<div dir="ltr">Pete,<div><br></div><div>I think it is easier if you look at a half-bridge using just two transistors with a bi-polar power supply. </div><div><br></div><div>Let's assume we have +/12V on the power rails with one terminal of the motor grounded and the other connected to your half-bridge output. We run the motor forward by turning on the top FET and applying +12V to the motor terminal and run it backward by turning on the bottom FET and applying -12V to the motor output. In this case when you cut the power to the motor the body diode of the FET that was NOT conducting acts as the catch diode for the motor (the body diode of the FET that was used to apply power does not conduct any current). So if you decide to drive the motor in only one direction and remove one of the FETs, you will have to add a catch diode since you removed the body diode of the 2nd FET which was acting as your catch diode. <span style="line-height:1.5">This is why h-bridge and half-bridge circuits with BJTs include catch diodes and ones with MOSFET typically do not.</span><span style="line-height:1.5"> </span><span style="line-height:1.5">I like to imaging my explanations makes sense but I am never sure. So, did that make sense to you?</span></div><div><br></div><div>You can use external catch diodes with a MOSFET full or half-bridge but you need to ensure the external diodes have a lower Vf than the FET body diodes to ensure the external diodes conduct before the body diodes. You may also see fast external diodes used with a FET to clamp inductive current spikes faster than the FET body diode can conduct, clamping the current spikes a lower voltage.</div><div><br></div><div>Shane</div></div><br><div class="gmail_quote"><div dir="ltr">On Thu, Mar 10, 2016 at 11:51 PM Pete Soper <<a href="mailto:pete@soper.us">pete@soper.us</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">
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?</div><div bgcolor="#FFFFFF" text="#000000"><br>
-Pete</div><div bgcolor="#FFFFFF" text="#000000"><br>
<div>On 03/10/2016 05:59 PM, Shane Trent
wrote:<br>
</div>
<blockquote 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 href="mailto:triembed@triembed.org" target="_blank">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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