Learning Mosfets...RdsON Warm Up?
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From: D from BC on 17 Dec 2009 14:17 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.
From: langwadt on 17 Dec 2009 15:53 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
From: J.A. Legris on 17 Dec 2009 16:07 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. 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? Can you post a shot of the waveforms? -- Joe
From: D from BC on 17 Dec 2009 17:47 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. > >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? Can you post a >shot of the waveforms? Waves coming up.. Have to set up scope+PC interface and try out the software for the first time.
From: Joerg on 17 Dec 2009 18:37
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. >> >> 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? Can you post a >> shot of the waveforms? > > Waves coming up.. 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 -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |