MOSFET keeps burning out
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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 have the PWM output wired *directly* to the Gate pin of the MOSFET.
The Gate is also wired to the ground via a 1000 ohm resistor. This is
intended to prevent static electricity from turning the MOSFET on.
The Source pin of the MOSFET is also wired to the ground.
The Drain pin is wired to the motor, and the other motor lead goes to
the + side of the 7.4 volt battery which drives the motor.
The motor control circuit is grounded on the ground strip on the
developers board and the "-" battery lead goes to the ground strip
also.
When this is all set up, it works fine for a while, but the MOSFET
gets hot and burns out, and then I have to unplug the battery to make
the motor stop. The implication seems to be that the MOSFET isn't
turning on all the way, and therefore generates more heat than it
should. Does it sound like I'm doing something wrong which could
cause the MOSFET to fail?
The MOSFET is Fairchild Semi's #NDP603AL. I think it has all the
right specs but I'm new to this so I could be wrong. Here is the
documentation page: http://www.fairchildsemi.com/ds/ND/NDP603AL.pdf
Thanks for your help,
Nathan
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 have the PWM output wired *directly* to the Gate pin of the MOSFET.
The Gate is also wired to the ground via a 1000 ohm resistor. This is
intended to prevent static electricity from turning the MOSFET on.
The Source pin of the MOSFET is also wired to the ground.
The Drain pin is wired to the motor, and the other motor lead goes to
the + side of the 7.4 volt battery which drives the motor.
The motor control circuit is grounded on the ground strip on the
developers board and the "-" battery lead goes to the ground strip
also.
When this is all set up, it works fine for a while, but the MOSFET
gets hot and burns out, and then I have to unplug the battery to make
the motor stop. The implication seems to be that the MOSFET isn't
turning on all the way, and therefore generates more heat than it
should. Does it sound like I'm doing something wrong which could
cause the MOSFET to fail?
The MOSFET is Fairchild Semi's #NDP603AL. I think it has all the
right specs but I'm new to this so I could be wrong. Here is the
documentation page: http://www.fairchildsemi.com/ds/ND/NDP603AL.pdf
Thanks for your help,
Nathan
Comments
>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.
How are you heatsinking the MOSFET?
-Todd Peterson
E-Lab Digital Engineering, Inc.
"Integrated Circuits & PowerCube Motor Modules"
www.elabinc.com (816) 257-9954
> 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.
>
Look at the "On Characteristics" of the FET that you are using. Rds(on)
is .031 for Vgs=4.5V(logic level). This means that at 10amps drain current,
the power dissipated is 10amps^2 x .031 = 3.1watts. Looking at the thermal
characteristics for the FET indicates that the temp rise is 62.5degC/watt
above
ambient so 3.1 watts x 62.5C/watt = ~194degC(above ambient)......This will
definitley lead to 'blue smoke leakage' and as we all know, IF one lets this
blue
smoke out of the device, they quit working!!<VBG>
You might try a logic level FET with a much lower Rdson....they do exist
down to the .003 to .004 ohm level. You also might need an additional
freewheeling
diode across the FET to prevent the 'inductive kick' of the motor from
zapping
the FET. HTH.......been there......done that!!
Tom Fisher
BigD
>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
[noparse][[/noparse]Non-text portions of this message have been removed]
snip
>I have the PWM output wired *directly* to the Gate pin of the MOSFET.
I just read the PWM section of the PBASIC guide and it says this about
the PWM output:
"It outputs a rapid sequence of on/off pulses, as short as 1.6 µs in duration,
whose overall proportion over the course of a full PWM cycle of approximately
a millisecond is equal to the duty cycle "
Pulses as short as 1.6 uS duration? That's over 600 KHZ !
The stamp can't provide enough current to charge and discharge
the gate capacitance of your MOSFET at those frequencies...
You'll need to provide some *serious* current buffering to get
enough gate drive to overcome that huge gate capacitance.
Steve
[noparse][[/noparse]Non-text portions of this message have been removed]
> 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
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
> Check out MOSFET DRIVERS. these tiny chips handle 4 amps at your
> voltage to drive the FET.
