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Parallel Diodes Voltage Drop

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cduma

Thu Jul 28 2011, 02:21PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Im doing a high current 12V project and the diodes I am finding have way too high of a voltage drop to be usefull. If I put the diodes in parallel will this half the voltage drop? On a seperate note, would they be able to share the current?

Proud Mary

Thu Jul 28 2011, 02:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

cduma wrote ...

Im doing a high current 12V project and the diodes I am finding have way too high of a voltage drop to be usefull. If I put the diodes in parallel will this half the voltage drop? On a seperate note, would they be able to share the current?

So called 'current hogging' by one diode often occurs when two or more diodes are connected in parallel. If you connect a resistor in series with each diode, current hogging can be significantly reduced. The value of these series resistors must be calculated to meet the needs of your particular application in terms of current to be passed, acceptable voltage drop, heat to be dissipated, and so on.

Ash Small

Thu Jul 28 2011, 02:56PM

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

I have found that it is often possible to not use resistors. While current hogging does occur, they tend to share the current more as one heats up, however, there are no guarantees.

If you do try this without resistors, make sure all the diodes are identical and from the same batch.

Dr. Dark Current

Thu Jul 28 2011, 03:08PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yes the voltage drop decreases, however it probably wont be significant (depends on circuit details). If you put diodes from the same batch on the same heatsink, they will share current well.

Option #2: If the voltage is not too high (below 100-200 volts), consider using Schottky rectifiers.

cduma

Thu Jul 28 2011, 05:21PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Its 12V at 50A+. The purpose is to use a PSU to keep a charge on a car battery that is powering a stereo.

Proud Mary

Thu Jul 28 2011, 05:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

cduma wrote ...

Its 12V at 50A+. The purpose is to use a PSU to keep a charge on a car battery that is powering a stereo.

Why not buy something like this 100V 50A bridge rectifier, which is not expensive, and would save you from a great deal of needless labour.

Sulaiman

Thu Jul 28 2011, 06:08PM

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

Similar diodes on a common heatsink can be paralleled for higher current because as the datasheet graphs show the I vs V being close to the 'Ideal' diode at low currents but essentially resistive above that.
The intrinsic resistance of rectifier diodes is usually enough for the diodes to 'share' current fairly well.

Your amplifier may be rated at 50A, implying 600W dc input but;
Audio programme material usually has Peak:Average power of 20dB
Wikipedia

So a more modest (1A to 5A) battery charger would probably be acceptable.

The maximum output of automotive audio PAs are usually specified at Vbatt = 13.8
..... alternator output voltage / battery charging voltage.

Unless your power supply can give 13.8 Vdc at full current
(the battery gives nothing above 13V, thus is redundant)
you may not get full rated power output.

So, I think you need either;

A 13.8 V 50A power supply for full power rating
....(no need for a battery)
Or
A 1Adc to 5Adc, 12V battery charger with battery.

Ash Small

Thu Jul 28 2011, 06:11PM

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

Proud Mary wrote ...

Why not buy something like this 100V 50A bridge rectifier, which is not expensive, and would save you from a great deal of needless labour.

That will need a pretty big heatsink if you push 50 Amps through it, let alone 50+ Amps.

cduma

Thu Jul 28 2011, 08:22PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

Maybe I am going at this the wrong way. I would only need the diode to prevent the battery from backfeeding into the powersupply. Is there another route I could take?

Dr. Dark Current

Thu Jul 28 2011, 09:32PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

If your power supply has a rectifier on the output, there is no need to add another diode. Just make sure not to exceed the rating of the output diodes and capacitor(s). (That is, assuming, that it's a conventional power supply with laminated transformer, not a SMPS)

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