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The circuit below shows a three phase rectifier with 3 filter inductors at the input side, and a capacitor, a MOV, and fuse at the output side. I wonder what the differences are in protecting the AC side vs the DC side. I've seen designs where they use 3 fuses, 3 MOVs at the input side. This seems like an increase in cost if we compare it to using just one fuse and one MOV at the DC side. I've also seen the use of TVS diode after the bridge. What would the best approach be?

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  • \$\begingroup\$ Well, a fuse on the DC side isn't going to protect against something like a diode failing short on the AC side. Similarly, a MOV on the DC side won't protect against a line-to-line voltage spike blowing a diode. \$\endgroup\$ – DKNguyen Jul 17 '19 at 20:58
  • \$\begingroup\$ By the way, I think that fuse should go before the varistor, so clamping takes out the fuse, not the varistor. :) \$\endgroup\$ – rdtsc Jul 17 '19 at 21:45
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An MOV on the input side will protect the diodes from the voltage spike, with an MOV on the DC side you obviously save cost, but the current spike you'll get when the MOV conducts may damage the diodes. It'll protect whatever is downstream - which may be the costly parts.

AS for the fuses, it's a lot harder to find fuses rated to clear on DC, since you don't have the current zero that allows the arc to extinguish, and many fuses, thermal trips and breakers will only have an AC rating for that reason.

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