Forums

Rectifier Schottkies in parallel

Started by Piotr Wyderski October 27, 2014
Hello,

many Schottky rectifier diodes contain two diodes within a single
package, usually with the cathodes connected. Does it make sense
to connect the anodes too in order to get a diode which can withstand
higher current? Is a 2x30A diode more or less equivalent to a single
60A diode? I am thinking of VBT6045CBP if it helps.

The application: a high-current 50Hz rectifier.

    Best regards, Piotr
On Monday, October 27, 2014 2:19:23 PM UTC-4, Piotr Wyderski wrote:
> Hello, > > many Schottky rectifier diodes contain two diodes within a single > package, usually with the cathodes connected. Does it make sense > to connect the anodes too in order to get a diode which can withstand > higher current? Is a 2x30A diode more or less equivalent to a single > 60A diode? I am thinking of VBT6045CBP if it helps. > > The application: a high-current 50Hz rectifier. > > Best regards, Piotr
Is there any way of knowing if the two diodes are on the same piece of silicon? (That would help for thermal/ current balancing.) George H.
On a sunny day (Mon, 27 Oct 2014 11:54:43 -0700 (PDT)) it happened George
Herold <gherold@teachspin.com> wrote in
<69611644-5808-454b-8d88-29402a53ff54@googlegroups.com>:

>On Monday, October 27, 2014 2:19:23 PM UTC-4, Piotr Wyderski wrote: >> Hello, >> >> many Schottky rectifier diodes contain two diodes within a single >> package, usually with the cathodes connected. Does it make sense >> to connect the anodes too in order to get a diode which can withstand >> higher current? Is a 2x30A diode more or less equivalent to a single >> 60A diode? I am thinking of VBT6045CBP if it helps. >> >> The application: a high-current 50Hz rectifier. >> >> Best regards, Piotr > >Is there any way of knowing if the two diodes are on the same piece of silicon? (That would help for thermal/ current >balancing.) > >George H.
x-ray
On Mon, 27 Oct 2014 19:19:18 +0100, Piotr Wyderski wrote:

> Hello, > > many Schottky rectifier diodes contain two diodes within a single > package, usually with the cathodes connected. Does it make sense to > connect the anodes too in order to get a diode which can withstand > higher current? Is a 2x30A diode more or less equivalent to a single 60A > diode? I am thinking of VBT6045CBP if it helps. > > The application: a high-current 50Hz rectifier. > > Best regards, Piotr
I don't know if this is true of Schottky diodes (I think it is). With ordinary junction diodes you can't parallel them because the temperature coefficient of the forward voltage is negative -- that means that whatever diode happens to be getting more current will get hotter, lower its forward drop, and get even MORE current, until sooner or later it pops. You can do it, but you have to ballast the diodes with series resistance, and work hard to thermal match the diodes, and generally do a whole bunch more messing around than you'd do just getting a bigger diode. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
I don't see why not.  Disclaimer: offset in Vf, TJ, etc. manifests as a 
somewhat constant offset in current.  So, if you derate the total by the 
worst case difference (maybe, say, 10A), you still get a beefy sum (50A) 
without worrying about breaking things.

Advice of this nature applies most importantly to separate diodes, BJTs or 
IGBTs in parallel.  Probably, co-pack devices are close enough not to 
mind, let alone monolithic pairs (if they are such).

Here's an example from my junk bin:
http://seventransistorlabs.com/Images/SR3540PT_1.jpg
SR3540PT

http://seventransistorlabs.com/Images/SR3540PT_2.jpg
Cracked open, two dies.  Placed pretty far apart, good to know for 
heatsink design.  Possibly done to minimize lead length?

http://seventransistorlabs.com/Images/SR3540PT_3.jpg
Breaking the lead off, it's apparent the bond wire isn't so much a wire as 
a strip (of thin copper, apparently).  This looks to be soldered, welded 
or brazed to the lead itself, and soldered to the die (or some other sort 
of die-attach).  The die to base plate bond doesn't look like much, but 
may be solder as well.  (The brittleness of the bond-to-lead joint 
probably suggests die-attach rather than metallurgical methods?)

