[TriEmbed] N-MOSFET Symbol

Fri Mar 11 09:40:36 CST 2016

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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