[TriEmbed] N-MOSFET Symbol

Fri Mar 11 11:36:19 CST 2016

Pete,

Sorry,I do not understand the question.

Shane

On Fri, Mar 11, 2016 at 11:50 AM Pete Soper <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> 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> 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 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>
>>> <triembed at triembed.org>
>>> *To:* 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
>>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>> TriEmbed web site: http://TriEmbed.org <http://triembed.org/>
>>>
>>>
>>>
>>> _______________________________________________
>>> Triangle, NC Embedded Computing mailing list
>>> 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/20160311/4240f5f6/attachment.htm>