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'[EE]: IRL540 MOSFET Question'
2001\07\17@132943 by Dan Michaels

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Another question related to motor control. I am looking at
building a discrete hi-current h-bridge using IRL540 [n-chan]
and IRL5305 [p-chan] MOSFETs.

These devices have an internal "body diode" that is in the
exact configuration that, if 4 of these devices were hooked
up in a full h-bridge config [p-chan above, n-chan below],
that the diodes would fit together to form a perfect bridge
clamp.

The question is - the specs indicate these body diodes are
quite fast and able to handle large currents, and I just wanted
some outside verification that they are adequate to the inductive
spike clamping job. Yes ????//

thanks,
- dan michaels
http://www.oricomtech.com
=========================

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2001\07\17@165220 by steve

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> The question is - the specs indicate these body diodes are
> quite fast and able to handle large currents, and I just wanted
> some outside verification that they are adequate to the inductive
> spike clamping job. Yes ????//

I don't have the data for that particular FET but generally the
answer is yes. The part that you do have to be aware of is that the
diode will be dissipating power while current is flowing through it. If
you have an over-spec'd FET to get low Rds-on (and therefore lower
Pd), it may be negated by the power dissipated by the 0.7V diode
drop when the current is going the other way. This still heats up the
die and drops the working ratings of your FET.

I think there may be a worked example somewhere in the archives.

Steve.


======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
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2001\07\17@170202 by Spehro Pefhany

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At 08:46 AM 7/18/01 +1200, you wrote:

>I don't have the data for that particular FET but generally the
>answer is yes. The part that you do have to be aware of is that the
>diode will be dissipating power while current is flowing through it. If
>you have an over-spec'd FET to get low Rds-on (and therefore lower
>Pd), it may be negated by the power dissipated by the 0.7V diode
>drop when the current is going the other way. This still heats up the
>die and drops the working ratings of your FET.

In some situations you can turn the FET on  rather than let the
diode conduct the current. They will conduct in both directions...

Best regards,

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2001\07\17@173635 by Brent Brown

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> The question is - the specs indicate these body diodes are
> quite fast and able to handle large currents, and I just wanted
> some outside verification that they are adequate to the inductive
> spike clamping job. Yes ????//

Yes. As Steve said there should be some more on this in the
archives. I have used MOSFETs purely for their inherent diode by
strapping the gate to the source to keep the transistor off. If the
data specs look right then go for it. With a full bridge I guess you
may be doing high freq PWM into an inductive load or motor. In that
case the current is more or less constant in both the diode and
transistor, main power losses are I x V in the diode and I^2 x R in
the transistor. Depending on your control scheme you may have the
option of turning on the MOSFET's that are behaving like diodes and
reduce their power dissipation. Hope this helps.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brownspamKILLspamclear.net.nz

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2001\07\17@183637 by Olin Lathrop

face picon face
> The question is - the specs indicate these body diodes are
> quite fast and able to handle large currents, and I just wanted
> some outside verification that they are adequate to the inductive
> spike clamping job. Yes ????//

Usually yes, but you have to consider what is on the other side of those
diodes too and how it might handle a "backwards" current spike.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam.....embedinc.com, http://www.embedinc.com

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2001\07\18@012144 by Roman Black

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Yep! They're designed for driving inductive loads
and the diodes match the FETs for performance.
The diodes are generally schottky or fast recovery
and have low on drop, 0.2v at medium currents is
common. But as someone said, often it's more
efficient to turn the FET on for flyback and let
the FET handle the reverse electrons.
-Roman


Dan Michaels wrote:
{Quote hidden}

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2001\07\18@122343 by Hartung, Greg

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  I have been trying to a get 4 of these working in an H-bridge also, but
perhaps I've been lead astray.  I was lead to believe that the gate voltage
needed to be 5v+Vds.  Is 5v enough to turn a 540 on completely?

{Original Message removed}

2001\07\18@160529 by Dan Michaels

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Hartung, Greg wrote:
>   I have been trying to a get 4 of these working in an H-bridge also, but
>perhaps I've been lead astray.  I was lead to believe that the gate voltage
>needed to be 5v+Vds.  Is 5v enough to turn a 540 on completely?
>

Greg,

From what I can tell from the datasheet, the answer is yes. The Vgs
threshold is 4v max, so if you tie the 540 source to gnd, then 5v out of
a PIC should do it. However, if you are also using a 540 for the upper
transistor in the bridge, then you will not be able to turn it on without
a larger voltage.