>
http://www.onsemi.com/pub/Collateral/MC34152-D.PDF
> 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, what do you get when you do the thermal calculations
for this FET when there is no heatsink? Vgs = 4.5V & Rdson=.031
ande drain current = 10A
Tom Fisher
Your problem is a lack of saturation. Most MOSFETs
require 10 volts or so to fully saturate (Drain ~=
Source). With only ~5 volts (output from a Stamp pin)
the MOSFET is in the "active range" i.e. has a high
resistance to current flow. This means lots of heat,
and eventually a catastophic shorting of the device
(motor runing full-on). You either need a 12 volt FET
Gate drive circuit, or Logic Level FETs. Logic level
FETs will fully saturate (.031 ohms) with just a 5
volt input. Most super-duty FETs require the 12 volt
system.
DD
--- Dave Mucha <davemucha@j...> wrote:
> --- 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
>
>
>
>
>
>
>
> >
> >
> > [noparse][[/noparse]Non-text portions of this message have been
> removed]
>
>
>
> To UNSUBSCRIBE, just send mail to:
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> Text in the Subject and Body of the message will be
> ignored.
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> > and the other motor lead goes to
> > the + side of the 7.4 volt battery
> > which drives the motor.
>
>
> Dave,
> what do you get when you do
> the thermal calculations
> for this FET when there is no heatsink?
> Vgs = 4.5V & Rdson=.031
> ande drain current = 10A
> Tom Fisher
Looking at the data sheet, it appears the unit is saturated with
4.5V, so it would be on fully.
Assuming a GS voltage of 4.5V also means a Source voltage of 10V with
your Gate voltage of 5 volts, the SD voltage is 10 volts.
The battery is 7.4 volts so the GS is only about 3 volts (assuming a
4.5v gate voltage)
7.4 volts at 0.031 ohms is 238ma or 1.766 watts.
This seems to fit with the GS voltage of 10 volts and a max of 15A
for 10V.
I'm assuming a steady state ON condition or 100% duty cycle.
The chart on page 5 lists the max safe operating area of 10 volts and
a pure DC condition of only 5 amps. The chart shows a 100mS pulse
would be rated for 10 amps.
The Thermal resistance of the junction to ambient is 62.5 deg C per
watt and at the 1.766 watts, the case would be getting a 110C rise.
Since this is on a PWM output, I would assume the unit is not on pure
DC (100% duty cycle), but on a lower duty cycle.
I would agree that assuming a 25 deg C ambient and a 110 C rise, the
package being at something approaching 150 C or more (open board
would be cooler, a enclosed board would get hotter) heat sinks would
be recommended. It is not quite reaching (theoretically) the case
limit of 175 deg C, but it is getting HOT none-the-less.
But, the safe amp operating area is of a concern also.
Wadda-ya-think ?
Dave
Ps: my current pet peeve is that the MOSFET data sheets do elaborate
more on selection. The unit is `rated' for 25A and 30V, but with a
steady state ON condition, it cannot come anywhere near that.
> your Gate voltage of 5 volts, the SD voltage is 10 volts.
>
> The battery is 7.4 volts so the GS is only about 3 volts (assuming a
> 4.5v gate voltage)
>
I thought the originator of the thread stated that the motor draws 10A.
P = I^R so Power = 10^2 * .031 = 3.1 watts.....
Working backwards, if we want to limit the case temp to 175C with 25C
ambient then 150C rise is max. 150/62.5 = 2.4watts
2.4watts/3.1watts = 77% max duty cycle.........I bet a cup of StarBucks
that this is the problem.
Tom Fisher
....also remember that Rdson *increases* with junction temp making
that blue smoke appear even faster!!<G>
>
> The Thermal resistance of the junction to ambient is 62.5 deg C per
> watt and at the 1.766 watts, the case would be getting a 110C rise.
>
> Since this is on a PWM output, I would assume the unit is not on pure
> DC (100% duty cycle), but on a lower duty cycle.
>
> I would agree that assuming a 25 deg C ambient and a 110 C rise, the
> package being at something approaching 150 C or more (open board
> would be cooler, a enclosed board would get hotter) heat sinks would
> be recommended. It is not quite reaching (theoretically) the case
> limit of 175 deg C, but it is getting HOT none-the-less.
>
> But, the safe amp operating area is of a concern also.
>
> Wadda-ya-think ?
>
> Dave
>
> Ps: my current pet peeve is that the MOSFET data sheets do elaborate
> more on selection. The unit is `rated' for 25A and 30V, but with a
> steady state ON condition, it cannot come anywhere near that.
>
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
> P = I^R so Power = 10^2 * .031 = 3.1 watts.....
Ooops....I^2 x R
Tom Fisher
<tboonefisher@s...> wrote:
> > Assuming a GS voltage of 4.5V also means a Source voltage of 10V
with
> > your Gate voltage of 5 volts, the SD voltage is 10 volts.
