It's dead, Jim

A project log for 1 Square inch 4-quadrant floating power supply

A 4-quadrant power supply built on a 1sqin PCB, +/-12V, 100mA, and floatable for multiple paralleling/series. 4layer, $10 OSHPark PCB.

Martin HeldMartin Held 10/01/2018 at 07:030 Comments

While testing the current measuring portion of the system, something happened. I'm not sure what, but this what time-constrained failure looks like.  I was testing the feedback loop on the DAC to set the output to 0V.  I wanted it at zero volts, so that I could put a resistive loaded power supply onto the output, when I saw the serial port output looked funny, it was reading -4V at the output stage.  After resetting, I noticed my bench power supply supplying the board was reading 350mA.  I felt the board, and the regulator was hot.  I isolated a few portions, but I'm relatively certain the MCU is the culprit.  Applying 3.3V to just the power supply, it is drawing 180mA.  Because this was my first time testing the voltage feedback loop (I had just gotten it to measure voltage when another voltage was applied to the input, without the DAC stage connected), I'm not fully sure what happened, but my guess is one of a few things.  

*The DAC got overloaded

*The ADC went out of range (unlikely culprit, I've measured +/- 12V with this thing)

*Some little gremlin took a crap in my processor.

I dunno, it's kinda odd.  Usually when things fail, they fail pretty dramatically and I get a short circuit.

It's not the op-amp chip, pulling vcc has no difference.  And VCC isn't hooked up to the 3.3V rail anyway.  Powering just the 3.3V rail yields 180mA with the +/-15V rail floating.

So just replace it you say.  Not so fast, it's a QFN32 package.  I have a hot air station, but not the time of day right now to replace it without risking more destruction.  I'll have to take it to work in the morning.  

I'll have to decide tomorrow if it's worth furthering this entry to the contest.  Given I'm now unlikely to produce a functioning board (a specific requirement for the contest) with my time constraints, I'm going to bed.  If only work hadn't sent me to the Philippines for 2 weeks plus vacation...