Using FETS on DC motors
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Posts: 46,084
Can you suggest a "high speed recovery diode" to be used with a MOSFET
driving a motor (and source)?
>No where in the previous messages did I see anyone
>mention the need for a high speed recovery diode across
>the DC motor.
>Without one, the DC motor will develop one high voltage
>spike which will fry the MOSFET when trying to turn off
>the motor.
>The diode needs to be rated at least 30% of the nominal
>current rating of the motor and have low nanosecond
>recovery time.
>The diode clamps the EMF generated on turn off and
>keeps current flowing in the motor.
[noparse][[/noparse]Non-text portions of this message have been removed]
driving a motor (and source)?
>No where in the previous messages did I see anyone
>mention the need for a high speed recovery diode across
>the DC motor.
>Without one, the DC motor will develop one high voltage
>spike which will fry the MOSFET when trying to turn off
>the motor.
>The diode needs to be rated at least 30% of the nominal
>current rating of the motor and have low nanosecond
>recovery time.
>The diode clamps the EMF generated on turn off and
>keeps current flowing in the motor.
[noparse][[/noparse]Non-text portions of this message have been removed]
Comments
No where in the previous messages did I see anyone
mention the need for a high speed recovery diode across
the DC motor.
Without one, the DC motor will develop one high voltage
spike which will fry the MOSFET when trying to turn off
the motor.
The diode needs to be rated at least 30% of the nominal
current rating of the motor and have low nanosecond
recovery time.
The diode clamps the EMF generated on turn off and
keeps current flowing in the motor.
Sincerely,
Ron
'********************* Previous Messages
*********************
Message: 4
Date: Wed, 30 Jun 2004 21:39:07 -0000
From: "Dave Mucha" <davemucha@j...>
Subject: Re: MOSFET keeps burning out
--- In basicstamps@yahoogroups.com, laurasdog@w...
wrote:
> At 08:02 AM 6/30/2004, nathanchronister wrote:
>
>
> >Hi. I'm using the microcontroller to control a DC
motor. I have a
PWM
> >output hooked up to an N-channel MOSFET. (I'm not
using an H-
> >bridge because I don't need reverse.) Even though
the MOSFET is
rated
> >25 amps and should easily handle the ~10 amp motor
current, it
keeps
> >burning out. After this happens the motor won't shut
off.
>
>
> I'm not real familiar with using the stamp PWM
feature, but I'd be
curious
> what frequency it switches on and off at...
>
> MOSFET's have a very high DC input impedance on the
gate. so it
takes
> almost no gate current to turn on with a *DC signal*
but there can
also be
> a very large gate capacitance, which means at high
frequencies it
can take
> a LOT of gate current to charge and discharge that
capacitance.
> The data sheet for your device shows an input
capacitance of 1100
pF which
> is pretty large.
>
> The stamp is probably unable to source enough current
to fully turn
the
> MOSFET on and off at the PWM frequency....
>
> Steve
The is very true. the gate has a large capatacance
component that
will not be satisfied with the logic signal from the
Stamp.
Check out MOSFET DRIVERS. these tiny chips handle 4
amps at your
voltage to drive the FET.
The problem stems from how the FET works. As the
voltage on the gate
slowly (in FET speed) begins to allow the FET to pass
current, the
slow voltage does not saturate the FET, but begins to
activate parts
of it. The result is your circuit attemps to pass the
full current
on a tiny portion of the FET and burns out sections.
This cascades
across the rest of the FET and your chip smokes.
Dave
Good point!... inherently there is a diode formation
from the substrate of the MOSFET anyway, but this is
usually considered a "weak" diode and should not be
relied upon for absorbing EMF spikes in any application.
BTW) The diode could be placed across the S/D of the
MOSFET and/or the terminals of the motor.
>FYI,
>
>No where in the previous messages did I see anyone
>mention the need for a high speed recovery diode across
>the DC motor.
>
>Without one, the DC motor will develop one high voltage
>spike which will fry the MOSFET when trying to turn off
>the motor.
>
>The diode needs to be rated at least 30% of the nominal
>current rating of the motor and have low nanosecond
>recovery time.
>
>The diode clamps the EMF generated on turn off and
>keeps current flowing in the motor.
>
>Sincerely,
>Ron
>
>'********************* Previous Messages
>*********************
>
>Message: 4
> Date: Wed, 30 Jun 2004 21:39:07 -0000
> From: "Dave Mucha" <davemucha@j...>
>Subject: Re: MOSFET keeps burning out
>
>--- In basicstamps@yahoogroups.com, laurasdog@w...
>wrote:
> > At 08:02 AM 6/30/2004, nathanchronister wrote:
> >
> >
> > >Hi. I'm using the microcontroller to control a DC
>motor. I have a
>PWM
> > >output hooked up to an N-channel MOSFET. (I'm not
>using an H-
> > >bridge because I don't need reverse.) Even though
>the MOSFET is
>rated
> > >25 amps and should easily handle the ~10 amp motor
>current, it
>keeps
> > >burning out. After this happens the motor won't shut
>off.
> >
> >
> > I'm not real familiar with using the stamp PWM
>feature, but I'd be
>curious
> > what frequency it switches on and off at...
> >
> > MOSFET's have a very high DC input impedance on the
>gate. so it
>takes
> > almost no gate current to turn on with a *DC signal*
>but there can
>also be
> > a very large gate capacitance, which means at high
>frequencies it
>can take
> > a LOT of gate current to charge and discharge that
>capacitance.
> > The data sheet for your device shows an input
>capacitance of 1100
>pF which
> > is pretty large.
> >
> > The stamp is probably unable to source enough current
>to fully turn
>the
> > MOSFET on and off at the PWM frequency....
> >
> > Steve
>
>
>The is very true. the gate has a large capatacance
>component that
>will not be satisfied with the logic signal from the
>Stamp.
>
>Check out MOSFET DRIVERS. these tiny chips handle 4
>amps at your
>voltage to drive the FET.
>
>The problem stems from how the FET works. As the
>voltage on the gate
>slowly (in FET speed) begins to allow the FET to pass
>current, the
>slow voltage does not saturate the FET, but begins to
>activate parts
>of it. The result is your circuit attemps to pass the
>full current
>on a tiny portion of the FET and burns out sections.
>This cascades
>across the rest of the FET and your chip smokes.
>
>Dave
>
>
>
>
>
>
>
>
>
>
>
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>
Beau Schwabe Mask Designer II National Semiconductor Corporation
500 Pinnacle Court, Suite 525
Mail Stop GA1
diode in the fet, being faster than the one in parallel, will take the brunt
of the energy, and nothing will be gained<G>. Just like the FYI person
explained. Standard 60 Hz diodes, 1N4007, for example, are not fast enough
to ensure that they will work.
Original Message
From: Beau Schwabe [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=w3uZE7-4S68bMTvT9zDqH1_B2UyBucQXA-fAlngIr9N2GDxsGuYW72EDmIo-bF_Nk48KeChPt9tcaWwDSZdeOWc]bschwabe@a...[/url
Sent: Thursday, July 01, 2004 5:10 PM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Using FETS on DC motors
Ron,
Good point!... inherently there is a diode formation
from the substrate of the MOSFET anyway, but this is
usually considered a "weak" diode and should not be
relied upon for absorbing EMF spikes in any application.
BTW) The diode could be placed across the S/D of the
MOSFET and/or the terminals of the motor.
>FYI,
>
>No where in the previous messages did I see anyone
>mention the need for a high speed recovery diode across
>the DC motor.