> >
> > The battery is 7.4 volts so the GS is only about 3 volts
(assuming a
> > 4.5v gate voltage)
> >
>
> I thought the originator of the thread stated that the motor
draws 10A.
> P = I^2 * R so Power = 10^2 * .031 = 3.1 watts.....
> Working backwards, if we want to limit the case temp to 175C with
25C
> ambient then 150C rise is max. 150/62.5 = 2.4watts
> 2.4watts/3.1watts = 77% max duty cycle.........I bet a cup of
StarBucks
> that this is the problem.
> Tom Fisher
> ....also remember that Rdson *increases* with junction temp making
> that blue smoke appear even faster!!<G>
>
We do know that the data sheet lists the max current of the device in
100% duty cycle as less than 5 amp.
We also know the application is in switching a motor.
Maybe we can get the origional poster to offer some other
characteristics of use ? was it a low dutycycle project ? was it a
current switch ? just turning on a motor and letting it run ?
But, as a solution, I think we are both saying that a heatsink is a
good idea and may solve the problem.
Dave
logic FET http://www.fairchildsemi.com/pf/IS/ISL9N302AP3.html
to drive a diesel glowpin that draws >20A DC(0.5ohm res). The BS2p24
PWMPAL output is connected to the gate via a 220ohm resistor
and a 10K pulldown resistor to ground. Supply voltage is ~13V,
freq is 50Hz and the duty cycle is 50%. The FET doesn't even get
warm...the glowpin gets VERY hot and works great.
Tom Fisher
Original Message
From: "TBooneFisher" <tboonefisher@s...>
To: <basicstamps@yahoogroups.com>
Sent: Wednesday, June 30, 2004 10:56 PM
Subject: Re: [noparse][[/noparse]basicstamps] Re: MOSFET keeps burning out
> > Assuming a GS voltage of 4.5V also means a Source voltage of 10V with
> > your Gate voltage of 5 volts, the SD voltage is 10 volts.
> >
> > The battery is 7.4 volts so the GS is only about 3 volts (assuming a
> > 4.5v gate voltage)
> >
>
> I thought the originator of the thread stated that the motor draws 10A.
> P = I^R so Power = 10^2 * .031 = 3.1 watts.....
> Working backwards, if we want to limit the case temp to 175C with 25C
> ambient then 150C rise is max. 150/62.5 = 2.4watts
> 2.4watts/3.1watts = 77% max duty cycle.........I bet a cup of StarBucks
> that this is the problem.
> Tom Fisher
> ....also remember that Rdson *increases* with junction temp making
> that blue smoke appear even faster!!<G>
>
>
>
> >
> > The Thermal resistance of the junction to ambient is 62.5 deg C per
> > watt and at the 1.766 watts, the case would be getting a 110C rise.
> >
> > Since this is on a PWM output, I would assume the unit is not on pure
> > DC (100% duty cycle), but on a lower duty cycle.
> >
> > I would agree that assuming a 25 deg C ambient and a 110 C rise, the
> > package being at something approaching 150 C or more (open board
> > would be cooler, a enclosed board would get hotter) heat sinks would
> > be recommended. It is not quite reaching (theoretically) the case
> > limit of 175 deg C, but it is getting HOT none-the-less.
> >
> > But, the safe amp operating area is of a concern also.
> >
> > Wadda-ya-think ?
> >
> > Dave
> >
> > Ps: my current pet peeve is that the MOSFET data sheets do elaborate
> > more on selection. The unit is `rated' for 25A and 30V, but with a
> > steady state ON condition, it cannot come anywhere near that.
> >
> >
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the Subject
and
> Body of the message will be ignored.
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
>
>
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
> Yahoo! Groups Links
>
>
>
>
>
It has been said time and time again that the PWM signal coming
from the stamp is not exactly designed for motor speed control,
but more over a way to provide an analog reference voltage from
the Stamp.
In my former life working with prosthetics, I found that PWM
works just fine as long as you have a fixed, or semi fixed base
frequency to your PWM. To minimize PWM noise within the motor
it is necessary to gain relative speed feedback from the motor
and adjust your base frequency accordingly. The PWMPAL achieves
a fixed base frequency however the Stamp creates a pseudo random
base frequency mixed in with the actual PWM that is meant to
produce a nice average voltage if filtered correctly.
Not that the PWM from the Stamp won't work to drive a motor, it
just won't be optimal, and may stress the MOSFET that is trying
to keep up with the high frequency components produced from the
Stamp PWM in combination with the high inductive motor load.
