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Schottkys in parallel

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IamSmooth

Mon Jan 18 2010, 09:04PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

if I have one schottky diode that can handle 30A, can I put two in parallel for 60A? These are ultra-fast for blocking the body diode of a mosfet. I just need to know so I can order some; otherwise, I need a bigger, single diode.

Zeus

Mon Jan 18 2010, 09:40PM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

Just remember what happens to capacitance in parallel. If the circuit is capacitance
critical I wouldn't advise you to.

[Edit: typo]

101111

Mon Jan 18 2010, 11:15PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

It works, go for it.
Just make sure the wirering give same amount of current to both devices.

Myke

Mon Jan 18 2010, 11:30PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I don't know how good the current sharing is on schottky diodes but I would still derate the parallel diodes so that one doesn't see 40A while the other only sees 20A.

You could test out how well they share current by measuring the current through both diodes at the same time. I'm not sure how much voltage a single meter would drop across it though.

radiotech

Mon Jan 18 2010, 11:57PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I you laid out the pcb with the diodes spaced apart lengthwise on a pair of traces and connected your object circuit to a point centered between them you might be able to get load sharing by shifting the
center between them while watching on a scope. Consider a (microstripline as well as) series resistance balast that is used to
force parallel stuff with nonlinear e/i slopes to share.

MinorityCarrier

Tue Jan 19 2010, 12:39AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

I believe silicon tempco makes paralleling silicon diodes not as effective as you'd like. One diode is almost certain to conduct more than the other.

Be sure the reverse breakdown voltage of your schottkeys is high enough.

IamSmooth

Tue Jan 19 2010, 01:04AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I think I will go with Mouser's 844-63CPT100

It is 60A/100v reverse

I will incorporate this into my design unless someone thinks this one is not appropriate.

Andri

Tue Jan 19 2010, 02:01AM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

PN-junctions such as diodes have a negative temperature coefficient. I.e. conductivity increases as current rises. This may lead to one diode conducting considerably more current than the other just because it was warmer to begin with. You could think of this as positive feedback.

Most devices (such as resistors) have PTC so the current share is regulated by resistance increasing with increased current, thus shifting the current over to the cooler resistor, ad infinitum. You can think of this as negative feedback.

It may well be that the diodes in question have enough series resistance to counter the effect, so that the PTC overcomes the NTC but you're better safe than sorry.

hboy007

Tue Jan 19 2010, 12:00PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I've paralleled two 40V schottky diodes when I modified an ATX power supply for 0.8 .. 5V @ 50A output. It was just a quick&dirty mod and I didn't consider resistive or inductive balancing because there was just the place for one more diode in the layout (maybe they mount these for higher power though). They are to some extent balanced by their logarithmic forward voltage drop but this requires pairs that are well matched over the given temperature range. I guess I was just lucky. Maybe the small heat sink helped because the diodes were located next to each other and heated at similar rates.

If the diodes are used in a switching topology, the forward voltage drop characteristics are even more pronounced due to the higher current pulses. Resistive balancing would introduce further losses but you could try a bifilar wound choke with a few 100nH to ÂµH (frequency dependent, of course) to couple the currents.

Steve Conner

Tue Jan 19 2010, 12:12PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As I said in Iamsmooth's other thread (wish he wouldn't keep starting new ones at random) many of these dual schottky diodes are rated for parallel operation. The datasheet usually says "suitable for parallel operation."

The tempco is negative, sure, but the two diode dice are soldered to the same tab, keeping them at nearly the same temperature.

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