Mosfet switching times ?
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From: Hammy on 10 Jul 2010 14:55 On Sat, 10 Jul 2010 11:14:53 -0700, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >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 Thanks I never found that one. >... 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. I've never heard it refereed to as intrinsic to the device specifically in a data sheet. The ST device shows the test set-up and Rg is shown external. To add to the confusion this AN from TI says pretty much that the intrinsic Rg is has negligible impact on switching speeds except at RF? see pg4 http://focus.ti.com/lit/ml/slup169/slup169.pdf >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. That is the other confusing thing . Given how they are so loose with high current ratings. >Jon
From: Tim Williams on 10 Jul 2010 15:40 "Hammy" <spam(a)spam.com> wrote in message news:l2ah36l4s8ntgjdg8o8njolk7hqfd5rlsc(a)4ax.com... > 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. This is true for BJTs, which tend to have switching times in the ballpark of the device, regardless of how you drive it. This comes from base transit time and stored charge effects. Base spreading resistance is generally negligible, I guess. > I've always just used Qg and my drivers sink/source ability to > estimate times. For FETs, this is correct. There is a gate spreading resistance, and it's occasionally specified, usually in the 1 ohm range. This suggests that you can't really make it go faster than a 1 ohm Rg equivalent. Now, you could drive it with a precompensated waveform, so the current overshoots to kick the RC around, but now you're getting into territory where you need serious power input for marginal output power and efficiency. For most power transistors, I would guess this is in the low VHF range. I made a 1MHz generator with TC4420's, FDP26N40's and 1 ohm gate resistors. The '4420s actually get hot at this frequency! Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Joerg on 10 Jul 2010 15:51 Hammy 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. > No, not missing much. Switch times aren't very meaningful in those datasheets. > I've always just used Qg and my drivers sink/source ability to > estimate times. > Most people put gate resistors in because they are afraid to blow the EMC cert or that it could oscillate. Which spoils the power of the nice PWM chip in front of it, like driving a Porsche at 35mph. I tend to drive them hard but there's always the internal gate path resistance which you cannot overcome. So I tend to drive them with lots of gusto, preferably well north of 10V which helps in that domain. It also helps to drive the gate negative a few volts, -5V or so. That speeds up turn-off. > >> 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. ;-) > John likes to floor it, wonder how long his new Audi will last :-) [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jon Kirwan on 10 Jul 2010 18:53 On Sat, 10 Jul 2010 14:55:05 -0400, Hammy <spam(a)spam.com> wrote: >On Sat, 10 Jul 2010 11:14:53 -0700, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>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 > >Thanks I never found that one. It's one of a few I've read, with what understanding I may, that seem to say similar things to me. >>... 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. > >I've never heard it refereed to as intrinsic to the device >specifically in a data sheet. I'm sorry. I was thinking about Spice models when I wrote that. I believe, now that you bring it up, that I've seen it on some datasheets. But that wasn't on my mind at the time I wrote. >The ST device shows the test set-up and Rg is shown external. Yes. >To add to the confusion this AN from TI says pretty much that the >intrinsic Rg is has negligible impact on switching speeds except at >RF? > > see pg4 > >http://focus.ti.com/lit/ml/slup169/slup169.pdf ><snip> Since the RC ladder is low-pass, that makes some sense to me on first blush. The areas further away from the bond point would never reach sufficient voltage, so I think that would be a real problem, as frequencies rise (no firm turn off or turn on, just finding a quiescent point and sitting there neither on nor off.) But I haven't given this much thought. But your phrasing of TI's apnote comment seems reasonable to me and not conflicting. There is still an Rg to the device. It's just that, because of the RC ladder, at high frequencies there is huge problem caused by the inability to switch the whole gate area around that fast. But I have only just now given this two minutes thought. TI's apnote is probably correct. If Rg were important, it would probably be on all of the datasheets rather than a few. It's not always there, though. (It obviously _is_ important for the Spice models, though.) The gate-source charge, the Miller charge, and overdrive charges are important (overdrive extracted by subtracting the other two from the total charge, I gather.) So they are spec'd. Jon
From: ian field on 11 Jul 2010 12:09 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:0ibh365789vlqn1hqm31m8clbobvv1v2kp(a)4ax.com... > 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. Recently I was asking in various groups what frequency people thought I might shove through a 2N7000 in grounded gate - some people thought the limit might be around 100MHz. Do you have an estimate on this?
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