> Also, FYI, I'm currently using an ISL9N302AP3
>logic FET http://www.fairchildsemi.com/pf/IS/ISL9N302AP3.html
>to drive a diesel glowpin that draws >20A DC(0.5ohm res). The BS2p24
>PWMPAL output is connected to the gate via a 220ohm resistor
>and a 10K pulldown resistor to ground. Supply voltage is ~13V,
>freq is 50Hz and the duty cycle is 50%. The FET doesn't even get
>warm...the glowpin gets VERY hot and works great.
>Tom Fisher
>
>
>
Original Message
>From: "TBooneFisher" <tboonefisher@s...>
>To: <basicstamps@yahoogroups.com>
>Sent: Wednesday, June 30, 2004 10:56 PM
>Subject: Re: [noparse][[/noparse]basicstamps] Re: MOSFET keeps burning out
>
>
> > > Assuming a GS voltage of 4.5V also means a Source voltage of 10V with
> > > your Gate voltage of 5 volts, the SD voltage is 10 volts.
> > >
> > > The battery is 7.4 volts so the GS is only about 3 volts (assuming a
> > > 4.5v gate voltage)
> > >
> >
> > I thought the originator of the thread stated that the motor draws 10A.
> > P = I^R so Power = 10^2 * .031 = 3.1 watts.....
> > Working backwards, if we want to limit the case temp to 175C with 25C
> > ambient then 150C rise is max. 150/62.5 = 2.4watts
> > 2.4watts/3.1watts = 77% max duty cycle.........I bet a cup of StarBucks
> > that this is the problem.
> > Tom Fisher
> > ....also remember that Rdson *increases* with junction temp making
> > that blue smoke appear even faster!!<G>
> >
> >
> >
> > >
> > > The Thermal resistance of the junction to ambient is 62.5 deg C per
> > > watt and at the 1.766 watts, the case would be getting a 110C rise.
> > >
> > > Since this is on a PWM output, I would assume the unit is not on pure
> > > DC (100% duty cycle), but on a lower duty cycle.
> > >
> > > I would agree that assuming a 25 deg C ambient and a 110 C rise, the
> > > package being at something approaching 150 C or more (open board
> > > would be cooler, a enclosed board would get hotter) heat sinks would
> > > be recommended. It is not quite reaching (theoretically) the case
> > > limit of 175 deg C, but it is getting HOT none-the-less.
> > >
> > > But, the safe amp operating area is of a concern also.
> > >
> > > Wadda-ya-think ?
> > >
> > > Dave
> > >
> > > Ps: my current pet peeve is that the MOSFET data sheets do elaborate
> > > more on selection. The unit is `rated' for 25A and 30V, but with a
> > > steady state ON condition, it cannot come anywhere near that.
> > >
>
>
Beau Schwabe Mask Designer II National Semiconductor Corporation
500 Pinnacle Court, Suite 525
Mail Stop GA1
attempting to use the variable PWM to control the motor from 0 to
full throttle. I'm not sure what the duty cycle was when the motor
was burning out, because the processor reads a potentiometer and
adjusts the duty cycle according to the wing position, sort of like
in a servo except that it only goes in one direction at varying speed.
There is no heat sink. I need to save weight so I was hoping it
wouldn't be necessary. Tom, thanks for describing your deisel engine
project. I'll look into that other MOSFET. Hopefully I can get by
without a heat sink then.
Thank you all for such a detailed analysis of this problem. Can
anyone recommend a separate motor control device I might use instead
of my own MOSFET? I considered using a model airplane speed control,
but it receives a control pulse only fifty times a second which might
make the motor respond too slowly for my purposes.
Nathan
--- In basicstamps@yahoogroups.com, "Beau Schwabe" <bschwabe@a...>
wrote:
> I think that there is some merit here with using the PWMPAL.
> It has been said time and time again that the PWM signal coming
> from the stamp is not exactly designed for motor speed control,
> but more over a way to provide an analog reference voltage from
> the Stamp.
>
> In my former life working with prosthetics, I found that PWM
> works just fine as long as you have a fixed, or semi fixed base
> frequency to your PWM. To minimize PWM noise within the motor
> it is necessary to gain relative speed feedback from the motor
> and adjust your base frequency accordingly. The PWMPAL achieves
> a fixed base frequency however the Stamp creates a pseudo random
> base frequency mixed in with the actual PWM that is meant to
> produce a nice average voltage if filtered correctly.
