Tuesday, 4 April 2017

Bridge rectifier in parallel?


Is it possible to safely double the current limit using 4 bridge rectifiers? In one forum I found someone wrote:




Use four bridges, one for each leg. Tie the AC input (cathode to anode) connections across on each bridge. This now makes a single equivalent diode, at twice the PIV and current. Two diodes in series; two in parallel.



I tried to draw a schematics to these instructions but it doesn't make sense to me how it can work.



Answer



In most cases, paralleling diodes is not a good idea. Their forward voltage drops Vf can be different, if they are not from the same batch. As a result one diode with the lowest Vf in the bank will conduct more current than the rest and more than it's rated for. That diode will get burned. Then the next one will get overcurrent in the same fashion. Untill all of the parallel diodes in the bank are burned. I'm afraid that the quote in the O.P. is describing this [wrong] approach.


But a bridge rectifier could lend itself to a possible workaround. Suppose, you've got 4x bridge rectifier ICs. You must assume that they are not from the same batch. But each of the diodes inside the bridge is from the same batch, because they are on the same die.


enter image description here


Notice that there are parallel diode connections within the bridges, but not between the bridges. No more than 2x bridges can be paralleled this way.


Proceed with caution. Best of all, just get a single bridge with sufficient current rating.



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