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'[EE]: FET component evaluation'
2002\10\15@203029 by Ray Gallant

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My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The opto-coupler output is 5 volts.  The FET's high level input is 5 volts.  Will this opto drive the FET properly? (y/n) Cost is not an issue, but performance is.  I will address filtering later!
Can some of you FET-guruzzz kindly offer your feedback.  I'm concerned about current requirements.  Any suggestions with discretes would also be appreciated  Tx, {slewrate}
www.irf.com/product-info/datasheets/data/ips511.pdf
http://www.nve.com/isopdf/il710.pdf

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2002\10\15@210958 by hard Prosser

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Looking at the risetime & slew rate limits you may be pushing it at 20kHz.
Just the switching on & off will take up most oif thye available time -
which will push the power loss up also.

The IL710 shouldn't have any trouble driving the FET module at this speed
though - it's just the speed of the FET & its internal driver that would
concern me.
Also - the FET module is rated to 50V only - presumably for a 24V solenoid
you are running at about 50% PWM. Make sure your flyback diode is fast
enough. Once the solenoid is pulled in you can normally reduce the PWM to a
lower hold in value which can reduce power requirements considerably.
(Unless you are just pulsing it anyway).


Richard P



My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
opto-coupler output is 5 volts.  The FET's high level input is 5 volts.
Will this opto drive the FET properly? (y/n) Cost is not an issue, but
performance is.  I will address filtering later!
Can some of you FET-guruzzz kindly offer your feedback.  I'm concerned
about current requirements.  Any suggestions with discretes would also be
appreciated  Tx, {slewrate}
www.irf.com/product-info/datasheets/data/ips511.pdf
http://www.nve.com/isopdf/il710.pdf

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2002\10\15@215907 by Russell McMahon

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My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
opto-coupler output is 5 volts.  The FET's high level input is 5 volts.
Will this opto drive the FET properly? (y/n) Cost is not an issue, but
performance is.  I will address filtering later!

FET would need to be a "lohic level" device to be driven by 5 volts. ie look
at Vth spec and rating curves. Device should be fully turned on at Vgs=5v
AMD have a low enough Rdson to meet your need. For gate drives not much
above Vth FETs will suually exhibit a "current source" operation at far less
than their ratewd current. Can be useful if you want it and very annoying if
you are not expecting it.

FET needs essentially zero current when operated but needs current to
charge/discharge gate capacitance. This is typically arouind 1 mA at 29 kHz
foir typical FETS as a VERY ROUGH guide. (See data sheet for gate charge for
your FET to calculate real value).

Why are you PWMing a solenoid?  - it surely will not be responding at 20 KHz
:-) Back EMF will need to be accounted for.


               RM

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2002\10\15@221937 by hard Prosser

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Russel
The FET is not just a FET as such - it is a complete module including a
driver circuit. The load is connected to the source of the FET and there is
a separate ground connection for the logic. Input control is 0-5V logic
levels only.

The spec does, however, give limits for risetime & slew rate that I feel
may be marginal for 20kHz operation.
Presumably he is trying to drive a 24V solenoid from a higher voltage
supply or is attempting to use drive voltage or current to control the
position of the plunger.

Richard P




My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
opto-coupler output is 5 volts.  The FET's high level input is 5 volts.
Will this opto drive the FET properly? (y/n) Cost is not an issue, but
performance is.  I will address filtering later!

FET would need to be a "lohic level" device to be driven by 5 volts. ie
look
at Vth spec and rating curves. Device should be fully turned on at Vgs=5v
AMD have a low enough Rdson to meet your need. For gate drives not much
above Vth FETs will suually exhibit a "current source" operation at far
less
than their ratewd current. Can be useful if you want it and very annoying
if
you are not expecting it.

FET needs essentially zero current when operated but needs current to
charge/discharge gate capacitance. This is typically arouind 1 mA at 29 kHz
foir typical FETS as a VERY ROUGH guide. (See data sheet for gate charge
for
your FET to calculate real value).

Why are you PWMing a solenoid?  - it surely will not be responding at 20
KHz
:-) Back EMF will need to be accounted for.


               RM

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2002\10\16@022409 by Ray Gallant

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----- Original Message -----
From: "Russell McMahon"

