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Converting large bridge rectifiers into a single diode!

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LAN_403

Signification

Mon Oct 26 2015, 03:48PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Years ago, I got my hands on an industrial microwave oven (BIG sucker!)...'looked' like one from the 70's, but a later model. I noticed a HUGE bridge rectifier (single phase, 4-pin standard) that was wired in a strange way--only the "+" and "-" pins were connected (no ratings)! All I could assume, after drawing it, was that it was being used as a single high power rectifier diode! NOW...I have use for such a thing--this is what I had in mind:
Using the 'cheap and common' full wave bridge (FWB) i.e. the 50A @ 1kV squares.

I was planning to try using one of these as a "100A" single rectifier. BTW, since a FWB should have it's internal diodes rated at double the peak voltage (for 'R' loads), that would make the voltage rating 2kV due to two 1kV diodes in series (but this may be already taken into account)--each pair in parallel remains 1kV, but they're two sets of these for a 2p2s array of 100A. I realize that they may not be exactly matched, but how much closer can you come with four diodes from the same FWB pak? So does anyone see why I should NOT expect a 2kV @ 100A single diode from a 1kV @ 50V FWB using only the "+" and "-" pins (in the reversed sense here)?
...couple more things...
* I could use two FWB's for 2kV @ 200A (or perhaps 1kV @ 200A)...I'm more concerned with the Amps.
* would you choose to connect the "AC" connections together in the combined--or single-- FWB(s)? In any case there will be only two wires out.
I see cases where this would be more convenient than a stud rectifier.

DerAlbi

Mon Oct 26 2015, 05:24PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

since a FWB should have it's internal diodes rated at double the peak voltage (for 'R' loads)

No, dont count on that. It is a good thought from an engineering point of view... however there are maketing guys on top of that who wont leave out the chance to push some numbers. You allways need a 1kV-rectifier if you want to handle 500VAC.

Paralleling diodes of a Bridge recitfier is probably not the best idea imho. I mean.. you can simply try it how much they actually stray apart by measuring them one by one. Bridge rectifiers are made mainly with high overload capacity in mind. No particular matching.. just a fat junction. They are optimized for low frequency operation and there might be even a snubber-capacitor included. Without datasheet who knows.
Youve got the advantage that the diodes are thermally coupled. Its such a big advantage so you could be lucky.
A simple test for a single piece of art would be that you put the recitfier under load (full spec current) and just monitor the voltage difference during the heat up at the 2 open legs. if the voltage difference grows, you are fucked. if its converging to 0V then its a good indicator that the bridge splits the current equally. (its basically a wheatstone bridge)
If you want to double check the breakdown voltage, use a 100k-resistor in series with the diode and measure the reverse leakage current of the diodes with an appropiate high voltage supply. Just use a soldering iron (or heat gun) to heat up the junction(s) because breakdown voltages are allways higher at colder temperatues - so make sure you dont measure marketing worthy values.

Electra

Wed Oct 28 2015, 10:35PM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156

At those currents I would have thought you would want to avoid having 2x the
voltage drop across a diode as the heat dissipation and loss will be
twice as much.

I can see why it would be attractive at lower current levels,
isolated package and commonly available.

If no such part exists at the required voltage and current, then you
have no choice other than to combine a lot of smaller devices and the
same rules apply, you have to allow some margin for imperfect
balance.

I also thought the voltage rating marked on a bridge rectifier was same
as that of the PIV of each internal diode.

Sulaiman

Wed Oct 28 2015, 11:10PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Signification wrote ...

All I could assume, after drawing it, was that it was being used as a single high power rectifier diode!

So does anyone see why I should NOT expect a 2kV @ 100A single diode from a 1kV @ 50V FWB using only the "+" and "-" pins (in the reversed sense here)?

* would you choose to connect the "AC" connections together in the combined--or single-- FWB(s)?

I have seen this in industrial equipment but never had to replace one,
so whatever 'rule of thumb' or design technique used, it must be conservative.

The wording is ambiguous, if the question was
"would four modified bridges wired as a single bridge have twice the voltage and current rating of a single bridge ?"
then I would say yes,
used as a 1/2-wave rectifier I would say no.

The ones that I have seen had the ac terminals un-connected.
this is logical as then current sharing is better
(assuming random distribution of differences in current vs. voltage)
you could parallel each diode with a fixed resistance (and optionally capacitance) to balance reverse voltages
but it seems that is not necessary.

Plus, the power rating and/or dissipation of a bridge of bridges
operating at twice the voltage and current of a single bridge
would dissipate four times the power,
so a 1kV 50A bridge can be used as a diode in a 2kV 100A bridge

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