[TriEmbed] N-MOSFET Symbol

kschilf at yahoo.com kschilf at yahoo.com
Wed Mar 9 13:51:56 CST 2016


Hi Brian,
All transistors have parasitic elements.  There are parasitic elements among the Gate, Drain, and Source as well as between the Drain and Substrate and Source and Substrate.  The "body" diode is one of the latter.  It is formed at the PN junctions (definition of a diode) that exist between the Drain / Substrate and the Source / Substrate.  Parasitics are undesired and scale with device size.  Larger devices --> larger parasitics.  An accurate board symbol showing all of them would be overwhelming and unnecessary, but semiconductor manufacturers do analyze them.
So how big to make the transistors?
Bigger --> more current capability
Bigger --> greater ESD immunityBUT
Bigger --> switches more slowly (bad for logic) with more switching distortion (bad for power loads)
Bigger --> consume more die size, more expensive

Hobbyist: switching small loads, slowly (turning LED's on and off at a few KHz, a few mA at a time) - Don't CareYou aren't pushing switching speed.  Transient distortion will not be noticed.  Size for load.

Logic Designer:  switching very small loads, very quickly (tiny currents at GHz)
Need for speed.  Need for speed.  Need for speed.  Smaller transistors.

Power Designer:  switching large loads, slowly (power supplies - large currents at Hz, KHz)Must have larger transistors to handle the mechanical stress of passing and switching large currents.  You must manage parasitic distortion of your switching waveforms.  Ripples in a bathtub are fun.  Twenty foot tidal surges are not.  You have the undying admiration of us "Logic" guys!  :-)

Bottom LIne:  The body diode in the symbol is a visual to tell you that this is a large transistor for higher power.  It is like the horse on the hood.  A ferrari, a car, can be used to go to Harris Teeter, but it has other capabilities.  If you really have special requirements for this transistor, you should clearly show them in your documentation (minimum power, gate charge, etc.)  A schematic should be a helpful document.

Good Luck,Kevin Schilf



      From: Shane Trent via TriEmbed <triembed at triembed.org>
 To: Carl Nobile <carl.nobile at gmail.com>; Grawburg <grawburg at myglnc.com> 
Cc: Triangle Embedded Devices <TriEmbed at triembed.org>
 Sent: Wednesday, March 9, 2016 10:43 AM
 Subject: Re: [TriEmbed] N-MOSFET Symbol
   
Brian,
My understanding is that all power MOSFETs have a body diode due to the design and method of fabrication. However, small-signal MOSFETs typically do not have a body diode. I like to see the body diode on the symbol just to remind me that it is there.
Shane
On Wed, Mar 9, 2016 at 10:26 AM Carl Nobile via TriEmbed <triembed at triembed.org> wrote:

Brian,
>From my experience many symbols don't put in the diode even though they are actually in the part. In other cases they are not in the part. Sorry to say it seems to be a decision made by whoever creates the symbol. My own personal opinion is to put the diode in the symbol if the part actually has it and leave it out if it doesn't actually have it. I'm fairly sure that as of now most manufacturers put the diode in the part now, but there may be exceptions.
~Carl

On Wed, Mar 9, 2016 at 9:42 AM, Grawburg via TriEmbed <triembed at triembed.org> wrote:

I'm making an electronic symbols sheet for a summer Raspberry Pi class. Sometimes I see symbols that include the diode and sometimes without.  The kids will be using a FQP30N06L or similar in one of their projects. Are there some MOSFETS that do not have the internal diode? Do I need to include both symbols and how would I differentiate them if I do?

Thanks,
Brian Grawburg

--
[This was sent from a PC running Debian 7, 64-bit Linux. No Microsoft products were used.]





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