Learning Mosfets...RdsON Warm Up?
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From: Bill Sloman on 18 Dec 2009 03:05 On Dec 18, 5:03 am, D from BC <myrealaddr...(a)comic.com> wrote: > On Thu, 17 Dec 2009 15:37:53 -0800, Joerg <inva...(a)invalid.invalid> > wrote: > > >Take a look at figure 4: > > >http://www.irf.com/product-info/datasheets/data/irfs3806pbf.pdf > > Yup. Rdson is sensitive to temperature. > I'm just surprised to Rdson variation within a 100uS on time. > If that's what I'm seeing. > That or something goofy somewhere. The thermal mass of a drain channel isn't all that high. Eventually the heat distribution acros the transistor evolves into a nice smooth thermal gradient from channel to heat sink, but initially you've got a lot of room temperature silicon to take up the first few micro-joules of dissipation. Jim might know representative dimensions for drain channels. Silicon has a heat capacity of about 700 joules per kilogram at room temperature. The data sheet suggests limiting avalanche energy to 70 mJ, and stops allowing you to increase the single pulse avalanche power for pulse- widths below 6usec - which has probably got more to do with the onset of channeling than anything specific about the thermal mass of the channel. Figure 13 of the data sheet shows a three-componenet thermal model, with the shortest time constant at 260usec, followed by 1.228msec and 8.12msec. A more physically realistic model, with concentric shells of silicon getting warmed up in succession, could probably be resolved into exponential components including some with shorter time constants. -- Bill Sloman, Nijmegen
From: Joerg on 18 Dec 2009 10:33 D from BC wrote: > On Thu, 17 Dec 2009 13:07:37 -0800 (PST), "J.A. Legris" > <jalegris(a)sympatico.ca> wrote: > >> On Dec 17, 2:17 pm, D from BC <myrealaddr...(a)comic.com> wrote: >>> I got a 10 ohm carbon switched by a mosfet(TO-220). >>> Spec sheet: Rdson=0.17ohms at 9.8amps with <=300uS pulse 2% duty. >>> R to drain. >>> Source to gnd. >>> 20V regulated supply >>> >>> On the scope I've noticed that Vdson is still increasing in voltage >>> (Ids still increasing) after a 50uS mosfet turn on time. >>> (Freq=10xper second.) >>> I do spot the initial current signature due to loop inductance but >>> following that Id morphs to a slope too gradual to be inductance. >>> Id levels off and stays constant after about 100uS. >>> >>> Is it thermal? >>> Is the mosfet material is heating up and becoming more resistive? >>> Then it's able to cool down before the next pulse.. >>> >>> If it's a thermal dynamic, then I suppose this may not show up on a >>> simulator. >> Which MOSFET, and how are you driving the gate? Please describe the >> entire circuit. > > Using function generator to drive the mosfet. > > Using obsolete mosfet: > http://www.irf.com/product-info/datasheets/data/irfb17n20d.pdf > > Dead bug picture. > http://members.shaw.ca/chainsaw/SED/circuit > 603KB JPEG > Yikes! You really need some sort of ground structure. Even if it's just a piece of copper clad. > A 12ohm resistor from drain to V+ = 15V. > Source to ground. > Gate to function gen. > Scope 10:1 probe on drain. > Ceramic cap is 0.1uF. > >> And what do you mean when you say that "Vdson is still increasing in >> voltage (Ids still increasing) after a 50uS mosfet turn on time"? Do >> you mean they hit bottom and are now moving up again? > > Yup. > I'm watching Vds after the Vds negative edge. > Upon gate +pulse, Vds drops down, goes a bit negative, rises with a > steep slope (circuit inductance) then morphs to a lesser slope(from > heat???). > (Scratching head..) > http://members.shaw.ca/chainsaw/SED/vds > 2.4MB bmp...<< smaller by request. > Try using the PNG format next time, much better, much smaller. My scope uses just 5k-10k for a pic of similar detail. >> Can you post a >> shot of the waveforms? > > I was expecting Vds to drop to Ids*Rds during the on period. > All nice a square like seen on a simulator. > A little puzzling. > Well, it has no heat sink at all. The channel will get hot very fast. But without a ground structure it's hard to say how much of the measurement is for real. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 18 Dec 2009 10:36 John Larkin wrote: > On Thu, 17 Dec 2009 22:07:01 -0800, Archimedes' Lever > <OneBigLever(a)InfiniteSeries.Org> wrote: > >> On Thu, 17 Dec 2009 12:53:43 -0800 (PST), "langwadt(a)fonz.dk" >> <langwadt(a)fonz.dk> wrote: >> >>> On 17 Dec., 20:17, D from BC <myrealaddr...