[TriEmbed] N-MOSFET Symbol

Shane Trent shanedtrent at gmail.com
Thu Mar 10 16:59:45 CST 2016


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> 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 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>
> <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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