[TriEmbed] N-MOSFET Symbol

Pete Soper pete at soper.us
Thu Mar 10 15:24:27 CST 2016


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 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>
> *To:* 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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