[TriEmbed] N-MOSFET Symbol

Pete Soper pete at soper.us
Thu Mar 10 22:51:35 CST 2016


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?
-Pete
On 03/10/2016 05:59 PM, Shane Trent wrote:
> Pete,
>
> 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.
>
> 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.
>
> 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.
>
> Shane
>
> On Thu, Mar 10, 2016 at 4:24 PM Pete Soper via TriEmbed 
> <triembed at triembed.org <mailto:triembed at triembed.org>> wrote:
>
>     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.
>
>     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:
>
>     https://en.wikipedia.org/wiki/Power_MOSFET#Body_diode
>
>     Here is the transistor Brian's kids are going to use:
>
>     https://www.fairchildsemi.com/datasheets/FQ/FQP30N06L.pdf
>
>     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.)
>
>     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!
>
>     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. :-)
>
>
>     -Pete
>
>
>
>     On 03/09/2016 06:44 PM, kschilf at yahoo.com
>     <mailto:kschilf at yahoo.com> wrote:
>>     Hi Pete,
>>
>>     Good note about warning flags.
>>
>>     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.  :-)
>>
>>     Sincerely,
>>     Kevin Schilf
>>
>>
>>     ------------------------------------------------------------------------
>>     *From:* Pete Soper via TriEmbed <triembed at triembed.org>
>>     <mailto:triembed at triembed.org>
>>     *To:* triembed at triembed.org <mailto:triembed at triembed.org>
>>     *Sent:* Wednesday, March 9, 2016 5:25 PM
>>     *Subject:* Re: [TriEmbed] N-MOSFET Symbol
>>
>>     I'm pretty sure about 70% of Brian's interest in this subject
>>     involves
>>     dealing with inductive loads. The body diode in the schematic
>>     symbol is
>>     a merciful hint.  If his kids can remember that the lack of a
>>     body diode
>>     is a red flag they might avoid blowing up their BJTs or adding
>>     redundant
>>     components.
>>
>>     -Pete
>>
>>
>>
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>>
>
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