Paralleling Schottky Rectifiers

Parallel some 3A diodes? I've often paralleled them, seems a common thing to do, in that I see it often in stuff I dismantle.

Another reason for paralleling is to get the smaller on voltage.

Grant.

For 4A dc what's wrong with anyone of the several DPAK or D2PAK.

Here's just one shows less then 2W dissipation for 4ADC.

If you want Vishays got some new SCHOTTKY's in a new package 5A devices.

You would defiantly need to use two of them.

Its not a very high current - hard to justify the cost unless the size and efficiency are important issues in the application. A 4A active rectifier could be as small as an SO8 - controllers/drivers are commercially available.

RL

snipped-for-privacy@yahoo.com schrieb:

Hello,

they are within the same case, therefore they should have nearly the same temperature.

I saw a high current rectifier built with a lot of parallel diodes mounted on an large heatsink. Series resistors for equal current distribution might be necessary.

Bye

:

Thanks, that's not bad. Better than the parts I'd looked at.

It seems, basically, I can do it with a schottky for about 2W. I can get about 50=BAC/W with 200mm^2 copper, for Tj=3DTa+100=BAC, or about 150= =BAC in my case. Tj(max) =3D 150-175=BAC, depending on the device.

The Vishay 8TQ080 (8A 80V) device above shows an "average" power loss of 1.6-1.7W @ 4A (Fig. 6), giving a rise of 85=BAC and Tj=3D135=BAC. That'= s better, but those parameters aren't guaranteed, so I can't rely on them.

So, it works, but it bugs me to run dice that hot.

This time I had to grunt out something that worked on the spot, so I went with a P-FET, worst-case dissipation =3D 1.2W, worst-case rise =3D

60=BAC, Tj=3D110=BAC.

But, you gave me food for thought for sure. Thanks.

-- Cheers, James Arthur

Size is limited, so that's a driver.

As for a 4A active rectifier--that's more expensive than a plain FET, right?

-- Cheers, James Arthur

I've seen ballasting resistors in a few cases. I'm trying to save heat here though--everything's tiny and jammed together.

In the old days I'd just throw a TO-220 on a heatsink and not give it a second thought.

(An early PC I worked on circa 1978 had a 5V 35A linear supply, uA723

• 2n3055's (darlington'd, IIRC).)

-- Cheers, James Arthur

I once spent about $60,000.00 of a client's money trying to develop a compact switcher for wheelchair chargers. The project died because I don't know how to design a switcher; I was going by app notes and telephone advice (this was in the early 1990's before 600W PC supplies came down to $29 bucks.)

Anyway, one problem I _didn't_ have was with parallel diodes - they weren't Schottkys (Schottkies?) but "Ultrafast recovery" diodes, that came two in a package - I presume that being in the same package helped thermally stabilize them to each other.

Thanks, Rich

e:

thing

t

Man, I wish I could've helped you. That would've been fun. I'd done several switchers by then.

e

Same package helps, close matching helps too. If they're on the same chip you can do magic but that's Jim's trade, not ours.

-- Cheers, James Arthur

don't

to

Well gee, you could've at least tried something. Burning $60,000 of = transistors is better than "what'd you spend it on? Uh, nothing".

I'm proud to say, in about five years of building my own switching = supplies, I have yet to fill even a small bucket with all the = transistors I've burned, and I've hardly spent $1000 doing so. = Resources are definietly better today, though my equipment's the same. = Actually you probably had better equipment, all I have is a rusty Tek =

475.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

negative temperature coef, ? bad news as one will hog current. Same case helps a lot you can balance it out somewhat with a low series (each diode) resistance. SiC has a positive temp coef, which is great!

Don't you guys open any mass produced gear to see what they do?

Paralleling Schottky diodes is common practice, the technique works, even when they're not in the same package, but roughly run the same temperature, bolted to the same heatsink, or next to each other.

I suggest you look at how steep the forward voltage vs current is, compared to thermal effects.

Grant.

Did you consider the new SiC Schottky diodes from Infineon?

SiC diodes are relatively new technology, so there's no second source (AFAIK) and you get all the other wonderful benefits of being on the leading edge. ;-)

The IDT05S60CXK can handle 5A continuous (42 A surge) with a maximum PIV of 600V and maximum power dissipation of 55 watts. This is one of the smaller models -- there are models that can handle up to 16 amps of continuous current.

That's in a TO-220 package, not surface mount. There are a couple models in surface mount, but the rating is 3 or 4 amps. One of those might work for you if you can relax your spec.

Jay Ts

Why would you spend almost $10.00 on a diode optimised for hard switching to use it in a DC application?

He said DC. Before you dump that on diode you might as well get a PFET switch it from an open collector( basic ) or use an integrated load switch controller still cheaper then any SIC.

There are sevaral DPAK and d2pak Schottky's some with dual rectifiers that would dissapate less then 2W.

Shottky's are optimised either for low leakage or low Vf or compromise in between. For DC leakage is irelavant. Search for the lowest Vf DPAK device in 8A-12A and you could probably find one that dissapates about

1.5W at 4Adc. A dpak D2pak can easily dissapate 1.5 to 2W with modest copper heatsink. >

Some interesting reading on max junction limits and mechanisms for thermal runaway.

Tj MAX LIMIT OF SCHOTTKY DIODES

THE THERMAL RUNAWAY LAW IN SCHOTTKY USED IN OR-ing APPLICATION

The Infineon IDT05S60CXK I referred to is just $2.51 in single quantities at Mouser, not $10.

However, it was my mistake that I did not realize that there were Si Schottky diodes that can handle 5 amps in a single configuration, and cost much less. Oops. ;-)

Jay Ts

Price isn't the only reason why a SIC is a bad choice for a dc APP. They have Vf of 1.2 to 1.6v with a positive tempco.Which when compared to a 8-12A Si Shottkey running at 4Adc with a Vf of 0.395 to 0.45V at a working junction temp you would easily have three to four times the losses with a SIC.

SIC's were made to reduce losses in the switching mosfet mainly by eliminating Reverse recovery which can cause ringing and the FET also has to eat the reverse recovery charge every switching transition.In high power app's this can contribute significantly to switching looses in the FET.

They are really only beneficial when switching losses can be reduced enough to cancel out the higher conduction losses that a SIC has which is when you get up into higher power 1kW or so. They can also reduce ringing EMI/rfi which is another benefit which may justify there use at lower power levels.