[TriEmbed] N-MOSFET Symbol

Pete Soper pete at soper.us
Fri Mar 11 16:03:40 CST 2016

Oh. I thought BVDss was the Vsource - Vsource when the body diode 
conducts.  Sorry about that.

Rather than trust Wikipedia we might believe Infineon's app note AN 
2014-03 on page 6:

  " In switching converters, body diodes are common instruments used to 
provide the freewheeling capability at no extra component cost. It is an 
advantage in this case where no external diode is required."


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>
> *To:* Shane Trent <shanedtrent at gmail.com>; "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 
>> <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
>>>>>             _______________________________________________
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>>>>>             <http://triembed.org/>
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