Learning Mosfets...RdsON Warm Up?
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From: Hammy on 17 Dec 2009 20:31 On Thu, 17 Dec 2009 15:37:53 -0800, Joerg <invalid(a)invalid.invalid> wrote: > >Hey, don't make too many waves up there in Canada :-) > > >> Have to set up scope+PC interface and try out the software for the >> first time. >> > >Take a look at figure 4: > >http://www.irf.com/product-info/datasheets/data/irfs3806pbf.pdf To bad they dont have 500 or 600V Fets like that. Low gate charge and Rdson.
From: D from BC on 17 Dec 2009 22:38 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 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.
From: D from BC on 17 Dec 2009 23:03 On Thu, 17 Dec 2009 15:37:53 -0800, Joerg <invalid(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.
From: Archimedes' Lever on 18 Dec 2009 01:07 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.
From: John Larkin on 18 Dec 2009 01:08
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. John |