[TriEmbed] N-MOSFET Symbol

Pete Soper pete at soper.us
Fri Mar 11 18:19:57 CST 2016


Well that couldn't be written any more clearly. I jumped to the 
conclusion that this very beefy transistor was designed to tolerate 
reverse bias breakdown up to a much greater current (for reasonably 
short times) compared to more wimpy FETs. Likewise, various test 
circuits in the datasheets tricked me. Thanks for your patience.
-Pete

On 03/11/2016 07:05 PM, Shane Trent wrote:
> 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 <mailto: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
>     <mailto: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>
>>     <mailto:triembed at triembed.org>
>>     *To:* Shane Trent <shanedtrent at gmail.com>
>>     <mailto:shanedtrent at gmail.com>; "triembed at triembed.org"
>>     <mailto:triembed at triembed.org> <triembed at triembed.org>
>>     <mailto: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
>>>     <mailto: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
>>>>         <mailto: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
>>>>>             <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
>>>>>>
>>>>>>
>>>>>>
>>>>>>                 _______________________________________________
>>>>>>                 Triangle, NC Embedded Computing mailing list
>>>>>>                 TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>>>>>>                 http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>>>>>                 TriEmbed web site: http://TriEmbed.org
>>>>>>
>>>>>>
>>>>>
>>>>>                 _______________________________________________
>>>>>                 Triangle, NC Embedded Computing mailing list
>>>>>                 TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>>>>>                 http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>>>>                 TriEmbed web site: http://TriEmbed.org
>>>>>
>>>>
>>>
>>
>>
>>     _______________________________________________
>>     Triangle, NC Embedded Computing mailing list
>>     TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>>     http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>     TriEmbed web site: http://TriEmbed.org
>>
>>
>
>     _______________________________________________
>     Triangle, NC Embedded Computing mailing list
>     TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>     http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>     TriEmbed web site: http://TriEmbed.org
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20160311/e67fea70/attachment-0001.html>


More information about the TriEmbed mailing list