Learning Mosfets...RdsON Warm Up?
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From: D from BC on 18 Dec 2009 15:30 On Fri, 18 Dec 2009 07:33:44 -0800, Joerg <invalid(a)invalid.invalid> wrote: >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. >> >> 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 >> > >Yikes! You really need some sort of ground structure. Even if it's just >a piece of copper clad. This 'octopus circuit' is glued to a pcb copper side down. Grounding the plane doesn't make any changes to Vds. > > >> 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. >> > >Try using the PNG format next time, much better, much smaller. My scope >uses just 5k-10k for a pic of similar detail. I've yet to read up on the PNG format. I don't know yet if it's supported by net browsers. > > >>> 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. >> > >Well, it has no heat sink at all. The channel will get hot very fast. >But without a ground structure it's hard to say how much of the >measurement is for real. I move all the wires around to check for E field and M field coupling. No Vds signal change. afaik parasitics to the ground plane reduce E-field coupling and I don't think that was needed for this circuit. I'm not familiar with conductor inductance difference with and without a ground plane. I was playing with a strip line calculator and the conductor inductance went up as the conductor got closer to the ground plane???
From: D from BC on 18 Dec 2009 15:48 On Fri, 18 Dec 2009 07:54:13 -0800 (PST), "J.A. Legris" <jalegris(a)sympatico.ca> wrote: >On Dec 17, 10:38�pm, D from BC <myrealaddr...(a)comic.com> wrote: >> On Thu, 17 Dec 2009 13:07:37 -0800 (PST), "J.A. Legris" >> >> >> >> <jaleg...(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. > >New 'scope 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
From: D from BC on 18 Dec 2009 16:00 On Fri, 18 Dec 2009 09:42:19 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"D from BC" <myrealaddress(a)comic.com> wrote in message >news:v28li5tsd6k24nd6hpbompftalcmokmj7i(a)4ax.com... >> http://members.shaw.ca/chainsaw/SED/vds > >Aside from that twiddle at the beginning, it looks like two exponentials, >one fairly fast (tau ~ 2us?), the other a lot slower, closer to 15-20us. > >You're clipping off a lot of waveform, do you know what the recovery time is >on your scope? At that scale, the ADCs should be overloaded, I wonder how >well they do. > >> 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. > >Pfffbt, simulator, schmimulator. ;-) I dunno.. In this case, the simulator was more right than my testing. It was my probe changing the signal. :( When I adjusted the probe comp screw, I get other curves in this area.
From: Joerg on 18 Dec 2009 16:03 D from BC wrote: > On Fri, 18 Dec 2009 07:33:44 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> 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. >>> 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 >>> >> Yikes! You really need some sort of ground structure. Even if it's just >> a piece of copper clad. > > This 'octopus circuit' is glued to a pcb copper side down. Grounding > the plane doesn't make any changes to Vds. > Yeah, but it makes it difficult to predict exactly what you are measuring. When hunting very subtle or strange effects the first thing to do should be establishing a well defined environment. A piece of copper clad ain't that expensive and can be re-used :-) >> >>> 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. >>> >> Try using the PNG format next time, much better, much smaller. My scope >> uses just 5k-10k for a pic of similar detail. > > I've yet to read up on the PNG format. I don't know yet if it's > supported by net browsers. > They do. TIFF can be a problem, or usually is. I use PNG with all my clients and never had an issue. A click on the file and it either opens right up in the browser or a viewer. >> >>>> 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. >>> >> Well, it has no heat sink at all. The channel will get hot very fast. >> But without a ground structure it's hard to say how much of the >> measurement is for real. > > I move all the wires around to check for E field and M field coupling. > No Vds signal change. > afaik parasitics to the ground plane reduce E-field coupling and I > don't think that was needed for this circuit. > I'm not familiar with conductor inductance difference with and without > a ground plane. I was playing with a strip line calculator and the > conductor inductance went up as the conductor got closer to the ground > plane??? > 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 18 Dec 2009 16:39
D from BC wrote: > On Fri, 18 Dec 2009 07:54:13 -0800 (PST), "J.A. Legris" > <jalegris(a)sympatico.ca> wrote: > >> On Dec 17, 10:38 pm, D from BC <myrealaddr...(a)comic.com> wrote: >>> On Thu, 17 Dec 2009 13:07:37 -0800 (PST), "J.A. Legris" >>> >>> >>> >>> <jaleg...(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. >> New 'scope 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! > Neato! Notice how the lettering is crisp and clear while it was fuzzy in your huge bitmap before. > 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.. :( > That's one reason why I prefer a direct coax link to the scope for stuff like this. I've once scratched my head for a few minutes, knowing I had cal'd the probe an hour ago. My mistake was that I answered a nature call, couldn't find the bathroom right away because it was a client building and later when I found the probe cal out of whack one of the guys said "Oh, maybe it's because I just dropped it?" -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |