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Is it possible to parallel freewheeling diodes using series resistors?

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Ash Small

Fri Dec 24 2010, 01:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've been puzzling over this all night.

I need a 5 amp freewheeling diode across a transistor.

I have half a dozen 1 amp diodes.

If I add resistors in series with the diodes, I can share the current across the diodes, but I get the voltage drop across the resistors across the transistor, thus defeating the object of adding freewheeling diodes in the first place.

Am I correct in thus assumption?

Or would using, say 5 X 1 ohm resistors (total resistance 0.2 ohms) still provide protection?

Steve Conner

Fri Dec 24 2010, 01:29PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Depends on whether the transistor is a MOSFET or not.

If it's a MOSFET then its own parasitic diode will conduct given half a chance, making your added diodes pointless. This can happen even without any series resistance. You usually don't want it to, because the parasitic diode is usually terribly slow.

If it's an IGBT or BJT, then it can take about 5-7V in reverse before anything bad happens, so you'll get away with it.

I was once building a valve amp and wanted fast recovery diodes in the power supply for extra audiophile cred. I couldn't find any that would take the required voltage and current, so I had to choose between seriesing them and paralleling them. I ended up paralleling 2kV ones with 1 ohm current sharing resistors, and it has worked fine ever since.

You can probably go to the diode shop and get a single 5 amp one.

Ash Small

Fri Dec 24 2010, 02:00PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the advice Steve, Going to the 'diode shop' is against the rules for this project. Apart from it being more fun, I'm learning more. Anyway, I've not found a local diode shop yet anyway (not been living here that long yet). I'll be ordering some, as well as some zeners and mosfets or IGBTs after christmas.

What puzzles me though, is I'll get 0.7 volt drop across the diodes, the rest of the voltage will be across the resistors, which will ALSO be across the transistor (initially a 2N3055H but I have a few others to try as well)

Or am I missing something?

Steve Conner

Fri Dec 24 2010, 02:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, you're missing what I said in the last post.

You can take the emitter of a NPN BJT 5-7V negative with respect to the base and collector, before anything unwanted happens. Any more and the base-emitter junction breaks down like a zener diode and may blow up.

Therefore, as long as the diode drop plus the resistor drop adds up to less than 5V you're OK.

This is of course assuming your freewheel diode is connected in antiparallel between collector and emitter, to conduct reverse current, like the damper diode in a TV HOT.

If you're building a buck converter, the freewheel diode goes between the emitter and ground, and its volt drop simply adds to the off-state Vce seen by the transistor. There's no issue there.

radiotech

Fri Dec 24 2010, 06:18PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Ash Small wrote:
If I add resistors in series with the diodes, I can share the current across the diodes

Do you think each of the diodes will start conducting at the same time and at the same voltage drop?

Ash Small

Fri Dec 24 2010, 06:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

radiotech wrote ...

Ash Small wrote:
If I add resistors in series with the diodes, I can share the current across the diodes

Do you think each of the diodes will start conducting at the same time and at the same voltage drop?

No, but I've read several articles that say that the resistors will basically 'balance things out' across the diodes, allowing them all to kick in. Others here agree with this (Steve does, for one).

Opinions do seem to be divided on whether it's better to use resistors or just hope the diodes are matched closely enough, so I'm doing some basic experiments to try to establish exactly what happens.

Basically, I'll wire four up in parallel, then see if one gets hotter than the others. If this is the case, I'll add resistors and try again.

I used wire that was too thick the first time. I'll re-solder some thinner wire onto the diodes later (I suspect I had a couple of 'dry joints' adding extra resistance on my first attempt)

klugesmith

Fri Dec 24 2010, 07:09PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Ash Small wrote ...
I need a 5 amp freewheeling diode across a transistor.
I have half a dozen 1 amp diodes.

Any chance the 5 amp forward current requirement is a peak value, rather than average? Then you might be overdesigning.

If you do use a bunch of parallel 1A diodes and balancing resistors, don't pack them too closely together. The D and R power ratings assume free circulation of air not already heated by neighboring components.

Ash Small

Fri Dec 24 2010, 08:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Already thought of that Rich, that's why I'm soldering on long thick wires for extra cooling (as well as to keep them further apart.), if I keep them in the circuit I'll probably add a PC cooling fan as well.

The basic plan is to try it and see how hot they get, and whether they heat up evenly, or one gets hotter.

I've already put more than two amps through one, it got hot but didn't blow.

I'll get some decent ones after christmas, but I'll probably build a ZVS circuit enentually, I've a couple of large (>1kW) N27 ferrite cores I'm planning to use eventually. Just having some fun learning the basics at the moment.

Ash Small

Sat Dec 25 2010, 03:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, with four parallel diodes with around 3.5 amps through them they are all at different temperatures.

One is hot, two are warm, and one is cold.

All four are from the same board, so presumably from the same batch.

I've not tried adding series resistances yet. Was thinking of using the heating element from a fan heater, cut up into 1 ohm lengths, the theory being that if one gets hotter, it's resistance will increase, thereby hopefully diverting more current to the others.

I might try putting five amps through them as they are first, and see if any blow. I'm sure I can find some more.

EDIT: just tried 5 amps, they 'seem' to be sharing the current better. I might just try them in the circuit as they are.

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