(a)comic.com> wrote: >>>> I got a 10 ohm carbon switched by a mosfet(TO-220). >>>> Spec sheet: Rdson=0.17ohms at 9.8amps with <=300uS pulse 2% duty. >>>> R to drain. >>>> Source to gnd. >>>> 20V regulated supply >>>> >>>> On the scope I've noticed that Vdson is still increasing in voltage >>>> (Ids still increasing) after a 50uS mosfet turn on time. >>>> (Freq=10xper second.) >>>> I do spot the initial current signature due to loop inductance but >>>> following that Id morphs to a slope too gradual to be inductance. >>>> Id levels off and stays constant after about 100uS. >>>> >>>> Is it thermal? >>>> Is the mosfet material is heating up and becoming more resistive? >>>> Then it's able to cool down before the next pulse.. >>>> >>>> If it's a thermal dynamic, then I suppose this may not show up on a >>>> simulator. >>> have a look at the datasheet for a random mosfet, look for tj vs. Ron >>> looks like it generally doubles from 20C to 100C >>> >>> >>> -Lasse >> For really good devices check out Zetex. > > Purchased by Diodes Inc. Funny, I would have predicted the other way > around. > IRF is usually the leader of the pack. But IIRC they have sold rights to some devices to others (Vishay?). Disinvested or whatever the MBA speak might be. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Tim Williams on 18 Dec 2009 10:42 "D from BC" <myrealaddress(a)comic.com> wrote in message news:v28li5tsd6k24nd6hpbompftalcmokmj7i(a)4ax.com... > http://members.shaw.ca/chainsaw/SED/vds Aside from that twiddle at the beginning, it looks like two exponentials, one fairly fast (tau ~ 2us?), the other a lot slower, closer to 15-20us. You're clipping off a lot of waveform, do you know what the recovery time is on your scope? At that scale, the ADCs should be overloaded, I wonder how well they do. > I was expecting Vds to drop to Ids*Rds during the on period. > All nice a square like seen on a simulator. > A little puzzling. Pfffbt, simulator, schmimulator. ;-) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: J.A. Legris on 18 Dec 2009 10:54
On Dec 17, 10:38 pm, D from BC <myrealaddr...(a)comic.com> wrote: > On Thu, 17 Dec 2009 13:07:37 -0800 (PST), "J.A. Legris" > > > > <jaleg...(a)sympatico.ca> wrote: > >On Dec 17, 2:17 pm, D from BC <myrealaddr...(a)comic.com> wrote: > >> I got a 10 ohm carbon switched by a mosfet(TO-220). > >> Spec sheet: Rdson=0.17ohms at 9.8amps with <=300uS pulse 2% duty. > >> R to drain. > >> Source to gnd. > >> 20V regulated supply > > >> On the scope I've noticed that Vdson is still increasing in voltage > >> (Ids still increasing) after a 50uS mosfet turn on time. > >> (Freq=10xper second.) > >> I do spot the initial current signature due to loop inductance but > >> following that Id morphs to a slope too gradual to be inductance. > >> Id levels off and stays constant after about 100uS. > > >> Is it thermal? > >> Is the mosfet material is heating up and becoming more resistive? > >> Then it's able to cool down before the next pulse.. > > >> If it's a thermal dynamic, then I suppose this may not show up on a > >> simulator. > > >Which MOSFET, and how are you driving the gate? Please describe the > >entire circuit. > > Using function generator to drive the mosfet. > > Using obsolete mosfet:http://www.irf.com/product-info/datasheets/data/irfb17n20d.pdf > > Dead bug picture.http://members.shaw.ca/chainsaw/SED/circuit > 603KB JPEG > > A 12ohm resistor from drain to V+ = 15V. > Source to ground. > Gate to function gen. > Scope 10:1 probe on drain. > Ceramic cap is 0.1uF. > > > > >And what do you mean when you say that "Vdson is still increasing in > >voltage (Ids still increasing) after a 50uS mosfet turn on time"? Do > >you mean they hit bottom and are now moving up again? > > Yup. > I'm watching Vds after the Vds negative edge. > Upon gate +pulse, Vds drops down, goes a bit negative, rises with a > steep slope (circuit inductance) then morphs to a lesser slope(from > heat???). > (Scratching head..)http://members.shaw.ca/chainsaw/SED/vds > 2.4MB bmp...<< smaller by request. > > > Can you post a > >shot of the waveforms? > > I was expecting Vds to drop to Ids*Rds during the on period. > All nice a square like seen on a simulator. > A little puzzling. New 'scope right? Try adjusting the little screw on the probe. -- Joe |