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I'm looking through my local components distributor's online catalogue, and bridge rectifiers are significantly cheaper for a given current rating than individual diodes. I'm likely to need two or more diodes anyway, and it's okay for them to be connected at one side or the other, so I'd only be "wasting" half the bridge and not three quarters of it.

But: is it safe to "rectify" a DC input? My intent is to protect string(s) of solar panels from reverse power, so the individual diodes in the bridge would be forward-biased for hours at a time. Do hot-spots form in the junctions if the input frequency is too low? Presumably a 35A bridge has big junctions; if it's safe to pass those 35A through that big junction for 10ms at a time, every 20ms (50Hz AC input), to me it doesn't necessarily follow that the junction can safely handle 17.5A (or anything roughly half of 35A) continuously, for similar sorts of reasons that you can't just parallel BJTs without adding emitter resistors. I've just had a look at two random datasheets and there's no hint of any time dependence in the maximum average forward current. One example: the KBPC35xx series

Actually this question applies equally well to individual rectifier diodes, it's just that bridges have so much higher current ratings relative to the package size that I'm more suspicious in that context.

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    \$\begingroup\$ Bridge rectifiers (on AC) have diodes that take turn at conducting therefore, their maximum current can be apparently greater than what the data sheet says for a single diode. \$\endgroup\$ – Andy aka Aug 30 '15 at 23:54
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    \$\begingroup\$ Bernd, could you please post the link to the datasheet of the bridge rectifier(s) in question (for the sake of completeness). \$\endgroup\$ – Nick Alexeev Aug 31 '15 at 0:18
  • \$\begingroup\$ As Andy says, they're rated at 50% for each pair of diodes, so it's definitely safe to use them for DC at 50% of rated RMS current. Beyond that.. \$\endgroup\$ – Spehro Pefhany Aug 31 '15 at 0:44
  • \$\begingroup\$ Good point re 50%, but what I'm thinking of is more of an intra-diode phenomenon than an inter-diode one. I'll edit to clarify. \$\endgroup\$ – Bernd Jendrissek Aug 31 '15 at 8:40
  • \$\begingroup\$ Widely used for reverse polarity protection, but double the power and voltage is lost in the bridge rectifier than with just a series diode, because the current now has to pass through two diodes. Anyway in intermittent operation of a diode or bridge, consult power derating, transit thermal impedance and rated surge forward current curves of the device. \$\endgroup\$ – GR Tech Aug 31 '15 at 12:36
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BURNING BRIDGES.

In your solar application you dont have to worry about surge current because solar cells have a short circuit current thats not much greater than the MPPT current.

Not using all the bridge is helpful but there are a couple of things to consider.

  1. The thermal impedence of many of these bridges is much higher than say a TO247 package so you need a good low thermal resistance heatsink for you to get anywhere near its rating at an acceptable junction temp like 100celcius, in other words the bridge could use more space than the alternative discretes.
  2. The slow standard silicone diodes that appear to be the only thing available in these bridge packages can waste a little more than 1V per diode! You may want to do better by using a Mosfet or a schottky, then the heatsink issue isn't an issue and you aren't wasting expensive solar capability.
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