[TriEmbed] N-MOSFET Symbol

Shane Trent shanedtrent at gmail.com
Fri Mar 11 18:05:11 CST 2016


Pete,

In this case the body diode will not conduct unless the inductive kick
generates a voltage high enough to cause a breakdown failure of the FET or
body diode. A freewheel/snubber diode's job is to provide a current path
between the inductor terminals to allow the current to recirculate (can be
called a recirculation diode) until the losses in the circuit dissipate the
energy stored in the magnetic field. The FET body diode does not provide a
path between the inductor terminals. If the body diode could conduct (say
you instantly reverse the body diode polarity) then the coil is simply
energized through a forward biased diode and says on.

Interestingly you can kill a MOSFET at relatively low currents if using low
gate voltages. A MOSFET is actually not a single switching element but a
large array of parallel devices. Under typical use conditions (full gate
drive) each element in the FET has a resistance with a positive temperature
coefficient meaning that the elements carrying the most current get hotter
than their neighbors and therefore develop a higher resistance and this
reduces the current through them, allowing the large arrange of elements to
effectively share their load. This is the big benefit that MOSFETs have
over BJTs. BJTs have a negative temperature coefficient and require
external bias resistors to prevent one transistor from getting hot,
reducing its resistance, taking more current and getting hotter until the
smoke gets out.

But at low gate voltages, MOSFETs have a negative temperature coefficient
like BJTs so you can get localized heating of a cell, lowering it's
resistance, stealing current from neighbors cells until the cell has a
meltdown. This is why using MOSFET's in linear applications must be done
with great care (ensuring you stay above the Zero Temperature Coefficient
point. Google Spirito Effect for more details that you could ever want.

Shane

On Fri, Mar 11, 2016 at 6:31 PM Pete Soper via TriEmbed <
triembed at triembed.org> wrote:

> Double oops. Yes, BVDss is exactly what I thought: Vdrain - Vsource.
> Beyond this there is conduction, right? For this particular part this
> happens in an "all at once" fashion and the heat is spread evenly so it
> takes a relatively large, sustained current to risk damaging the chip,
> right?
> So if the body diode has avalanched it's close to a short circuit and so
> the current goes like this:
>
>         +V <<----|
>
>         coil     |
>
>         source   |
>
>         drain    |
>
>         ground >>|
>
> Now the body diode is a snubber/freewheeler/<insert five other synonyms>.
> If the supply voltage, inductance, turn off time , transistor selection are
> all OK then an external diode is a waste of money.
>
> What am I missing here?
>
>
> -Pete
>
>
> On 03/11/2016 04:31 PM, kschilf at yahoo.com wrote:
>
> Hi Pete,
>
> The voltage at each of the three terminals of the transistor (gate, drain,
> source) is a function of the rest of the circuit.  You can bias (set
> voltages, and draw currents) the transistor anyway you want, once you
> understand its behavior (and limits) at whatever operating point you set.
>
> It is possible to bias the source such that Vsource > Vdrain (Vds < 0).
> BVDss the maximum voltage difference (Vdrain - Vsource) exerted before you
> possibly damage the part.  This value is temperature dependent.
>
> Born before Wikipedia, I still believe in books.  :-)
>
> Since textbooks ain't cheap, borrow a sophomore level circuits text (NCSU
> library, etc.).  Peruse the chapter on BJT's and MOSFET's.  That should
> clear up some of the mystery.  :-)
>
> Don't let the smoke out (at least while anybody is looking!)  :-)
>
> Sincerely,
> Kevin Schilf
>
>
>
>
> ------------------------------
> *From:* Pete Soper via TriEmbed <triembed at triembed.org>
> <triembed at triembed.org>
> *To:* Shane Trent <shanedtrent at gmail.com> <shanedtrent at gmail.com>;
> "triembed at triembed.org" <triembed at triembed.org> <triembed at triembed.org>
> <triembed at triembed.org>
> *Sent:* Friday, March 11, 2016 12:38 PM
> *Subject:* Re: [TriEmbed] N-MOSFET Symbol
>
> If the transistor shorts out at 60 volts it's hard to get the source above
> 60 volts, right?
> -Pete
>
> On 03/11/2016 12:36 PM, Shane Trent wrote:
>
> Pete,
>
> Sorry,I do not understand the question.
>
> Shane
>
> On Fri, Mar 11, 2016 at 11:50 AM Pete Soper <pete at soper.us> wrote:
>
> 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:
>
>    http://datasheet.octopart.com/FQP30N06L-Fairchild-datasheet-82531.pdf
>
> 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?
>
>
>
> -Pete
>
>
>
> On 03/11/2016 10:40 AM, Shane Trent wrote:
>
> Pete,
>
> I think it is easier if you look at a half-bridge using just two
> transistors with a bi-polar power supply.
>
> 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. This is why h-bridge and half-bridge circuits with BJTs
> include catch diodes and ones with MOSFET typically do not. I like to
> imaging my explanations makes sense but I am never sure. So, did that make
> sense to you?
>
> 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.
>
> Shane
>
> On Thu, Mar 10, 2016 at 11:51 PM Pete Soper <pete at soper.us> wrote:
>
> 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> 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> <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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