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Good afternoon all,<br>
<br>
By limiting the gate current during turn-on and turn-off, you can
reduce or eliminate back EMF spikes from inductive loads. Not
knowing the mass or velocity of the motor at turn-off keeps me from
offering suggested series resistor values, but does introduce a
method of tuning.<br>
<br>
Connect an oscilloscope between the FET Drain and Source ... if the
source is floating above ground, use two scope channels in A+B mode,
and leave the probe grounds off.<br>
<br>
When switching on and off, you will see a square wave with a
(hopefully) minor spike. By increasing the resistor value from the
gate drive source, you can slow the gate turn-on (treat the gate as
the capacitor it is). Don't go too high, or you'll overheat the
transistor by staying in the linear (not hard-switched) mode.<br>
<br>
If you need different turn-on and turn-off values, connect two
resistors, with one end of both resistors connected to your gate
drive, the other end of each resistor is connected to a diode (high
speed please), one resistor to one diode's anode, the other resistor
to the other diode's cathode. The free ends of the two diodes then
connect to the gate.<br>
<br>
By adjusting the turn-on and turn-off resistance values, you can
greatly reduce the spikes induced by the rapid DI/DT (change in
current over time) created by the mosfet switch. If you have a known
load, you can also connect a series resistor/capacitor across the
drain and source ... you'll have to research snubbers to figure out
the appropriate values.<br>
<br>
For difficult snubber issues, such as when you are burning up
snubber RC networks, you can install a TVS between the drain and the
gate, and then a resistor from gate to ground (at the mosfet side of
any current limiting resistors). Choose the TVS voltage wisely,
greater than your typical VCC, but less than the maximum Vds. This
creates a self-snubbing circuit, where if the spike is big enough to
trigger the TVS, the FET will turn back on momentarily, and
thus"eating" the spike. You really do need to install a second TVS
between the gate and source, with a turn-on voltage less than the
Vgs rating of the mosfet. Be careful to ensure you don't set up a
nice, noisy power oscillator (yea, been there ... done that!).<br>
<br>
If you see that your spikes are minimal to non-existent ...
congratulations. <br>
<br>
Regards,<br>
<br>
Rick<br>
<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 3/10/2016 4:24 PM, Pete Soper via
TriEmbed wrote:<br>
</div>
<blockquote cite="mid:56E1E60B.2020102@soper.us" type="cite">
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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" class="moz-txt-link-freetext"
href="https://en.wikipedia.org/wiki/Power_MOSFET#Body_diode">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" class="moz-txt-link-freetext"
href="https://www.fairchildsemi.com/datasheets/FQ/FQP30N06L.pdf">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. :-)<br>
<br>
-Pete<br>
<br>
<br>
<div class="moz-cite-prefix">On 03/09/2016 06:44 PM, <a
moz-do-not-send="true" class="moz-txt-link-abbreviated"
href="mailto:kschilf@yahoo.com"><a class="moz-txt-link-abbreviated" href="mailto:kschilf@yahoo.com">kschilf@yahoo.com</a></a> wrote:<br>
</div>
<blockquote
cite="mid:945883586.5592057.1457567048429.JavaMail.yahoo@mail.yahoo.com"
type="cite">
<div style="color:#000; background-color:#fff;
font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial,
Lucida Grande, sans-serif;font-size:16px">
<div id="yui_3_16_0_1_1457562766890_7123">Hi Pete,</div>
<div id="yui_3_16_0_1_1457562766890_7122"><br>
</div>
<div id="yui_3_16_0_1_1457562766890_8027">Good note about
warning flags.</div>
<div id="yui_3_16_0_1_1457562766890_7471"><br>
</div>
<div id="yui_3_16_0_1_1457562766890_7543" 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 id="yui_3_16_0_1_1457562766890_7542" dir="ltr"><br>
</div>
<div id="yui_3_16_0_1_1457562766890_8028" dir="ltr">Sincerely,</div>
<div id="yui_3_16_0_1_1457562766890_8029" dir="ltr">Kevin
Schilf<br>
</div>
<div id="yui_3_16_0_1_1457562766890_7318"><span></span></div>
<div id="yui_3_16_0_1_1457562766890_7319"
class="qtdSeparateBR"><br>
<br>
</div>
<div style="display: block;"
id="yui_3_16_0_1_1457562766890_6997" class="yahoo_quoted">
<div id="yui_3_16_0_1_1457562766890_6996"
style="font-family: HelveticaNeue, Helvetica Neue,
Helvetica, Arial, Lucida Grande, sans-serif; font-size:
16px;">
<div id="yui_3_16_0_1_1457562766890_6995"
style="font-family: HelveticaNeue, Helvetica Neue,
Helvetica, Arial, Lucida Grande, sans-serif; font-size:
16px;">
<div id="yui_3_16_0_1_1457562766890_7071" dir="ltr"> <font
size="2" face="Arial">
<hr size="1"> <b><span style="font-weight:bold;">From:</span></b>
Pete Soper via TriEmbed <a moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:triembed@triembed.org"><triembed@triembed.org></a><br>
<b><span style="font-weight: bold;">To:</span></b> <a
moz-do-not-send="true"
class="moz-txt-link-abbreviated"
href="mailto:triembed@triembed.org"><a class="moz-txt-link-abbreviated" href="mailto:triembed@triembed.org">triembed@triembed.org</a></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 id="yui_3_16_0_1_1457562766890_6994"
class="y_msg_container"><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 class="yqt7247335099" id="yqtfd91986"><br
clear="none">
<br clear="none">
<br clear="none">
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<br>
<br>
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<pre wrap="">_______________________________________________
Triangle, NC Embedded Computing mailing list
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