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message 
news:m2m2b7$3tu$1@node1.news.atman.pl...
> Hello, > > many Schottky rectifier diodes contain two diodes within a single > package, usually with the cathodes connected. Does it make sense > to connect the anodes too in order to get a diode which can withstand > higher current? Is a 2x30A diode more or less equivalent to a single > 60A diode? I am thinking of VBT6045CBP if it helps. > > The application: a high-current 50Hz rectifier. > > Best regards, Piotr
On Monday, October 27, 2014 3:31:58 PM UTC-4, Tim Wescott wrote:
> On Mon, 27 Oct 2014 19:19:18 +0100, Piotr Wyderski wrote: > > > Hello, > > > > many Schottky rectifier diodes contain two diodes within a single > > package, usually with the cathodes connected. Does it make sense to > > connect the anodes too in order to get a diode which can withstand > > higher current? Is a 2x30A diode more or less equivalent to a single 60A > > diode? I am thinking of VBT6045CBP if it helps. > > > > The application: a high-current 50Hz rectifier. > > > > Best regards, Piotr > > I don't know if this is true of Schottky diodes (I think it is). With > ordinary junction diodes you can't parallel them because the temperature > coefficient of the forward voltage is negative -- that means that whatever > diode happens to be getting more current will get hotter, lower its > forward drop, and get even MORE current, until sooner or later it pops. > > You can do it, but you have to ballast the diodes with series resistance, > and work hard to thermal match the diodes, and generally do a whole bunch > more messing around than you'd do just getting a bigger diode.
You're basically correct, but the problem is mitigated in dual Schottky diodes by way of them being roughly isothermal--the hotter diode can't get that much hotter than the cooler diode, which limits the hogging. Depending on the operating point the diodes are somewhat resistive, which may help too. ISTR Motorola spec sheets for dual Schottkies rated some for 2x current if the diodes were paralleled, while other spec sheets had no such guarantee. YMMV. Cheers, James Arthur
On Monday, October 27, 2014 11:19:23 AM UTC-7, Piotr Wyderski wrote:
> Hello, > > many Schottky rectifier diodes contain two diodes within a single > package, usually with the cathodes connected. Does it make sense > to connect the anodes too in order to get a diode which can withstand > higher current?
It's done, a lot. And, it fails, a lot (in switching power supplies, this is a key check for shorts and failures). The old recommendation on high current paralleling of diodes is to put a little series inductance in each; this makes a current hog diode have a little less terminal voltage, and fights thermal imbalance. For this application, you do NOT want a lossy ferrite.
On Monday, October 27, 2014 5:20:05 PM UTC-4, whit3rd wrote:
> On Monday, October 27, 2014 11:19:23 AM UTC-7, Piotr Wyderski wrote: > > Hello, > > > > many Schottky rectifier diodes contain two diodes within a single > > package, usually with the cathodes connected. Does it make sense > > to connect the anodes too in order to get a diode which can withstand > > higher current? > > It's done, a lot. And, it fails, a lot (in switching power supplies, this > is a key check for shorts and failures). > The old recommendation on high current paralleling of diodes > is to put a little series inductance in each; this makes a current hog > diode have a little less terminal voltage, and fights thermal imbalance. > For this application, you do NOT want a lossy ferrite.
The OP's part is here: http://www.vishay.com/diodes/list/product-89374/ 30A @ Vf=0.54v (typ) is 18 milliohms, or 9 milliohms for two perfectly matched sections in parallel. Your inductor in series is a neat trick for SMPS. At 50Hz, I'd be tempted to make a MOSFET synchronous rectifier if ultimate low loss is important, or just parallel the Schottkies and add some small ballast resistors if he really wants 60A and loss doesn't matter. For 30A I'd parallel the two bare diodes and be glad for whatever heat-spreading it gave me for free. Cheers, James Arthur
On Mon, 27 Oct 2014 14:31:53 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Mon, 27 Oct 2014 19:19:18 +0100, Piotr Wyderski wrote: > >> Hello, >> >> many Schottky rectifier diodes contain two diodes within a single >> package, usually with the cathodes connected. Does it make sense to >> connect the anodes too in order to get a diode which can withstand >> higher current? Is a 2x30A diode more or less equivalent to a single 60A >> diode? I am thinking of VBT6045CBP if it helps. >> >> The application: a high-current 50Hz rectifier. >> >> Best regards, Piotr > >I don't know if this is true of Schottky diodes (I think it is). With >ordinary junction diodes you can't parallel them because the temperature >coefficient of the forward voltage is negative -- that means that whatever >diode happens to be getting more current will get hotter, lower its >forward drop, and get even MORE current, until sooner or later it pops.
That's qualitative. Just because there's some positive feedback doesn't mean that the system runs away.
> >You can do it, but you have to ballast the diodes with series resistance, >and work hard to thermal match the diodes, and generally do a whole bunch >more messing around than you'd do just getting a bigger diode.
As the current goes up, the ohmic loss inside the diode growns bigger relative to the ideal PN diode drop. The PN drop has a negative tempco but the ohmic drop tempco is positive. At some current, the TC is zero, and at higher currents it's positive. The crossover current is sometimes at a useful point, sometimes not, but the ohmic positive-TC effect mitigates the current-hogging effect. Some big potted diode assemblies are multiple chips in parallel. A single large diode will not be isothermal, so arguments about runaway can apply to regions of a single diode. I'd go for 50 amps for the case at hand. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Tim Wescott wrote:

> > I don't know if this is true of Schottky diodes (I think it is). With > ordinary junction diodes you can't parallel them because the temperature > coefficient of the forward voltage is negative
** Scottkys have a similar negative tempco, about -1.6mV/C. -- that means that whatever
> diode happens to be getting more current will get hotter, lower its > forward drop, and get even MORE current, until sooner or later it pops.
** You are merely theorising aren't you ? If you ever tried it you will find it works rather well.
> You can do it, but you have to ballast the diodes with series resistance,
** Nope. Just soldering the leads into the same PCB area or twisting them together provides all the thermal coupling needed. .... Phil