SMPS blew up at Switch-ON

[treez]

Hi,

I completed an EHT insulation test on an SMPS. -Then, when subsequently switched ON, -this SMPS blew up, and would not re-start as the capacitor which blew up apparently blew up the adjacent buck converter controller.

In fact, it was a 1000V capacitor occurring near the Buck Converter that blew up.

Please find here the diagram of my connections for this EHT insulation test. (Unfortunately i have forgotten to include the Buck Converter in between the PFC and the H-Bridge).......

Apparently, this SMPS had been running fine before i did this EHT insulation test, so engineers have said that whilst doing the EHT insulation test, i must have forgotten to switch the Prime Power switch to the ON position....then, they say, due to this switch being OFF, the internal circuitry must have floated up to a high negative voltage, which they say weakened components, resulting in explosion at subsequent switch ON.

Am i correct in assuming that the -16KV at the output, -due to the transformer primary-secondary capacitance, made the primary circuitry jump to -16kv when the High voltage was connected, and that then there was serious leakage currents flowing through various components to earth (eg the capacitor that blew up)?

 

[VEBill]

If it has to meet that spec, then wouldn't it also have to meet it when switched off?

 

[treez]

Thankyou for your thoughts on this.

The output voltage of this SMPS can be up to 6KV...so I am presuming they want to assure that there is adequate isolation between primary and secondary sides when the output is energised.

I am not sure why 16KV was chosen for the EHT voltage source.

Presumably with the switch open there is definetely sufficient isolation between output terminals and the Mains input terminals. -Though that can't be the reason for this test as both the prime power source and the output voltage are lethal. -I'm guessing this test is for looking at leakage currents under high voltage, and is a particularity of high voltage power supplies.

(I forgot to mention that the Mains input at the left of the above figure is the AC rms mains).

 

[VEBill]

I'd thought maybe it was a test for common mode surges as from lightning or similar.

Well, if it is on and then turned off, there will some number of microseconds while the output is still on even while the switch has just be turned off. But it would obviously be at 6kV as opposed to 16kV.

Based on your schematic, I'd follow the ground wire to see if it leads anywhere near the affected capacitor. The hot and neutral are open by the switch, so the ground circuit is probably the first place to look for an unexpected path.