[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,
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>
> *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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>>>>>
>>>>>
>>>>
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>>>
>>
>
>
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