Update to revision 2.1

Yet another silly mistake, but its all in the pursuit of learning. I’ve fixed the ferrite bead mix up from the last update, and that appears to have fixed the 44V instead of 180V issue, but allowed a new issue to surface! In my infinite wisdom, I selected an 0603 resistor for R39 (the 1.5 mega ohm resistor in charge of the feedback voltage divider). The 0603, if you didn’t know, is rated for 75V. Which means when a voltage greater than 75V is present across the resistor, it essentially open circuits. This is not good news for the feedback into the MAX1771. As far as it’s concerned, the feedback voltage is zero! Time to increase the voltage! But wait… if the output voltage is greater than 75V (which it always will be when aiming for 180V), then the output voltage is going to keep increasing forever! Well, not forever, just until the IC saturates and can’t give any more. So now instead of producing 180V or even 44V, we have 500V!!! Well over what the components I selected are rated for.

Lesson learnt, schematic design is not the only important thing, component selection and layout are VERY VERY IMPORTANT too!

I plan to fix this with a bodge resistor which is properly rated, but this won’t be suitable for the final design, so I will add this to revision 2.1 to increase the size of R39 to 2512, rated for well over 250V.