Mosfet switching times ?
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From: Hammy on 10 Jul 2010 11:14 When Mosfet switching times are shown in the data sheets i.e ton,toff and tdelay. They specify an Rg value. Is this Rg value the driver resistance or the internal mosfet mesh resistance? The reason I'm asking is that every FET sheet uses a different value of Rg for testing. The ones with the lower Rg have the quickest switching times. For example two similar devices a Fairchild http://www.fairchildsemi.com/ds/FQ/FQP18N50V2.pdf And a ST device. http://www.st.com/stonline/products/literature/ds/9118/stw20nk50z.pdf The ST device uses an Rg of 4.7 oHms while the Fairchild device uses an Rg of 25oHms. The ST devices shows much faster transitions but has twice the gate charge. I would prefer to use the Fairchild device if I know that Rg is set by the driver I would know that the same speeds could be obtained as the ST device. Is this correct? If it is the driver resistance, why would they test with a high Rg ,it's not flattering to the mosfet?
From: John Larkin on 10 Jul 2010 12:28 On Sat, 10 Jul 2010 11:14:59 -0400, Hammy <spam(a)spam.com> wrote: >When Mosfet switching times are shown in the data sheets i.e ton,toff >and tdelay. They specify an Rg value. Is this Rg value the driver >resistance or the internal mosfet mesh resistance? It's the test generator impedance, usually. > >The reason I'm asking is that every FET sheet uses a different value >of Rg for testing. The ones with the lower Rg have the quickest >switching times. Makes sense. > >For example two similar devices a Fairchild > >http://www.fairchildsemi.com/ds/FQ/FQP18N50V2.pdf > >And a ST device. > >http://www.st.com/stonline/products/literature/ds/9118/stw20nk50z.pdf > >The ST device uses an Rg of 4.7 oHms while the Fairchild device uses >an Rg of 25oHms. > >The ST devices shows much faster transitions but has twice the gate >charge. I would prefer to use the Fairchild device if I know that Rg >is set by the driver I would know that the same speeds could be >obtained as the ST device. Is this correct? > >If it is the driver resistance, why would they test with a high Rg >,it's not flattering to the mosfet? Don't know. Most mosfets will switch much faster than their datasheets suggest, if you just drive them hard. The Fairchild BSS123 datasheet cites a typical turn-on rise time of 9 ns, and 17 for turnoff. They will actually do this: http://www.highlandtechnology.com/DSimages/T760%20wave-web.jpg That's a 100 volt pulse into 50 ohms, transformer isolated. Given the complications of driver and fet inductances and such, and the state of the datasheets, the best thing to do is experiment. If you want to go fast, you should probably stick to one manufacturer and not assume that identical part numbers are always interchangable. Buy a reel. John
From: Hammy on 10 Jul 2010 13:22 On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: [snip] > >Most mosfets will switch much faster than their datasheets suggest, if >you just drive them hard. The Fairchild BSS123 datasheet cites a >typical turn-on rise time of 9 ns, and 17 for turnoff. I never paid much attention too data sheet switching times until recently. Mainly from the emphasis some people here and in other forums place on them. So I was beginning to think I may be missing something, but I guess not. I've always just used Qg and my drivers sink/source ability to estimate times. >They will actually do this: > >http://www.highlandtechnology.com/DSimages/T760%20wave-web.jpg > >That's a 100 volt pulse into 50 ohms, transformer isolated. Your exceeding the 25C pulsed current rateing a tad. ;-) >Given the complications of driver and fet inductances and such, and >the state of the datasheets, the best thing to do is experiment. If >you want to go fast, you should probably stick to one manufacturer and >not assume that identical part numbers are always interchangable. Buy >a reel. > >John Yes your right I plan on experimenting with both and maybe a couple others I have in the applications.
From: John Larkin on 10 Jul 2010 13:34 On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <spam(a)spam.com> wrote: >On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >[snip] >> >>Most mosfets will switch much faster than their datasheets suggest, if >>you just drive them hard. The Fairchild BSS123 datasheet cites a >>typical turn-on rise time of 9 ns, and 17 for turnoff. > >I never paid much attention too data sheet switching times until >recently. Mainly from the emphasis some people here and in other >forums place on them. So I was beginning to think I may be missing >something, but I guess not. > >I've always just used Qg and my drivers sink/source ability to >estimate times. I generally assume that mosfet silicon is infinitely fast, and that only capacitances and wirebond inductances get in the way. Seems to work so far. > > >>They will actually do this: >> >>http://www.highlandtechnology.com/DSimages/T760%20wave-web.jpg >> >>That's a 100 volt pulse into 50 ohms, transformer isolated. > >Your exceeding the 25C pulsed current rateing a tad. ;-) Life in the fast lane! John
From: Jon Kirwan on 10 Jul 2010 14:14 On Sat, 10 Jul 2010 11:14:59 -0400, Hammy <spam(a)spam.com> wrote: >When Mosfet switching times are shown in the data sheets i.e ton,toff >and tdelay. They specify an Rg value. Is this Rg value the driver >resistance or the internal mosfet mesh resistance? I've taken it as a device parameter, pretty much as discussed here: http://www.fairchildsemi.com/an/AN/AN-7017.pdf .... except that Rg is a term applied elsewhere, too. For example, I've seen Rg_i used for the mosfet's Rg, then Rg used for an explicit/implicit series Rg that is external to the device, and then Rg_hi and Rg_lo also for the high side and low side equivalent driving resistance of the gate driver. So I think context is important. But on a datasheet, unless specified as part of the testing setup, I don't think those external values are included in the Rg value ascribed to the part, itself. Of course, I'm not an expert reader, either. But that's the impression I've taken. >snip> >If it is the driver resistance, why would they test with a high Rg >,it's not flattering to the mosfet? Few would intentionally shoot themselves in the foot. Jon
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