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Old 23rd August 2006, 07:37 AM   #1
jarthel is offline jarthel  Australia
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Default paralleling diodes?

situation:

I need a 100V 4A diodes to make a bridged diodes. But I only got 100V 2A ones.

would I get the same effect as a 100V 4A diode if I parallel 2x 100V 2A?

I was thinking along the lines of resistors. possibly incorrect thinking.

Thank you.

ps. no answer please of just to buy a 100V 4A. this thread's purpose is to gain some knowledge on the idea of paralleling diodes.
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Old 23rd August 2006, 09:54 AM   #2
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The problem with your solution is that the two parallel diodes will have a slightly different forward voltage. The one with the lowest forward voltage will conduct more than the other, so you may end up with one diode carrying 80% of the total current and the other 20%.

One solution is to use small series resistors for each diodes, like 0.25 ohms, which will add (at nominal 2A per diode) some 0.5 volts to each forward voltage, equalising the current division better (but still not 50-50), the disadvantage is a small loss of voltage (across the resistors) and some extra heat.

Since you told us not to mention the obvious solution, I won't do that .

Jan Didden
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Old 23rd August 2006, 10:56 AM   #3
jarthel is offline jarthel  Australia
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but if the diodes are of the same make/brand, why would it have different forward voltage?

ps. thank you for the explanation above
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Old 23rd August 2006, 11:01 AM   #4
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The advice here is: avoid paralleling diodes unless the are double diodes in the same package. Under those conditions the two diodes are at the same temperature and also rather equal.
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Old 23rd August 2006, 11:02 AM   #5
sreten is online now sreten  United Kingdom
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Quote:
Originally posted by janneman
The problem with your solution is that the two parallel diodes will have a slightly different forward voltage. The one with the lowest forward voltage will conduct more than the other, so you may end up with one diode carrying 80% of the total current and the other 20%.

One solution is to use small series resistors for each diodes, like 0.25 ohms, which will add (at nominal 2A per diode) some 0.5 volts to each forward voltage, equalising the current division better (but still not 50-50), the disadvantage is a small loss of voltage (across the resistors) and some extra heat.

Since you told us not to mention the obvious solution, I won't do that .

Jan Didden
Hi,

The voltage drop across rectifier diodes at peak current (~2V)
is higher than the forward voltage, the currents will equalise.

Due to the very high peak currents resistors in the bridge is not good.

The safest option is to parallel 3 diodes, why not ?

/sreten.
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Old 23rd August 2006, 11:05 AM   #6
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Quote:
Originally posted by sreten

why not ?
Highly positive thermal coefficient.
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Old 23rd August 2006, 11:06 AM   #7
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I could just say - "'cos they do", but I think you want a slightly more in depth explanation.

Every single component is made to a tolerance, and no two devices will ever be identical. If you look at some of the threads about matching transistors, you will see that you can narrow the selction down by using devices with the same date code, or from the same batch, but even so, that just reduces the amount of deviation, it doesn't eliminate it. As for it's importance, imagine a pencil, balanced on it's point. The slightest disturbance will cause the pencil to fall over, much as the slightest imbalance in the diodes will cause one to take more current and go bang
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Old 23rd August 2006, 11:35 AM   #8
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Quote:
Originally posted by sreten
[snip]The voltage drop across rectifier diodes at peak current (~2V)
is higher than the forward voltage, the currents will equalise.
[snip]/sreten.

The voltage drop IS the forward voltage. Whatever the currents, they will surely NOT be the same, unless you go through the trouble of carefull matching at the specified current. Even then, in actual use, the temp will be different and the current sharing will drift.

Jan Didden
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Old 23rd August 2006, 02:05 PM   #9
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It is very easy to parallel diodes. It requires:

1. VF matching at the highest current the die will see. They will have to be matched to within 1 millivolt, they will have to be tested at the same ambient temperature, within minutes of each. Drift in the test equipment, even at a 10 minute time scale, can throw the match off.

2. They have to be from the same diffusion run, and typically, from the same wafer. This guarantees that they will have the same VI curve, and that they will respond in the same manner to current over the range of temperatures you desire.

3. The diodes have to be in intimate thermal contact with each other. Any slight difference, and the hotter one will run away with the current. This stability of course, depends on the diode type, as some will have different bulk silicon resistivity depending on speed and process.

4. True paralleling requires absolutely no series resistance difference between the two, but this flies in the face of what is possible for diy'ers, as they cannot typically get same diffusion run or same wafer devices. Jan is correct in that a small series resistance will work.

5. Thermally, they have to have identical cooling parameters..It is possible to create an isothermal island to get around this to some extent, but that compromised device life.

It is possible to run a full range of tests to find diodes which can be paralleled, but that test is not very easy to do. I know, I've done it..in between dodging the velociraptors..

Cheers, John
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Old 23rd August 2006, 04:42 PM   #10
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Are you sure you need 4A IFAV? The diodes are each only on half the time.
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