Using 6 Diodes for a 30 Amp Battery Charger

[John1397]

I was told once that two 1 amp tv type diodes parallel will not work because one will always carry the load, but I am working on a car 30 amp battery charger and it has 6 diodes 1/2" press in type all parallel same as from a 10DN series Delco alternator from the 60's where they used three positive and three negative off the windings this must of worked as this is the way they built it. You could probably just put one 200 amp diode instead of six 25 amp but the cost might be more for one than six not to mention if you have 6 and one burns out 5 are still good.

 

[CWatters]

It depends how well matched they are. Two diodes in parallel will have the same voltage across both. If their curves are identical then they share the current equally. If the curves are slightly different then there can be a large difference in the current that each carries.

 

[Merlin3189]

"Will not work" may be putting it a bit strongly. "May be a bad idea" or "needs more thought than you might think" might be a better one.

The current through a diode changes a lot for a small change in voltage. If there is a slight mismatch between the diodes, one can end up carrying much more current than another.
Since the characteristics change with temperature, there is also the possibility that the situation can be unstable - the diode which carries more current gets hotter and its V_f reduces, allowing it to take more of the current, and get even hotter, etc.

Addition of some Ohmic resistance in series with each diode helps balance the currents, by increasing the fwd voltage when the current increases. But of course this reduces efficiency, so may not be appropriate here.

Tying the diodes together thermally, ideally at the chip level, helps prevent thermal feedback exacerbating the imbalance.

This can also apply to putting diodes in series to get higher reverse voltage rating. If their characteristics are not identical, then one carries more of the voltage for each value of leakage current. So you can't assume two 1000 V diodes in series will withstand 2000 V. Maybe one will reach 1000 V while the other is at only 700 V, then any increase over 1700 V will exceed the rating of the first diode.

As CWatters says, it may work if they are well matched. And you avoid thermal problems. And increase your safety margins to cover whatever mismatch there is.

 

[Baluncore]

, but I am working on a car 30 amp battery charger and it has 6 diodes 1/2" press in type all parallel same as from a 10DN series Delco alternator from the 60's where they used three positive and three negative off the windings this must of worked as this is the way they built it.

Six diodes is normal in an alternator since they are full-wave rectifying three phases. Those diodes are not in parallel.
A full wave rectifier on a transformer secondary will have four diodes in a bridge. Again they are not in parallel.
How many phases are being rectified in the battery charger?

 

[John1397]

I am not even sure myself I will have to take front cover off. I do know there are three diodes on each side all six are connected to negative output and each pair of three's is connected to a 9 gauge wire this seems like there are two secondary windings. I thought it is half wave. It has 6, 8 and 12 volt output but not sure how this is obtained. It is not like alternator where three diodes are used for positive and three for negative. I think the picture I found of a battery charger looks like mine except where they use two diodes mine has 3 on each line off the tranformer. Seems like you could put twenty 10 amp schotty diodes and have the same thing?

 

[Windadct]

Rarely is paralleled diodes a good design practice - even when the Vf is "perfectly" matched, there is a strong affect of temperature on the Vf. So 2 well matched diodes ALSO need nearly identical cooling situations, and to many EE's this seems a trivial issue, however in most or many real world scenarios - it really is not.

I post 5 - really without an accurate schematic of this case it seems difficult to advise further. If your system has 3 different voltage outputs - I then highly suspect the diodes are not in parallel, 3 diodes, 3 voltages... hmmm...

 

[Baluncore]

Seems like you could put twenty 10 amp schotty diodes and have the same thing?

There are many very good reasons why power rectifier diodes should not be wired directly in parallel. You need to stop leaping to irrational conclusions based on false assumptions and a lack of knowledge.

Switching voltage converters with synchronous rectifiers are now more efficient than transformers with power diodes. That is because the voltage drop across a power MOSFET is now less than a junction diode.

The circuit attached to Post #5 shows a centre-tapped transformer using two diodes as a full-wave rectifier. The diodes are not in parallel. Current does not flow in the two diodes at the same time.

 

[John1397]

That's the way the designers built it works good again.

 

[Baluncore]

That's the way the designers built it works good again.

How come all six diodes are pressed into a heatsink with “case is ground” ? I would expect one heatsink plate to be the negative, the other the positive output. How do you know it will work at full current on a hot day? Do the replacement diodes have three times the required or original diode specifications?

Some old inefficient rectifier diodes had significant internal silicon resistance which at high currents dominated the voltage drop. That silicon resistance has a positive temperature coefficient that countered the negative tempco of the PN junction. Alternator rectifiers benefited from forced air cooling when operating.

Modern diodes do not have such high internal resistance and so will not balance current in parallel.

 

[Rive]

I was told once that two 1 amp tv type diodes parallel will not work because one will always carry the load,

It is called 'current hogging' (google it). It is a common problem for any semiconductor devices with negative temperature coefficient (transistors, diodes, FETs, MOSFETs and such) connected parallel.
It is cured by adding resistors series of each diodes.
Resistor might not be needed if the internal resistance of the diode or the resistance of the attached components is high enough.

Old electronics often has some nasty special tricks in them based on outdated component parameter ranges.