Learning Mosfets...RdsON Warm Up?
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From: kevin93 on 18 Dec 2009 19:40 On Dec 18, 12:48 pm, D from BC <myrealaddr...(a)comic.com> wrote: > On Fri, 18 Dec 2009 07:54:13 -0800 (PST), "J.A. Legris" .... right? Try adjusting the little screw on the probe. > > frick! > That's it! > Adjusting the probe comp screw is the only thing to affect the area of > interest. > > Vds is yellow trace. > Green trace is my other probe deliberately put out of calibration(more > than what is was) to emulate out of cal probe 1. > Green is on the cali pulse out on the scope and due to f diff I waited > for the signal to float by for stopping the acquisition. > > http://members.shaw.ca/chainsaw/SED/vdsandprobenotcali > 40k PNG and in color! > > I suppose if I had a fet probe/active probe, Vds would look more like > as seen on a simulator. > That or I learn how to discern what's probe signal and what's circuit > signal. > > groan.. :( > > Thanks You also have to be careful about scope overload when looking at this type of thing. Many scopes can take a long time to recover from the overload that occurs "off-screen" kevin
From: krw on 18 Dec 2009 20:10 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. AlwaysWrong, DI bought them out some time back and ditched a lot of the nice stuff.
From: John Larkin on 18 Dec 2009 20:14 On Thu, 17 Dec 2009 11:17:15 -0800, D from BC <myrealaddress(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. 100 us sounds fast to be thermal. But could be. Lots of scopes recover poorly from gross overloads. Scope probe compensation? John
From: D from BC on 20 Dec 2009 19:13 On Fri, 18 Dec 2009 00:05:03 -0800 (PST), Bill Sloman <bill.sloman(a)ieee.org> wrote: >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. Figures that I get jammed when the electronics take me into the semiconductor thermal physics. As soon I find myself crossing into another field of expertise, that's when the trouble begins.
From: D from BC on 20 Dec 2009 19:16
On Fri, 18 Dec 2009 13:03:25 -0800, Joerg <invalid(a)invalid.invalid> wrote: > >You might get lucky sometimes but I'd still do this on a piece of >copperclad. Source solder smack to the plane, coax (with divider if >needed) straight to the scope with it's shield also soldered to the >plane. I have a bunch of BNC with solder tabs in the drawer for that so >I can just plug it in. That also takes out any weird effects a probe can >have. Getting there.. |