I've just managed to fix a CPX200 power supply.
It was taking a huge current and blowing fuses and the inrush protector.
I found that the power input has power factor correction using an L4981 (https://www.st.com/en/power-management/l4981.html) and I realised that the TO220 Schottky diode on the output of the boost converter was dead short.
When I took that diode off, I found that the tab of the diode had shorted to its heatsink, which was connect to ground, so there is about 400 V between them the TO220 tab and the heatsink.
The TO220 was supposed to be insulated with a silicone insulator and a bush around the bolt, but the tube bit of bush was quite a lot shorter than the thickness of the TO220 tab resulting in just a tiny air gap with 400 V across it. There was a small amount of burning on the bush, the screw, the TO220 tab, the insulator and the heat sink, and there was basically a dead short across the 400 V supply.
It's working now with a replacement diode, mounting kit and inrush protector.
The replacement TO220 mounting kit bush has a tube a lot longer than the thickness of the TO220 tab, so the tube extends all the way into the heatsink. I've had to drill out the heatsink slightly to take the longer bush, but it seems like a more robust arrangement now.
It was taking a huge current and blowing fuses and the inrush protector.
I found that the power input has power factor correction using an L4981 (https://www.st.com/en/power-management/l4981.html) and I realised that the TO220 Schottky diode on the output of the boost converter was dead short.
When I took that diode off, I found that the tab of the diode had shorted to its heatsink, which was connect to ground, so there is about 400 V between them the TO220 tab and the heatsink.
The TO220 was supposed to be insulated with a silicone insulator and a bush around the bolt, but the tube bit of bush was quite a lot shorter than the thickness of the TO220 tab resulting in just a tiny air gap with 400 V across it. There was a small amount of burning on the bush, the screw, the TO220 tab, the insulator and the heat sink, and there was basically a dead short across the 400 V supply.
It's working now with a replacement diode, mounting kit and inrush protector.
The replacement TO220 mounting kit bush has a tube a lot longer than the thickness of the TO220 tab, so the tube extends all the way into the heatsink. I've had to drill out the heatsink slightly to take the longer bush, but it seems like a more robust arrangement now.