Better probably to have p-chan MOSFETs in the upper level, with "source"
wired to Vpower - ie, mirror image to n-chan MOSFET below - and both
gates wired together. Then the same 5v in will turn the upper one off, and
the n-chan in the lower position on - and vice versa for 0v in. This is
exactly how the output stage in any CMOS chip works - here the
PMOSFET/upper - NMOSFET/lower h-bridge is basically emulating same.

Note - this will only work for Vpower = 5v. If you want to run the
h-bridge at higher voltage, then you will need a level-shifter -
std npn inverter with collector pullup to Vpower and collector
also tied to the MOSFET gates should work, I think.

Also, you might look into using "logic-level" MOSFETs, with
Vgs threshold only ~2v, rather than 4v.

cheers,
- dan michaels
http://www.oricomtech.com
=========================

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2001\07\19@113725 by Dan Michaels

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Roman wrote:
>Yep! They're designed for driving inductive loads
>and the diodes match the FETs for performance.
>The diodes are generally schottky or fast recovery
>and have low on drop, 0.2v at medium currents is
>common. But as someone said, often it's more
>efficient to turn the FET on for flyback and let
>the FET handle the reverse electrons.


Thanks to everyone who responded to the IRF540 question.

Very interesting about the heat generation issue. Now
I am wondering whether I can reduce heat buildup in an
L293 h-bridge chip using similar fiddling. Next step,
on to the smoke tests.

thanks,
- dan michaels
http://www.oricomtech.com
=========================

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2001\07\19@171738 by Hartung, Greg

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  Well, I forgot to mention I am using the IRL540N, which is the logic
level version.  Anyway, I tore the circuit apart last night and started with
the basics.  Would 5v turn the motor on thru a single 540n?  Yes, but only
if the FET was sinking the motor, not sourcing it.  Attempting to source
current for the motor resulted in IRL meltdown.  So in a cascaded scenario
with the motor between source and drain, only one of the FETs works.  Vmotor
is a 7.2v RC battery.  Motors will draw about 20-30A.
  heeeeeeeeeeeeeeeeeeeeeelp

{Original Message removed}

2001\07\20@085912 by Eisermann, Phil [Ridg/CO]

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Remember that gate voltage of the upper FET has to be +Vt (+5) above the
source voltage. And the on-state resistance increases with decreasing gate
voltage (or decreases with increasing gate voltage). If the upper FET is
just barely on, then Rds will be relatively high, resulting in increased
power dissipation.

I didn't follow this thread. Are you using heatsinks? PWM? Do you only have
5V available? I would probably use a bjt level shifter to drive the gate at
10V or higher (charge pump would work well enough for this), or go to
p-channel fets for the high-side switches. Alternatively, International
Rectifier and others make a variety of H-bridge driver chips that will
generate the voltage for you.

I'd also be highly tempted to use a FET with a lower Rds rating, and drive
it with the maximum voltage possible. Look at the datasheet, and find where
it shows a graph of Ids vs. Vds for varying levels of Vgs (gate voltage).
Even though the FET will turn on with 5V and is rated for 36A, it's not
going to conduct 36V with 5V of gate drive. I'd estimate that each FET is
dropping well over 2V with your 5V gate drive. Don't use the curves for
Tj=25C, because the junction temperature will never be 25C under use,
especially if you've got 30A going through the device. Not unless you do
some heavy-duty cooling (and i don't mean just a heatsink), anyway.

> {Original Message removed}

2001\07\20@111626 by Dan Michaels

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Phil, as you see below, I already told him he does not have
enuf voltage to turn the upper MOSFET on when stacking 2
n-channels, but ????????????



Eisermann, Phil wrote:
{Quote hidden}

>> {Original Message removed}

2001\07\20@113319 by Roman Black

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Eisermann, Phil [Ridg/CO] wrote:
>
> Remember that gate voltage of the upper FET has to be +Vt (+5) above the
> source voltage.

> I didn't follow this thread. Are you using heatsinks? PWM? Do you only have
> 5V available?


