MOSFET acting weird, any ideas?
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Posted by on July 22, 2008, 1:19 pm
Please log in for more thread options Not a newbie, here, I went to electronics school about a decade ago, and although I've reeducated myself to this point, I seem to be stuck. Please help, anybody! I've got a mosfet that I'm using, driving an irf510 with a tc4420 cmos driver ic. I've got the driver being fed a square-wave with a 555. The Driver is putting out a clean 12volt p-p SW all the way to the gate leg on the mosfet. I'm trying to source battery voltage, 12.75v at way more potential amps than what my circuit is "supposed" to be drawing. around 6-8amps. The mosfet is only sourcing 2v p-p, and the same square-wave, good and clean, just not big enough! I've tried various pull-up resistor arrangements, I've tried using a transistor instead of the driver ic, and the driver ic works the best. but nothing over 2v p-p. I'm stumped. I need approximately 12v, I should be pulling directly form V++, B | ||||||||||||||||||||||
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Posted by Tim Wescott on July 22, 2008, 3:33 pm
Please log in for more thread options You didn't post a schematic or a link to one, so I'm guessing here: Is the MOSFET common drain or common source? If you're putting +12V on the gate and looking for voltage out the source (i.e. if you're using a common drain 'high side' circuit) then 2V is not unreasonable. You have to bring the gate 12V _more_ than the source. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html | ||||||||||||||||||||||
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Posted by thsuper1 on July 22, 2008, 4:46 pm
Please log in for more thread options > thsup...@gmail.com wrote:
> > Hi all!
> > Not a newbie, here, I went to electronics school about a decade ago, > > and although I've reeducated myself to this point, I seem to be stuck. > > Please help, anybody! >
> > I've got a mosfet that I'm using, driving an irf510 with a tc4420 cmos
> > driver ic. > > I've got the driver being fed a square-wave with a 555. > > The Driver is putting out a clean 12volt p-p SW all the way to the > > gate leg on the mosfet. > > I'm trying to source battery voltage, 12.75v at way more potential > > amps than what my circuit is "supposed" to be drawing. around > > 6-8amps. > > The mosfet is only sourcing 2v p-p, and the same square-wave, good and > > clean, just not big enough! >
> > I've tried various pull-up resistor arrangements, I've tried using a
> > transistor instead of the driver ic, and the driver ic works the best. > > but nothing over 2v p-p. I'm stumped. I need approximately 12v, I > > should be pulling directly form V++, >
> > B
>
> You didn't post a schematic or a link to one, so I'm guessing here: > > Is the MOSFET common drain or common source? If you're putting +12V on > the gate and looking for voltage out the source (i.e. if you're using a > common drain 'high side' circuit) then 2V is not unreasonable. > > You have to bring the gate 12V _more_ than the source. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" gives you just what it says. > See details athttp://www.wescottdesign.com/actfes/actfes.html So what you're saying, so that I understand, is that if I'm drawing "high side" or from the drain side? that 2v p-p is normal, and that I need to draw the gate to a full 12v higher than the source? That means to me that the device (mosfet)takes itself to the potential of the circuit, and that 2v p-p is all that's available from the potential of the circuit to the device "potential," so if I draw off of the "low-side", like in a common emitter amplifier? Please expound on this, I know all this stuff, it's just like reactivating it, you know? Thanks again for the input. B | ||||||||||||||||||||||
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Posted by Tim Wescott on July 22, 2008, 5:25 pm
Please log in for more thread options thsuper1 wrote:
>> thsup...@gmail.com wrote:
>>> Hi all!
>>> Not a newbie, here, I went to electronics school about a decade ago, >>> and although I've reeducated myself to this point, I seem to be stuck. >>> Please help, anybody! >>> I've got a mosfet that I'm using, driving an irf510 with a tc4420 cmos >>> driver ic. >>> I've got the driver being fed a square-wave with a 555. >>> The Driver is putting out a clean 12volt p-p SW all the way to the >>> gate leg on the mosfet. >>> I'm trying to source battery voltage, 12.75v at way more potential >>> amps than what my circuit is "supposed" to be drawing. around >>> 6-8amps. >>> The mosfet is only sourcing 2v p-p, and the same square-wave, good and >>> clean, just not big enough! >>> I've tried various pull-up resistor arrangements, I've tried using a >>> transistor instead of the driver ic, and the driver ic works the best. >>> but nothing over 2v p-p. I'm stumped. I need approximately 12v, I >>> should be pulling directly form V++, >>> B >> You didn't post a schematic or a link to one, so I'm guessing here:
>> >> Is the MOSFET common drain or common source? If you're putting +12V on >> the gate and looking for voltage out the source (i.e. if you're using a >> common drain 'high side' circuit) then 2V is not unreasonable. >> >> You have to bring the gate 12V _more_ than the source. >> >> -- >> >> Tim Wescott >> Wescott Design Serviceshttp://www.wescottdesign.com >> >> Do you need to implement control loops in software? >> "Applied Control Theory for Embedded Systems" gives you just what it says. >> See details athttp://www.wescottdesign.com/actfes/actfes.html >
