Paralleling Schottky Rectifiers
in [Design]
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From: Grant on 24 Jul 2010 02:15 On Fri, 23 Jul 2010 22:46:39 -0500, "John" <nospam(a)nospam.com> wrote: > ><dagmargoodboat(a)yahoo.com> wrote in message >news:f0c2d629-45b7-4331-ae81-03abd4134eb3(a)w31g2000yqb.googlegroups.com... >>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). >> > >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.
From: Jay Ts on 24 Jul 2010 04:08 dagmargoodboat 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. 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
From: Hammy on 24 Jul 2010 07:34 On 24 Jul 2010 08:08:07 GMT, Jay Ts <bitbucket(a)example.com> wrote: >dagmargoodboat 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. > >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. >
From: Hammy on 24 Jul 2010 08:11 On Sat, 24 Jul 2010 07:34:39 -0400, Hammy <spam(a)spam.com> wrote: >> > >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 http://www.st.com/stonline/products/literature/an/4940.pdf THE THERMAL RUNAWAY LAW IN SCHOTTKY USED IN OR-ing APPLICATION http://www.st.com/stonline/products/literature/an/8825.pdf
From: Jay Ts on 24 Jul 2010 09:01
Hammy wrote: > Jay Ts <bitbucket(a)example.com> wrote: >>dagmargoodboat 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. >> >>Did you consider the new SiC Schottky diodes from Infineon? >>[...] >> >>The IDT05S60CXK can handle 5A continuous (42 A surge) with a maximum PIV >>of 600V and maximum power dissipation of 55 watts. [...] >> > Why would you spend almost $10.00 on a diode optimised for hard > switching to use it in a DC 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 |