[TriEmbed] N-MOSFET Symbol

Pete Soper pete at soper.us
Fri Mar 11 17:31:07 CST 2016

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, 
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?


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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