>
> Not that the PWM from the Stamp won't work to drive a motor, it
> just won't be optimal, and may stress the MOSFET that is trying
> to keep up with the high frequency components produced from the
> Stamp PWM in combination with the high inductive motor load.
>
>
> > Also, FYI, I'm currently using an ISL9N302AP3
> >logic FET http://www.fairchildsemi.com/pf/IS/ISL9N302AP3.html
> >to drive a diesel glowpin that draws >20A DC(0.5ohm res). The
BS2p24
> >PWMPAL output is connected to the gate via a 220ohm resistor
> >and a 10K pulldown resistor to ground. Supply voltage is ~13V,
> >freq is 50Hz and the duty cycle is 50%. The FET doesn't even get
> >warm...the glowpin gets VERY hot and works great.
> >Tom Fisher
> >
> >
> >
Original Message
> >From: "TBooneFisher" <tboonefisher@s...>
> >To: <basicstamps@yahoogroups.com>
> >Sent: Wednesday, June 30, 2004 10:56 PM
> >Subject: Re: [noparse][[/noparse]basicstamps] Re: MOSFET keeps burning out
> >
> >
> > > > Assuming a GS voltage of 4.5V also means a Source voltage of
10V with
> > > > your Gate voltage of 5 volts, the SD voltage is 10 volts.
> > > >
> > > > The battery is 7.4 volts so the GS is only about 3 volts
(assuming a
> > > > 4.5v gate voltage)
> > > >
> > >
> > > I thought the originator of the thread stated that the motor
draws 10A.
> > > P = I^R so Power = 10^2 * .031 = 3.1 watts.....
> > > Working backwards, if we want to limit the case temp to 175C
with 25C
> > > ambient then 150C rise is max. 150/62.5 = 2.4watts
> > > 2.4watts/3.1watts = 77% max duty cycle.........I bet a cup of
StarBucks
> > > that this is the problem.
> > > Tom Fisher
> > > ....also remember that Rdson *increases* with junction temp
making
> > > that blue smoke appear even faster!!<G>
> > >
> > >
> > >
> > > >
> > > > The Thermal resistance of the junction to ambient is 62.5 deg
C per
> > > > watt and at the 1.766 watts, the case would be getting a 110C
rise.
> > > >
> > > > Since this is on a PWM output, I would assume the unit is not
on pure
> > > > DC (100% duty cycle), but on a lower duty cycle.
> > > >
> > > > I would agree that assuming a 25 deg C ambient and a 110 C
rise, the
> > > > package being at something approaching 150 C or more (open
board
> > > > would be cooler, a enclosed board would get hotter) heat
sinks would
> > > > be recommended. It is not quite reaching (theoretically) the
case
> > > > limit of 175 deg C, but it is getting HOT none-the-less.
> > > >
> > > > But, the safe amp operating area is of a concern also.
> > > >
> > > > Wadda-ya-think ?
> > > >
> > > > Dave
> > > >
> > > > Ps: my current pet peeve is that the MOSFET data sheets do
elaborate
> > > > more on selection. The unit is `rated' for 25A and 30V, but
with a
> > > > steady state ON condition, it cannot come anywhere near that.
> > > >
> >
> >
>
>
--
> Beau Schwabe Mask Designer II National Semiconductor
Corporation
> 500 Pinnacle Court, Suite 525
> Mail Stop GA1
>
--
This is really cool:
http://www.eeggs.com/items/27494.html
Secret message in the Stamp... You've been holding out on us, Jon :O)
TTYL
- Robert "8)
> It has been said time and time again that the PWM signal coming
> from the stamp is not exactly designed for motor speed control,
> but more over a way to provide an analog reference voltage from
> the Stamp.
>
Yep, the PWMPAL is really handy for my application which is
a controller for a 32KBtu/hr diesel fired coolant heater. I use one channel
to drive a fuel metering pump from 1.1Hz to 7.0Hz and another to
PWM the 12V supply voltage to an 8VDC glow pin for ignition.
I'd use another channel of the PWMPAL to drive the 12V/18A
combustion fan motor but the frequency is too low causing too
much noise, etc. The BS2p24 orchestrates all of this with ease
while monitoring fluid temp, over temp and flame temp thermistors
via the RCTIME command. The fan motor has a Hall effect RPM
sensor which is easily monitored with the PULSIN command.
Tom Fisher
============
Hey, guys and gals...
This is really cool:
http://www.eeggs.com/items/27494.html
Secret message in the Stamp... You've been holding out on us, Jon :O)
TTYL
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