> My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
> opto-coupler output is 5 volts.  The FET's high level input is 5 volts.
> Will this opto drive the FET properly? (y/n) Cost is not an issue, but
> performance is.  I will address filtering later!
_______________
> FET would need to be a "logic level" device to be driven by 5 volts. ie
look
> at Vth spec and rating curves. Device should be fully turned on at Vgs=5v
> AMD have a low enough Rdson to meet your need. For gate drives not much
> above Vth FETs will suually exhibit a "current source" operation at far
less
> than their ratewd current. Can be useful if you want it and very annoying
if
> you are not expecting it.
>
> FET needs essentially zero current when operated but needs current to
> charge/discharge gate capacitance. This is typically arouind 1 mA at 29
kHz
> foir typical FETS as a VERY ROUGH guide. (See data sheet for gate charge
for
> your FET to calculate real value).
>
> Why are you PWMing a solenoid?  - it surely will not be responding at 20
KHz
> :-) Back EMF will need to be accounted for.
>
>
>                 RM
____________
The solenoid:  --> in this proportional valve (uni-polar drive) pressure
compensated, involved to control hydraulic flow.  The output of the opto
offers the adaptation of couple (or various) driving circuit types.  Funny
for a guy who's handle is slewrate, I didn't closely observe all the
switching characteristics of the FET.  (puts face in hands!)
I am on a fast FET learning curve.  Great feedback, thanks to everyone.  I
will continue to seek for a better FET, perhaps an IRL530N
http://www.irf.com/product-info/datasheets/data/irl530n.pdf

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2002\10\16@033003 by Dwayne Reid

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At 09:27 PM 10/15/02 -0300, Ray Gallant wrote:
>My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
>opto-coupler output is 5 volts.  The FET's high level input is 5
>volts.  Will this opto drive the FET properly? (y/n) Cost is not an issue,
>but performance is.  I will address filtering later!

I've read some of your later responses and can offer my experience.

It sounds as if you are driving a hydraulic control valve.  I do this in
several products using a low frequency PWM current source based on a SG3526
PWM controller.  I mention that my feedback is current rather than voltage
- this reduces the inaccuracy caused by temperature changes in the solenoid
winding.  My PWM frequency is around 500 Hz - this low frequency causes
vibration which reduces the hysteresis caused by the solenoid core
'sticking' in place.

Now back to your FET driver.  You say that the opto-coupler output is
5V.  Where is the 5V supply coming from?

dwayne

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2002\10\16@053952 by Roman Black

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Ray Gallant wrote:

> > My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
> > opto-coupler output is 5 volts.  The FET's high level input is 5 volts.
> > Will this opto drive the FET properly? (y/n) Cost is not an issue, but
> > performance is.  I will address filtering later!

Since you seem to be having some difficulties with
the FET implementation, why not use a simpler system
with a power transistor?? I can understand the hassle
of a FET being worthwhile for a 20 amp h-bridge but
why to drive a 800mA solenoid??

For such a low power application you can use any common
decent performance transistor. These are my tested
figures for driving 1A loads etc and is a perfect
application for these parts;

part         Vcesat @ Ic @ Ib
BD203   NPN  183mV    1A   25mA    $0.50
TIP41C  NPN  343mV    1A   10mA    $0.55
TIP41C  NPN  213mV    1A   25mA    $0.55
MTP3055 FET  212mV    1A  (Vgs 5v) $0.85

All of these cheap parts will drive a 1A load and stay
cold even without a heatsink. They can all be driven
direct from a PIC pin (or opto) via one resistor.
PWM of 28v 800mA and 20kHz will not be a problem
with switching speed if driven from a PIC output.
-Roman

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2002\10\16@071734 by Ray Gallant

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----- Original Message -----
From: "Roman Black" <.....fastvidKILLspamspam@spam@EZY.NET.AU>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Wednesday, October 16, 2002 6:33 AM
Subject: Re: [EE]: FET component evaluation


> Ray Gallant wrote:
>
> > > My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).
The
> > > opto-coupler output is 5 volts.  The FET's high level input is 5
volts.
{Quote hidden}

I have used an A14 and 2N6045 (Motorola as per other discussions, right)
darlingtons combo which work great, but I am concerned about the ringing
affect and I had a little to much drop across Vce.  Your right, maybe I can
improve on this one. Tx,

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2002\10\16@084524 by Ray Gallant

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From: "Dwayne Reid"

Ray Gallant wrote:
> >My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
> >opto-coupler output is 5 volts.  The FET's high level input is 5
> >volts.  Will this opto drive the FET properly? (y/n) Cost is not an
issue,
> >but performance is.  I will address filtering later!
>
> I've read some of your later responses and can offer my experience.
>
> It sounds as if you are driving a hydraulic control valve.  I do this in
> several products using a low frequency PWM current source based on a
SG3526  PWM controller.  I mention that my feedback is current rather than
voltage this reduces the inaccuracy caused by temperature changes in the
solenoid  winding.  My PWM frequency is around 500 Hz - this low frequency
causes vibration which reduces the hysteresis caused by the solenoid core
> 'sticking' in place.
>
> Now back to your FET driver.  You say that the opto-coupler output is
> 5V.  Where is the 5V supply coming from?
>_________________
Things are going slow today: had to replace the thermal circuit breaker in
my coffee machine! grrrr.
All on the high side of the opto:  I have a 24VDC.  I plan, later, to add
filter stages with plenty of bandwidth utilizing operation voltage of 5VDC.
For the moment, I have used a LM317L regulator to provide 5VDC.

Tx for the feedback, I will definitely check out this concept/avenue.
{slewrate}

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