[TriEmbed] N-MOSFET Symbol
Pete Soper
pete at soper.us
Fri Mar 11 11:38:14 CST 2016
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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