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diodes

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j taylor

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can someone tell me how to parallel diodes and also can a full wave bridge rectifier be used as a half wave bridge rectifier and if so how to wire it
thanks
 
Hi,

Just a quick note here...

When you parallel diodes you dont always get half the current flowing
through each diode. This is because both diodes are not always
exactly the same as they are in circuit simulator programs.

For example, say we have two 5 amp diodes and we connect them in
parallel. Can we then pass 10 amps through the two diodes? The
answer to this is a resounding, "No", simply because the two diodes
can not be assumed to be exactly the same and so one diode will
pass a higher current than the other. For that example, one diode
might draw 7 amps and the other only 3, and that would burn up
one of the diodes.

We could probably assume a 1 to 2 ratio, where worst case one
diode will pass two times the current of the other. So for those
two diodes rated at 5 amps each, we might get away with
passing a TOTAL of 7.5 amps (one diode carries 5 while the other
carries 2.5), but certainly not 10 amps total.

The rule of thumb then would be:

Itotal=1.5*Id

where
Id is the current rating of one diode (in parallel with another diode of the same part
number).

For three diodes in parallel we could probably assume another 1/2 diode rating
added to the total current, or

Itotal=2*Id

but remember this is for THREE diodes in parallel.


I prefer to use one diode though with the proper rating instead of doing these
parallel connections, which just adds unneeded complexity.


As to the bridge rectifier question, i think you might have wanted to connect
a full wave bridge rectifier up as a half wave rectifier, but this doesnt work
at all unless you dont mind two diode drops in your half wave circuit, in which
case you can connect the AC to the (-) terminal and take the DC from the (+)
terminal. Again, note that there will be two diode drops instead of one so
more voltage lost then when using a single diode.
 
can someone tell me how to parallel diodes
To parallel diodes you need to put a resistor in series with each diode to balance the current between them. The problem with paralleling diodes is that if one draws more current than the other it gets hotter. The hotter the diode gets, the lower it's forward voltage gets, which causes it to pass more current, which makes it hotter. See the problem? Resistors help minimize this issue, but it is better just to use the proper sized diode.
and also can a full wave bridge rectifier be used as a half wave bridge rectifier and if so how to wire it
You could use just one diode in the bridge package. ie: connect the ~ terminals together and also to one lead of the transformer and use the + terminal as the output. You would leave the other terminal (-) on the bridge disconnected. Instead of wasting the other diodes in the bridge, either use it as a bridge or use a single discrete diode for your halfwave rectifier.
 
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You could also select diodes to get a better Forward Volt match & thermally couple them. (dual diodes are usually pretty well matched in both respects)
Some parasitic resistance (leads + tracks + wiring) will help.

In pulsed applications paralleling diodes works well due to the Vf/I characteristics (more current = more Vf)
I see this typically in switchmode supplies (schottky) & HF welders, both are pulse rectifier application.

tgau
 
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