> So what you're saying, so that I understand, is that if I'm drawing > "high side" or from the drain side? that 2v p-p is normal, and that I > need to draw the gate to a full 12v higher than the source? I'm not quite making out what you mean - I think your confusion is confusing me. If you're connected like the schematic below, and if your gate drive only goes 12V above ground, then the load not seeing the full 12V is expected. Only seeing 2V seems to be excessive drop, but if you're using a really low-resistance load I could maybe see it. Come to think of it, are you sure your supply voltage is really staying at 12V? Not just because the battery should, but because you've measured? 12V + | | ||-+ gate drive ||<- o----------||-+ | | to load '---------o (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) >
> That means to me that the device (mosfet)takes itself to the potential > of the circuit, and that 2v p-p is all that's available from the > potential of the circuit to the device "potential," so if I draw off > of the "low-side", like in a common emitter amplifier? Please expound > on this, I know all this stuff, it's just like reactivating it, you > know? The MOSFET needs to see lots-o-volts between the gate and source. In the circuit above, with the load connected to the source, raising the load voltage reduces the g-s voltage, and reduces the drive to the MOSFET. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html | ||||||||||||||||||||||
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Posted by thsuper1 on July 22, 2008, 6:20 pm
Please log in for more thread options > thsuper1 wrote:
> >> thsup...@gmail.com wrote:
> >>> Hi all! > >>> Not a newbie, here, I went to electronics school about a decade ago, > >>> and although I've reeducated myself to this point, I seem to be stuck. > >>> Please help, anybody! > >>> I've got a mosfet that I'm using, driving an irf510 with a tc4420 cmos > >>> driver ic. > >>> I've got the driver being fed a square-wave with a 555. > >>> The Driver is putting out a clean 12volt p-p SW all the way to the > >>> gate leg on the mosfet. > >>> I'm trying to source battery voltage, 12.75v at way more potential > >>> amps than what my circuit is "supposed" to be drawing. around > >>> 6-8amps. > >>> The mosfet is only sourcing 2v p-p, and the same square-wave, good and > >>> clean, just not big enough! > >>> I've tried various pull-up resistor arrangements, I've tried using a > >>> transistor instead of the driver ic, and the driver ic works the best. > >>> but nothing over 2v p-p. I'm stumped. I need approximately 12v, I > >>> should be pulling directly form V++, > >>> B > >> You didn't post a schematic or a link to one, so I'm guessing here: >
> >> Is the MOSFET common drain or common source? If you're putting +12V on
> >> the gate and looking for voltage out the source (i.e. if you're using a > >> common drain 'high side' circuit) then 2V is not unreasonable. >
> >> You have to bring the gate 12V _more_ than the source.
>
> >> --
>
> >> Tim Wescott
> >> Wescott Design Serviceshttp://www.wescottdesign.com >
> >> Do you need to implement control loops in software?
> >> "Applied Control Theory for Embedded Systems" gives you just what it says. > >> See details athttp://www.wescottdesign.com/actfes/actfes.html >
> > So what you're saying, so that I understand, is that if I'm drawing
> > "high side" or from the drain side? that 2v p-p is normal, and that I > > need to draw the gate to a full 12v higher than the source? >
> I'm not quite making out what you mean - I think your confusion is > confusing me. > > If you're connected like the schematic below, and if your gate drive > only goes 12V above ground, then the load not seeing the full 12V is > expected. Only seeing 2V seems to be excessive drop, but if you're > using a really low-resistance load I could maybe see it. > > Come to think of it, are you sure your supply voltage is really staying > at 12V? Not just because the battery should, but because you've measured? > > 12V > + > | > | > ||-+ > gate drive ||<- > o----------||-+ > | > | to load > '---------o > > (created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de) > > > > > That means to me that the device (mosfet)takes itself to the potential
> > of the circuit, and that 2v p-p is all that's available from the > > potential of the circuit to the device "potential," so if I draw off > > of the "low-side", like in a common emitter amplifier? Please expound > > on this, I know all this stuff, it's just like reactivating it, you > > know? >
> The MOSFET needs to see lots-o-volts between the gate and source. In > the circuit above, with the load connected to the source, raising the > load voltage reduces the g-s voltage, and reduces the drive to the MOSFET. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" gives you just what it says. > See details athttp://www.wescottdesign.com/actfes/actfes.html I See. So if: + 12v |______o <- load pos | _ ^ |______o <-load neg. gate | <-drive D _______G S | | = ground What would you expect the output to be if the input waveform is 12v p- p? The battery is not getting drawn down, yes I have measured, but thanks for asking. That's the kind of thinking I'm used to. What would you do to this to make a higher output waveform? take the load off at a different point? my load is a power step-up transformer. | ||||||||||||||||||||||
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> Not a newbie, here, I went to electronics school about a decade ago,
> and although I've reeducated myself to this point, I seem to be stuck.
> Please help, anybody!
>
> I've got a mosfet that I'm using, driving an irf510 with a tc4420 cmos
> driver ic.
> I've got the driver being fed a square-wave with a 555.
> The Driver is putting out a clean 12volt p-p SW all the way to the
> gate leg on the mosfet.
> I'm trying to source battery voltage, 12.75v at way more potential
> amps than what my circuit is "supposed" to be drawing. around
> 6-8amps.
> The mosfet is only sourcing 2v p-p, and the same square-wave, good and
> clean, just not big enough!
>
> I've tried various pull-up resistor arrangements, I've tried using a
> transistor instead of the driver ic, and the driver ic works the best.
> but nothing over 2v p-p. I'm stumped. I need approximately 12v, I
> should be pulling directly form V++,
>
> B