Paralleling Schottky Rectifiers
in [Design]
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From: dagmargoodboat on 22 Jul 2010 13:28 I never parallel rectifiers -- that's asking for thermal runaway. But, here, OnSemi says it's cool, even necessary to get full rating: http://www.onsemi.com/PowerSolutions/product.do?id=MBR20200CTG I guess those are matched with matched drops, so runaway won't be a problem. Hmm. I need a >5A 40V diode (to carry 4A d.c.), preferably surface mount. OTOH, that's gonna dissipate ~3W. Ouch. So it has to have a meaty package, and it needs some heat-spreading foil. Yuck. Might just use a FET instead, if permitted (it's not my call). -- Cheers, James Arthur
From: Grant on 22 Jul 2010 18:35 On Thu, 22 Jul 2010 10:28:09 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: >I never parallel rectifiers -- that's asking for thermal runaway. >But, here, OnSemi says it's cool, even necessary to get full rating: > >http://www.onsemi.com/PowerSolutions/product.do?id=MBR20200CTG > >I guess those are matched with matched drops, so runaway won't be a >problem. Hmm. > >I need a >5A 40V diode (to carry 4A d.c.), preferably surface mount. 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. > >OTOH, that's gonna dissipate ~3W. Ouch. So it has to have a meaty >package, and it needs some heat-spreading foil. Yuck. Another reason for paralleling is to get the smaller on voltage. > >Might just use a FET instead, if permitted (it's not my call). Grant.
From: Hammy on 22 Jul 2010 19:24 >On Thu, 22 Jul 2010 10:28:09 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: > >>I never parallel rectifiers -- that's asking for thermal runaway. >>But, here, OnSemi says it's cool, even necessary to get full rating: >> >>http://www.onsemi.com/PowerSolutions/product.do?id=MBR20200CTG >> >>I guess those are matched with matched drops, so runaway won't be a >>problem. Hmm. >> >>I need a >5A 40V diode (to carry 4A d.c.), preferably surface mount. > >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. >> >>OTOH, that's gonna dissipate ~3W. Ouch. So it has to have a meaty >>package, and it needs some heat-spreading foil. Yuck. > >Another reason for paralleling is to get the smaller on voltage. >> >>Might just use a FET instead, if permitted (it's not my call). > >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. http://www.vishay.com/docs/94266/94266.pdf If you want Vishays got some new SCHOTTKY's in a new package 5A devices. http://www.vishay.com/docs/88988/ss5p5.pdf You would defiantly need to use two of them.
From: Uwe Hercksen on 23 Jul 2010 04:21 dagmargoodboat(a)yahoo.com schrieb: > I never parallel rectifiers -- that's asking for thermal runaway. > But, here, OnSemi says it's cool, even necessary to get full rating: > > http://www.onsemi.com/PowerSolutions/product.do?id=MBR20200CTG > > I guess those are matched with matched drops, so runaway won't be a > problem. Hmm. 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
From: dagmargoodboat on 23 Jul 2010 13:13
On Jul 22, 6:24 pm, Hammy <s...(a)spam.com> wrote: > >On Thu, 22 Jul 2010 10:28:09 -0700 (PDT), dagmargoodb...(a)yahoo.com wrote: > > >>I never parallel rectifiers -- that's asking for thermal runaway. > >>But, here, OnSemi says it's cool, even necessary to get full rating: > > >>http://www.onsemi.com/PowerSolutions/product.do?id=MBR20200CTG > > >>I guess those are matched with matched drops, so runaway won't be a > >>problem. Hmm. > > >>I need a >5A 40V diode (to carry 4A d.c.), preferably surface mount. > > >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. > > >>OTOH, that's gonna dissipate ~3W. Ouch. So it has to have a meaty > >>package, and it needs some heat-spreading foil. Yuck. > > >Another reason for paralleling is to get the smaller on voltage. > > >>Might just use a FET instead, if permitted (it's not my call). > > >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. > > http://www.vishay.com/docs/94266/94266.pdf 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 50ºC/W with 200mm^2 copper, for Tj=Ta+100ºC, or about 150ºC in my case. Tj(max) = 150-175ºC, 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 85ºC and Tj=135ºC. 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 = 1.2W, worst-case rise = 60ºC, Tj=110ºC. But, you gave me food for thought for sure. Thanks. -- Cheers, James Arthur |