Oh dear! the smoke came out!

At the weekend I picked up the courage to plug the PWM filter connectors into the voltage control on the module, and it appeared to work, dialing the PWM output higher caused more voltage to come out, turning it down caused less. Yet there were problems...

  • That the relationship between the PWM value and the output voltage was decidedly non-linear.
  • The set voltage varied all over the place, especially when the output was turned up.
  • There was a very strange clicking noise when programming the micro which was accompanied by extremely large output excursions

I tried to get the voltage setting and voltage output to line up, to no avail, the PWM value would zero into a stable level, but then an excursion would completely upset the feedback loop. I tried several different methods to get this to work and yet it would not maintain stability. All the time, the strange clicking noise was getting more and more pervasive.

I was having one last ditch attempt before making a design change and tossing out the PWM idea last night when I noticed the distinctive smell of hot electronics. I immediately shut down the unit and went to find my IR thermometer. When I turned it back on, I scanned the unit to find the hot component while looking, the component announced itself with a plume of magic smoke. It looks like the component in question is a diode, but I don't know what else it has taken with it.

So I have ordered a new one and it will be here in a few weeks time.

In the meantime, I need to understand what caused this to happen. I came across this site https://wiki.beyondlogic.org/index.php?title=WD2002SJ_LTC3780_Synchronou... which appears to have parts of the circuit reverse engineered and drawn out. I was particularly interested in the diagram that has the two pots on it as this is the part of the circuit I am trying to interfere with. The 500kOhm variable resistor is what I had replaced with my PWM output and from the circuit diagram, it is not going to work at all as rather than using it as a complete voltage divider (which the PWM idea relied on) it is being used as the top half of a variable divider. I think I am going to need something with real resistance here.

While I am waiting for the new part to come in I will have to think about how I can solve this problem.

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