Learning Mosfets...RdsON Warm Up?
in [Design]
|
Prev: Favorite jellybean ~HF BJT?
Next: One zener diode vs two standard diodes for overvoltage protection of low-level voltage (analog and data) lines...
From: Joerg on 20 Dec 2009 21:07 krw wrote: > 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. ;-) Nope, dark blue :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: krw on 20 Dec 2009 21:09 On Sun, 20 Dec 2009 18:07:14 -0800, Joerg <invalid(a)invalid.invalid> wrote: >krw wrote: >> 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. ;-) > > >Nope, dark blue :-) Oh, that's much better. ?-/
From: Michael A. Terrell on 20 Dec 2009 21:53 Jim Thompson wrote: > > 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 ;-) He can't blow up any buttons that way. -- Offworld checks no longer accepted!
From: John Larkin on 20 Dec 2009 23:12 On Sun, 20 Dec 2009 16:20:50 -0800, D from BC <myrealaddress(a)comic.com> wrote: >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. Not many scopes recover cleanly from gross overload. And if you're looking for something small, like Vds in saturation, just after a big D-S voltage, both the scope and the probe can fool you. Some of the old Tek letter-series (W, Z) plugins, and the 7000-series stuff (7A13, 7A22) have astounding overload recovery. I tested a lot of power fets so that I could run realtime junction temperature simulations in a couple of big power amps, to push as much performance out of the fets as possible. ftp://jjlarkin.lmi.net/ExFets.jpg Most of them, on a heat sink, behaved roughly as if they had sort of a 1st order, 100 millisecond thermal time constant. John
From: Joerg on 21 Dec 2009 11:27
Michael A. Terrell wrote: > Jim Thompson wrote: >> 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 ;-) > > > He can't blow up any buttons that way. > That depends entirely upon the consumption of marzipan, pound cake, cheese cake, stollen and all sorts of other goodies. Comes a point where the button flies right off the shirt again upon a wee stretch :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |