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'[EE:] Current sharing resistors'
2004\06\24@175616 by steve

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Without going down the whole reverse engineering, desgn calculation
path, does someone have a simple, rule of thumb value for emitter
resistors to promote current sharing in paralleled bipolar transistors ?

Now that I actually need 5A out of my 5A bench supply (Wonix YW-
APS305 from Dick Smith for those who may also have one), I find that
they have 2 2N3055's in parallel (all three leads) and invariably, one of
them cooks and dies. Pity they put thermal sensing on the other one.

My immediate problem has been solved with the lead acid capacitor,
but it would be nice to eliminate the problem for next time. Any top of
the head values ?

Thanks,

Steve.

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2004\06\24@180857 by Spehro Pefhany

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At 09:55 AM 6/25/2004 +1200, you wrote:
>Without going down the whole reverse engineering, desgn calculation
>path, does someone have a simple, rule of thumb value for emitter
>resistors to promote current sharing in paralleled bipolar transistors ?
>
>Now that I actually need 5A out of my 5A bench supply (Wonix YW-
>APS305 from Dick Smith for those who may also have one), I find that
>they have 2 2N3055's in parallel (all three leads) and invariably, one of
>them cooks and dies. Pity they put thermal sensing on the other one.
>
>My immediate problem has been solved with the lead acid capacitor,
>but it would be nice to eliminate the problem for next time. Any top of
>the head values ?
>
>Thanks,
>
>Steve.

Try about 100mV across each ballast at full current. If that's at 2.5A
each, then about 0.04 ohms  at 1/4W (use 1/2W or better rated, non inductive).

For example, these are about $1.50 each US in one-off:
http://www.ohmite.com/catalog/pdf/10_series.pdf

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
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2004\06\24@180858 by Bob Blick

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> Without going down the whole reverse engineering, desgn calculation
> path, does someone have a simple, rule of thumb value for emitter
> resistors to promote current sharing in paralleled bipolar transistors ?

Hi Steve!

How about .1 to .5 ohms?

Basically, anything will work. The more resistance, the closer the current
will be.

Assuming -2.2 mV/degC, .1 ohm, 1 amp difference between the two
transistors, the two transistors, that's over 40 degC difference in their
junction temperatures.

Hopefully the transistors share the same heatsink! Then the transistors
will tend to track each other.

Cheerful regards,

Bob

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2004\06\24@181313 by Spehro Pefhany

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At 03:09 PM 6/24/2004 -0700, you wrote


>Hopefully the transistors share the same heatsink! Then the transistors
>will tend to track each other.

Yes, I just noticed I neglected to mention this...

MOSFETs have a positive tempco of Rds(on). You want them on seperate
heatsinks in order to share current best.

BJTs have a negative tempco of Vbe. You want them on the SAME heatsink.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\06\25@072750 by At van Wijk

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> Without going down the whole reverse engineering, desgn calculation
> path, does someone have a simple, rule of thumb value for emitter
> resistors to promote current sharing in parallel'ed bipolar transistors ?

> Now that I actually need 5A  .. //..    have 2 2N3055's in parallel ..

Hello Steve,

Take 1 ohm 5Watt resistors. That was always standard practice for me.
You can find resistive wire in heating elements or just take thin copper
wire (of the right length) if you don't have that kind of resistors.

At van Wijk.

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2004\06\25@075117 by hael Rigby-Jones

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{Quote hidden}

1 Ohm is way too high for high curent applications.  That's a 2.5volt drop
at maximum load in the OP application which could seriously eat into your
voltage overhead.  The resistors would also dissipate 6.25Watts each, so
would have to be 10Watt parts at a minimum.  0.047 Ohm would be my first
choice, at a push 0.1 Ohm as I'm more likely to have them in the junkbox.

Regards

Mike




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2004\06\25@081854 by Russell McMahon

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> Without going down the whole reverse engineering, desgn calculation
> path, does someone have a simple, rule of thumb value for emitter
> resistors to promote current sharing in paralleled bipolar transistors ?
>
> Now that I actually need 5A out of my 5A bench supply (Wonix YW-
> APS305 from Dick Smith for those who may also have one), I find that
> they have 2 2N3055's in parallel (all three leads) and invariably, one of
> them cooks and dies. Pity they put thermal sensing on the other one.


I hope that's not the DSE Q1760, 30v, 5A supply that I just bought :-)
CLAIMS to do 5A "at all selectable output voltages".

A bipolar transistor has ABOUT 0.6v Vbe when driven and more like 0.7V on a
power device under heavy drive.
If you cause 0.1v emitter drop at full current that would start to haven a
significant affect and 0.2v more so.

So if it's 5A/2 = 2.5A each then R = 0.2/2.5 = 0r8 per transistor should be
about right.
Either 0r68 or 1r0 are probably available at DSE.
Power = 2.5 x 0.2 = 0.5w so a 1w would do and 1 5w cheapo wire wound would
run cool. .

1r0 5w    R1602        in stock everywhere
0r47        R1601

No 0r68s


The 3055's dying at 2.5A is pretty unusual.
With proper heatsinking they take vast watts.
At say 30v max in and 2.5A that's 75W and non shared it's more. Even a TO3
with a good heatsink would be pressed at that level.



       Russell McMahon

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2004\06\25@082521 by hael Rigby-Jones

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Think you'd better recheck the calculations! :o)

0.2/2.5 = 0.08 Ohms

A 1R0 resistor would be dissipating 6.25 Watts!

Regards




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2004\06\25@114202 by Russell McMahon

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> Think you'd better recheck the calculations! :o)
>
> 0.2/2.5 = 0.08 Ohms
>
> A 1R0 resistor would be dissipating 6.25 Watts!

Hmmm - can I plead 1am ? :-)
I can see how I made the mistake but I'm annoyed that I did.
If I had used I^2R for my power (as I almost did) rather than VI it would
have been clear. So close :-(

OK  DSE 0r1 5W  = R1600
Also in stock everywhere.


http://www.dse.co.nz/cgi-bin/dse.storefront/40dc24ed018535d8273fc0a87f99070f/Product/View/R1600



       RM

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