<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 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 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 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 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 href="mailto:triembed@triembed.org" target="_blank"><triembed@triembed.org></a><br>
<b><span style="font-weight:bold">To:</span></b> <a 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">
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</div>
</div>
</blockquote>
<br>
</div>
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