[TriEmbed] N-MOSFET Symbol

Fri Mar 11 12:01:32 CST 2016

Pete,

Usually you can find a VDSmax and a recommended VDS working voltage. The
Toshiba FET below has a VDS max and Drain-Source breakdown voltage of 100V
and it is listed as 100V part. But in the datasheet it is characterized up
to a maximum of 80V (Figure 8.10). I can't put my finger on it but
somewhere Toshiba clearly states this FET has a recommended working voltage
of 80V. I suspect the Fairchild FET that Brian is using includes a safety
margin in the datasheet ratings. It would seem foolish to rate a chip for
60V operation if the expected Source-Drain breakdown voltage is 60V.

http://www.semicon.toshiba.co.jp/info/docget.jsp?type=datasheet&lang=en&pid=TPW4R50ANH

Shane

On Fri, Mar 11, 2016 at 12:38 PM Pete Soper <pete at soper.us> wrote:

> 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> 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
>>>>
>>>>
>>>>
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>>>>
>>>>
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>>>
>>>
>>
>
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