The only issue is that the drain pad gets buried under the device, but I was able to replace the PCB trace with just a 22 gauge solid wire kludged onto two of the drain pins and reaching over to the diode's cathode.

I was able to get a good look at the regulation and output ripple on my scope with various loads at 12V in and there weren't any big surprises. The MOSFET still gets quite warm when you hit it with a 2A load, but I stuck a thermometer on it and it topped out in the mid 50s Celsius, which I believe is acceptable - particularly without having the correct thermal solution. The output ripple was still no worse than 25 mV P-P, and the regulation was still well inside of 1% (voltages edged down as low as 4.963 volts, but never went higher than 4.980). Efficiency at 12 volts was 87% at 500 mA, 86% at 1A, 85% at 1.5A and 83% at 2A.

For those of you playing along at home, remember that even 10W resistors get super hot when you ask them to get rid of 5 watts of heat.

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