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Out in the world there are droves of H bridge motor control circuits
with beefy MOSFETS and no diodes in sight except the body diodes.
How is that possible?<br>
-Pete<br>
<div class="moz-cite-prefix">On 03/10/2016 05:59 PM, Shane Trent
wrote:<br>
</div>
<blockquote
cite="mid:CAAFjd3p4Oew7P6kJsh936B=KzNheVBOpVzYCQM2pQiQSPz75+Q@mail.gmail.com"
type="cite">
<div dir="ltr">Pete,
<div><br>
</div>
<div>I believe you still need the snubber even with the body
diode. A snubber is typically placed across the inductor
(motor or solenoid or relay coil) and not across the switching
element. </div>
<div><br>
</div>
<div>For example, if you turn off an N-FET supplying several
amps to a large solenoid, when you turn the FET off the
collapsing magnetic field of the coil will cause the voltage
across the solenoid terminals to increase. The N-FET will
neither forward conduct or reverse conduct via the body diode
until the transistors breakdown voltage (Vds max) is exceeded
and the FET fails. </div>
<div><br>
</div>
<div>The tradeoff with using a diode snubber (it seems to be
more of a voltage clamp) across the coil is that it will act
as a catch diode or recirculation diode and cause the solenoid
to turn off more slowly. You can strike a balance between
voltage and turn-off speed by combining a regular diode and
Zener diode to allow the voltage to increase across the
solenoid without exceeding the FET's maximum voltage rating.
But there are MANY ways to design inductive clamps. </div>
<div><br>
</div>
<div>Shane<br>
<br>
<div class="gmail_quote">
<div dir="ltr">On Thu, Mar 10, 2016 at 4:24 PM Pete Soper
via TriEmbed <<a moz-do-not-send="true"
href="mailto:triembed@triembed.org">triembed@triembed.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"> This may come
across as high-minded, but really I just want to pass it
along as something that's hopefully on target. This
topic forced me to go study and read and I'm looking for
confirmation I'm not misleading anybody.<br>
<br>
The specific motor control application that I think
might be relevant to Brian's kids is treated with the
"freewheeling diode"s link on this page:<br>
<br>
<a moz-do-not-send="true"
href="https://en.wikipedia.org/wiki/Power_MOSFET#Body_diode"
target="_blank">https://en.wikipedia.org/wiki/Power_MOSFET#Body_diode</a><br>
<br>
Here is the transistor Brian's kids are going to use:<br>
<br>
<a moz-do-not-send="true"
href="https://www.fairchildsemi.com/datasheets/FQ/FQP30N06L.pdf"
target="_blank">https://www.fairchildsemi.com/datasheets/FQ/FQP30N06L.pdf</a><br>
<br>
This transistor can handle 32 amps of avalanche current
and is specifically designed for inductive loads. The
body diode in this transistor qualifies as a snubber
when a motor is turned off and is "freewheeling". The
energy will go straight to ground without incident.
Searching for this part number and "motor" gives a
number of hits where hobby folks are putting rectifiers
across the motor windings. This strikes me as redundant.
(At this point one might think "but wait, this
transistor is only rated at 60 volts source to drain".
But when the coil field collapses and the source voltage
shoots up the transistor junction "avalanches" and
begins to conduct current very quickly, yanking the
voltage right down close to ground. The "avalanche
feature" of the transistor is manufacturing technique
that avoids "hot spots" that might ruin the part.)<br>
<br>
Sorry for assuming we more or less knew the application:
wimpy little low power motors with massive overkill
components. And I'm probably running the risk of
causing folks to blow up their parts by not simply
recommending a separate snubber. It may be going too
far to suggest that the body diode should be included in
the schematic when it can be considered a snubber, but I
confess this the frame of mind I'd developed before the
discussion woke me up. I'll be reading datasheets more
carefully in the future!<br>
<br>
Ah, but we haven't mentioned improperly switching the
transistor and having it sit in its linear zone. I claim
the local record for how fast a MOSFET can desolder
itself when this happens at six amperes to a small SMD.
:-)</div>
<div bgcolor="#FFFFFF" text="#000000"><br>
<br>
-Pete</div>
<div bgcolor="#FFFFFF" text="#000000"><br>
<br>
<br>
<div>On 03/09/2016 06:44 PM, <a moz-do-not-send="true"
href="mailto:kschilf@yahoo.com" target="_blank">kschilf@yahoo.com</a>
wrote:<br>
</div>
<blockquote type="cite">
<div
style="color:#000;background-color:#fff;font-family:HelveticaNeue,Helvetica
Neue,Helvetica,Arial,Lucida
Grande,sans-serif;font-size:16px">
<div>Hi Pete,</div>
<div><br>
</div>
<div>Good note about warning flags.</div>
<div><br>
</div>
<div dir="ltr">I have no idea about the
application. Current in an inductor can not
change instantaneously. If you are going to
interrupt the circuit, you should provide a path
to allow the inductor current to continue (catch
diode in a switching power supply) or diminish
(diode across a relay winding), etc. If not, you
let Mr. Murphy determine where the energy will go,
sometimes with exciting consequences. :-)</div>
<div dir="ltr"><br>
</div>
<div dir="ltr">Sincerely,</div>
<div dir="ltr">Kevin Schilf<br>
</div>
<div><span></span></div>
<div><br>
<br>
</div>
<div style="display:block">
<div style="font-family:HelveticaNeue,Helvetica
Neue,Helvetica,Arial,Lucida
Grande,sans-serif;font-size:16px">
<div style="font-family:HelveticaNeue,Helvetica
Neue,Helvetica,Arial,Lucida
Grande,sans-serif;font-size:16px">
<div dir="ltr"> <font face="Arial" size="2">
<hr size="1"> <b><span
style="font-weight:bold">From:</span></b>
Pete Soper via TriEmbed <a
moz-do-not-send="true"
href="mailto:triembed@triembed.org"
target="_blank"><triembed@triembed.org></a><br>
<b><span style="font-weight:bold">To:</span></b>
<a moz-do-not-send="true"
href="mailto:triembed@triembed.org"
target="_blank">triembed@triembed.org</a>
<br>
<b><span style="font-weight:bold">Sent:</span></b>
Wednesday, March 9, 2016 5:25 PM<br>
<b><span style="font-weight:bold">Subject:</span></b>
Re: [TriEmbed] N-MOSFET Symbol<br>
</font> </div>
<div><br>
I'm pretty sure about 70% of Brian's
interest in this subject involves <br
clear="none">
dealing with inductive loads. The body diode
in the schematic symbol is <br clear="none">
a merciful hint. If his kids can remember
that the lack of a body diode <br
clear="none">
is a red flag they might avoid blowing up
their BJTs or adding redundant <br
clear="none">
components.<br clear="none">
<br clear="none">
-Pete
<div><br clear="none">
<br clear="none">
<br clear="none">
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<br>
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