I’ve been working on a power supply product for a customer with a very tight limit on the AC mains fuse rating. One of the problems this causes is during differential mode surge testing.
When the metal oxide varistor (MOV) connected line-to-line fired, the resulting current was enough to blow the fuse after a couple of surges at the specified 1kV surge (1.2/50us, 2 ohm). Clearly there wasn’t enough headroom for the product to pass the test. A different MOV with a higher clamping voltage would have reduced the peak current but at the cost of higher voltage stress elsewhere in the circuit.
I decided to look at if the position of the varistor within the circuit made a difference to the surge current in the fuse. It started off in the middle of the mains filter (PCB routing convenience I suspect) but perhaps mounting it before the filters would help? What about at the end of the filter chain, then the X2 capacitors can go to work on the surge pulse first.
The easiest way to try these scenarios was to stick it into SPICE (I like SiMetrix) and have a look at the variables. I crudely modelled the input stage of the power supply as shown below. I guessed at many of the series impedances for the fuse and the capacitor. However the leakage inductances and DCR for the inductors I measured using my excellent Peak Electronics LCR45 component meter. The MOV was simply a 1N4004 diode with a 400V reverse breakdown and the surge was only applied in the +ve direction.
I varied the position of the “MOV” between positions A, B and C to see if there was a difference in the surge current through the fuse (R15). Interestingly enough, there was.
Red = A, Green = B, Blue = C
So the further down the filter chain that the MOV is placed, the less the peak surge current (56% lower) and the RMS current (23% lower) through the fuse.
The results were positive too. The power supply went from failing on the 5th strike at 1kV to passing 10 strikes at 1.75kV. A marked improvement resulting in a more robust product.