Learning Mosfets...RdsON Warm Up?
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From: D from BC on 20 Dec 2009 19:18 On Fri, 18 Dec 2009 16:40:21 -0800 (PST), kevin93 <kevin(a)whitedigs.com> wrote: >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 Thanks.. Will watch out for that.
From: D from BC on 20 Dec 2009 19:20 On Fri, 18 Dec 2009 17:14:29 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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 groan... Was scope compensation.. It's always the dumbest thing to blame over here.. Never some exotic complex little known phenomenon to go oooo ahhh over.
From: Joerg on 20 Dec 2009 20:24 D from BC wrote: > 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. > That's pretty much what my wife said after she found out that I had used silver wire and solder to put a button back onto a shirt :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 20 Dec 2009 20:54 On Sun, 20 Dec 2009 17:24:22 -0800, Joerg <invalid(a)invalid.invalid> wrote: >D from BC wrote: >> On Fri, 18 Dec 2009 00:05:03 -0800 (PST), Bill Sloman >> <bill.sloman(a)ieee.org> wrote: >> [auto-snip] >> >> 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. >> > >That's pretty much what my wife said after she found out that I had used >silver wire and solder to put a button back onto a shirt :-) Just get a "Buttoneer" and then you can play engineer and attach buttons better than sewing them on ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism properly!
From: krw on 20 Dec 2009 21:00
On Sun, 20 Dec 2009 17:24:22 -0800, Joerg <invalid(a)invalid.invalid> wrote: >D from BC wrote: >> 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. >> > >That's pretty much what my wife said after she found out that I had used >silver wire and solder to put a button back onto a shirt :-) I hope it wasn't a black shirt because it would clash. ;-) |