Seems that even in the few months i've been on
the piclist there seems to be an awful lack of
info on how to make a good h-bridge, and/or
how to use many of the good h-bridge chips that
are available. Most of the experienced guys can
do it but there is a never ending stream of
newbies with h-bridge questions.

Maybe someone should put up a h-bridge page??
Piclist h-bridge FAQs?? Suggestions?
-Roman

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2001\07\20@113744 by Eisermann, Phil [Ridg/CO]

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I was responding to his message about the upper FET not turning on. I didn't
follow the whole thread, so I didn't see your comment (nor did i read all of
the included responses). If you already told him, and he tried it again and
wondered why it didn't work, i guess it's worth mentioning again.

sorry for wasting everyone's bandwidth.


{Quote hidden}

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2001\07\20@115016 by Hartung, Greg

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  I apologize for asking the same question twice.  I'm just not very good
at this stuff.  It took me awhile to grasp the concept that it would take a
different voltage to turn the top on than the bottom.  I want to avoid the
complication of driver circuits or charge pumps (mostly for size).  Does the
IRL5602 look like a suitable p-channel for an upper stage?  Thanks for the
help!

{Original Message removed}

2001\07\20@115708 by Eisermann, Phil [Ridg/CO]

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       I'm sure there are lots of app-notes out there. I guess if someone
went through the trouble of a FAQ, it ought to be in very simple,
non-technical terms. It should definitely explain what the specs in the data
sheet mean. How to turn it on, how to turn it on/off fast (eg what's the
effect of gate capacitance and why that means you really do want lots of
current into the gate). Seems the things that most frequently confuses
people is 1) driving a high-side N-channel FET: gate-source voltage means
voltage from gate to source. if the source isn't sitting at ground, then
you'd better increase the voltage applied to the gate by that amount! and 2)
how to properly select a FET for your application. People most often look at
the max current and voltage ratings, perhaps the Rds(on) and select a device
based on that. Hmm. Maybe the FAQ needs a primer on power ratings and power
dissipation (especially during switching)? And maybe a few simple diagrams
to illustrate the concept? Or perhaps i'm making it out to be more
complicated than it really is?


> {Original Message removed}

2001\07\20@120113 by Roman Black

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Hi Greg, (re h-bridge questions) have you tried
doing a web search for "h-bridge motor" or
"h-bridge circuit" ??

I know there are some good circuits out there,
having come across them myself from time to time.
It would be nice if someone compiled these in one
place so the information was easily accessible.
I would do it but have been very busy the last
couple of weeks.
-Roman


Hartung, Greg wrote:
>
>    I apologize for asking the same question twice.  I'm just not very good
> at this stuff.  It took me awhile to grasp the concept that it would take a
> different voltage to turn the top on than the bottom.  I want to avoid the
> complication of driver circuits or charge pumps (mostly for size).  Does the
> IRL5602 look like a suitable p-channel for an upper stage?  Thanks for the
> help!
>

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2001\07\20@142816 by Dan Michaels

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Roman wrote:
>Hi Greg, (re h-bridge questions) have you tried
>doing a web search for "h-bridge motor" or
>"h-bridge circuit" ??
>
>I know there are some good circuits out there,
>having come across them myself from time to time.
>It would be nice if someone compiled these in one
>place so the information was easily accessible.
>I would do it but have been very busy the last
>couple of weeks.


Roman, I am surprised that "you" don't have this info on your
page. I did a search to find some ckts to reference on my page:

http://www.oricomtech.com/robolnk2.htm#Mot2

Of these, the ones I found most compelling were:

www.cadvision.com/blanchas/hexfet/h-bridge.htm
http://www.dprg.org/hbridge.html

[the last has a nice little ckt for sensing motor current].

These show a p-chan MOSFET stacked above an n-chan, but
unfortunately many show the drain of the p-chan connected
to +V, rather than the source, so I am wondering what gives.

- dan
===================

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2001\07\20@185709 by David P. Harris

picon face
Hi-

Me don't know much about this.  But I am into model trains - these days
they have brains in the form of PICs :-) and use H-bridges for the
motors.  Also, as the signal is transmitted on top of the track voltage,
we use H-bridges to do this too (1-10A is usual range).

