Monday, September 15, 2014

Pi Power destructive testing report

I took some time this evening and pressed Pi Power to, and beyond, its limits.

First, one thing I discovered is that the failure mode that I observed was that the MOSFET shorts. That causes the input voltage to pass through unmodified to the output voltage, which would likely destroy the Pi. I think a follow-on version of Pi Power will need to add an output over-voltage protection circuit of some sort. That will, however, raise the price.

Second, it looks like I am going to need to de-rate the top end of the voltage range. As the input voltage increases, so does the switching frequency. At 14 volts, the frequency is north of 300 kHz. That's enough that the capacitance of the MOSFET (that is, its maximum switching speed) starts to contribute excessive power dissipation, causing it to heat up. 2 amps at 15 volt input was enough to blow the MOSFET after a few minutes. At 14 volts, it was still hanging on.

I also pressed Pi Power with 9 volts input to see where the current overload protection would trip. When I tried it before, I was using a weaker 12 volt power supply which wasn't able to get that far. I found that the current protection kicks in at around 4.25 amps. The current limit resistor value of 39 kΩ came out of an online TI design tool. I did the math on the datasheet and came up with a value closer to 22 kΩ, and posed a question to StackOverflow about it and never got an answer. But it looks like my calculations were more correct than TI's. The question is whether to change the value in the design to reduce the current limit to be more in line with the limitations of the circuit, or leave it high so that it basically just protects against dead shorts.

Still, in most cases I've personally observed, the current draw of a reasonably loaded Raspberry Pi is closer to 1A than 2. And at that load, with an input voltage of 6-14 volts, Pi Power does just fine.

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