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That was very clear about the other transistor forward conducting.
One last question. Here's the datasheet for the transistor Brian
Grawburg started us with:<br>
<br>
<a class="moz-txt-link-freetext" href="http://datasheet.octopart.com/FQP30N06L-Fairchild-datasheet-82531.pdf">http://datasheet.octopart.com/FQP30N06L-Fairchild-datasheet-82531.pdf</a><br>
<br>
In the context of the simple case of one of these transistors
driving a motor what does it mean for the drain-source breakdown
voltage BVdss to be the same as the max drain-source voltage Vdss
together with the avalanche current and diode recovery specs?<br>
<br>
<br>
-Pete<br>
<br>
<br>
<div class="moz-cite-prefix">On 03/11/2016 10:40 AM, Shane Trent
wrote:<br>
</div>
<blockquote
cite="mid:CAAFjd3rzyw84YRuRv_gQzbyVH50uv3-6RM--BhYmy6zX55k5jg@mail.gmail.com"
type="cite">
<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
moz-do-not-send="true" 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 moz-do-not-send="true"
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 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">
_______________________________________________<br clear="none">
Triangle, NC Embedded Computing
mailing list<br clear="none">
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target="_blank">TriEmbed@triembed.org</a><br
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href="http://mail.triembed.org/mailman/listinfo/triembed_triembed.org"
target="_blank">http://mail.triembed.org/mailman/listinfo/triembed_triembed.org</a><br
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TriEmbed web site: <a
moz-do-not-send="true"
shape="rect"
href="http://triembed.org/"
target="_blank">http://TriEmbed.org</a><br
clear="none">
</div>
<br>
<br>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<br>
</div>
_______________________________________________<br>
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