[TriEmbed] N-MOSFET Symbol

Fri Mar 11 15:31:59 CST 2016

Hi Pete,
The voltage at each of the three terminals of the transistor (gate, drain, source) is a function of the rest of the circuit.  You can bias (set voltages, and draw currents) the transistor anyway you want, once you understand its behavior (and limits) at whatever operating point you set.
It is possible to bias the source such that Vsource > Vdrain (Vds < 0).BVDss the maximum voltage difference (Vdrain - Vsource) exerted before you possibly damage the part.  This value is temperature dependent.

Born before Wikipedia, I still believe in books.  :-)
Since textbooks ain't cheap, borrow a sophomore level circuits text (NCSU library, etc.).  Peruse the chapter on BJT's and MOSFET's.  That should clear up some of the mystery.  :-)
Don't let the smoke out (at least while anybody is looking!)  :-)
Sincerely,
Kevin Schilf

   From: Pete Soper via TriEmbed <triembed at triembed.org>
 To: Shane Trent <shanedtrent at gmail.com>; "triembed at triembed.org" <triembed at triembed.org> 
 Sent: Friday, March 11, 2016 12:38 PM
 Subject: Re: [TriEmbed] N-MOSFET Symbol

 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>
 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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