Anyway, after looking though lots of circuits (on the web, at
manufacturing sites etc), and looking at High-Side drivers etc, one of
the simplest H-bridges is on the MERG site in Britain.  It uses a
CD40106 hex driver and a pair of N and pair of P-Fets:
http://home.freeuk.net/merg/resources/bc1asch.pdf  (it even uses a PIC
;-)

NB- the PIC is doing some important timing so that the upper and lower
Fets are NOT TURNED ON TOGETHER, otherwise large currents will flow, but
only for a short time.

David

Dan Michaels wrote:

{Quote hidden}

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2001\07\20@192041 by Tom Handley

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  Roman, I should put up some MOSFET circuits on my page. Here are some
good links for motor control and H-Bridge theory and applications:

     *EXCELLENT* site for motor info at:
     http://www.us-epanorama.net/motorcontrol.html#general

     Chuck McMannis. Good theory and both Bipolar and MOSFET H-Bridge ckts:
     http://professionals.com/~cmcmanis//robotics/servo.html

     Good theory with circuit examples of different types of H-Bridges:
     http://www.cadvision.com/blanchas/hexfet/

     Bob Blick's site with a good Bipolar-based H-Bridge project:
     http://www.bobblick.com/bob/projects/

  - Tom

At 01:33 AM 7/21/01 +1000, Roman Black wrote:
{Quote hidden}

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New Age Communications
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2001\07\20@195249 by Tom Handley

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  Dan, I've been following this but I'm not sure if you are going to
use an IC or discrete bridge? If it's the latter, I'd recommend looking at
IR's HEXFETs with built-in diodes. They have versions with a Vgs of 2-4V for
logic compatibility. I've used them in several H-Bridges up to 20A. I'm
also using these devices in my robot. Let me know if I can help.

  - Tom

At 02:28 PM 7/20/01 -0400, Dan Michaels wrote:
{Quote hidden}

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2001\07\20@215713 by Dan Michaels

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Tom Handley wrote:
>   Dan, I've been following this but I'm not sure if you are going to
>use an IC or discrete bridge? If it's the latter, I'd recommend looking at
>IR's HEXFETs with built-in diodes. They have versions with a Vgs of 2-4V for
>logic compatibility. I've used them in several H-Bridges up to 20A. I'm
>also using these devices in my robot. Let me know if I can help.
>


Hi Tom, that is exactly the devices we have been talking about.
Also, I was trying to find some good links to help Greg Hartung
with his ckt that he is having some problems with - you might
take a look back at his comments.

best regards,
- dan michaels
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2001\07\21@035334 by Peter L. Peres

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You need the complementary FET for the 540 for the high side or a charge
pumpto provide the gate voltages for the high side.

Peter

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2001\07\21@035750 by Peter L. Peres

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Most mosfets have normal Si PN junction 'inverse' diodes, unless they have
an integrated Shottky (I've never seen one of those, except for switching
power supplies where the Shottky Anode has a separate lead). This means
that they drop 0.6V, not 0.2V.

The L293 has bipolar switches and I do not think that they will conduct in
reverse.

Peter

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2001\07\21@105051 by Roman Black

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Tom Handley wrote:
>
>    Roman, I should put up some MOSFET circuits on my page. Here are some
> good links for motor control and H-Bridge theory and applications:
>
>       *EXCELLENT* site for motor info at:
>       http://www.us-epanorama.net/motorcontrol.html#general
>
>       Chuck McMannis. Good theory and both Bipolar and MOSFET H-Bridge ckts:
>       http://professionals.com/~cmcmanis//robotics/servo.html
>
>       Good theory with circuit examples of different types of H-Bridges:
>       http://www.cadvision.com/blanchas/hexfet/
>
>       Bob Blick's site with a good Bipolar-based H-Bridge project:
>       http://www.bobblick.com/bob/projects/
>
>    - Tom


Hi Tom, have you put those 4 links up on your web
page? If so, that is a great place to start! :o)
We need all the good h-bridge info and links in
one place, so when a newbie posts a question we
can just point to the "h-bridge page"...
:o)
-Roman

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2001\07\21@161820 by Hans W

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"Peter L. Peres" wrote:

> The L293 has bipolar switches and I do not think that they will conduct in
> reverse.

Diodes are needed for the L298 see circuit diagram at my web site
http://hans-w.com the cnc page

If the voltage is high enough the bipolar semi will break down and the conduct
very well :-)

hansw

>
>
> Peter
>
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