[TriEmbed] N-MOSFET Symbol

Thu Mar 10 16:04:26 CST 2016

Good afternoon all,

By limiting the gate current during turn-on and turn-off, you can reduce 
or eliminate back EMF spikes from inductive loads. Not knowing the mass 
or velocity of the motor at turn-off keeps me from offering suggested 
series resistor values, but does introduce a method of tuning.

Connect an oscilloscope between the FET Drain and Source ... if the 
source is floating above ground, use two scope channels in A+B mode, and 
leave the probe grounds off.

When switching on and off, you will see a square wave with a (hopefully) 
minor spike. By increasing the resistor value from the gate drive 
source, you can slow the gate turn-on (treat the gate as the capacitor 
it is). Don't go too high, or you'll overheat the transistor by staying 
in the linear (not hard-switched) mode.

If you need different turn-on and turn-off values, connect two 
resistors, with one end of both resistors connected to your gate drive, 
the other end of each resistor is connected to a diode (high speed 
please), one resistor to one diode's anode, the other resistor to the 
other diode's cathode. The free ends of the two diodes then connect to 
the gate.

By adjusting the turn-on and turn-off resistance values, you can greatly 
reduce the spikes induced by the rapid DI/DT (change in current over 
time) created by the mosfet switch. If you have a known load, you can 
also connect a series resistor/capacitor across the drain and source ... 
you'll have to research snubbers to figure out the appropriate values.

For difficult snubber issues, such as when you are burning up snubber RC 
networks, you can install a TVS between the drain and the gate, and then 
a resistor from gate to ground (at the mosfet side of any current 
limiting resistors). Choose the TVS voltage wisely, greater than your 
typical VCC, but less than the maximum Vds. This creates a self-snubbing 
circuit, where if the spike is big enough to trigger the TVS, the FET 
will turn back on momentarily, and thus"eating" the spike. You really do 
need to install a second TVS between the gate and source, with a turn-on 
voltage less than the Vgs rating of the mosfet. Be careful to ensure you 
don't set up a nice, noisy power oscillator (yea, been there ... done 
that!).

If you see that your spikes are minimal to non-existent ... 
congratulations.

Regards,

Rick

On 3/10/2016 4:24 PM, Pete Soper via TriEmbed 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>
>> *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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