[TriEmbed] N-MOSFET Symbol
Pete Soper
pete at soper.us
Fri Mar 11 10:50:18 CST 2016
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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