[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
>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
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>>>>>>
>>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>
>>>
>>
>>
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>>
>
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