Symmetrical gate drive circuitry

Thread Starter

MikeJacobs

Joined Dec 7, 2019
156
Do any of you have some good reference links on symmetrical gate drive ideas. or even possibly books that discuss it?
I have not been able to find any text books that cover the information.

I am trying to understand more about what causes asymmetry in mosfet switching and ways around it.
 

Audioguru again

Joined Oct 21, 2019
1,772
Maybe asymmetrical Mosfet switching is because the gate-source of a Mosfet has a fairly high capacitance that needs high currents to charge and discharge quickly.
 

WBahn

Joined Mar 31, 2012
26,074
Do any of you have some good reference links on symmetrical gate drive ideas. or even possibly books that discuss it?
I have not been able to find any text books that cover the information.

I am trying to understand more about what causes asymmetry in mosfet switching and ways around it.
I'm not aware of any books, but there are papers out there on it.

Which types of asymmetries are you primarily interested in? Matching rise and fall times? Matching propagation delays? Matching parameters between the different logic gates?

How symmetric do they need to be? There's a lot of low hanging fruit just by considering differences between electron and hole mobilities and taking device parasitics, both of the channel and the drain/source regions, into account. Also, laying out the physical transistors so that they are identical, including in orientation, becomes important at some point.

When we needed highly symmetrical, well matched, and very high speed logic gates, we used an approach in which all gates output both the signal and it's complement and also required both the signal and the complement of each input. We got very good results, but the design effort was not trivial.
 

MrAl

Joined Jun 17, 2014
7,760
Do any of you have some good reference links on symmetrical gate drive ideas. or even possibly books that discuss it?
I have not been able to find any text books that cover the information.

I am trying to understand more about what causes asymmetry in mosfet switching and ways around it.
Asymmetrical drive is sometimes used to control spike amplitude and EMI. Turning on into an inductance is minor compared to turning off which can cause a high back EMF and so sometimes it is desirable to turn off a little slower. That ushers in the need for asymmetrical drive.

If you really want symmetrical drive then you need a reverse polarity supply voltage ideally of the same absolute amplitude as the positive drive pulse so you can turn off quickly.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
156
What is the different between propogation delay and a rise and fall time
I don't know if I truly understand the difference
Thank you for all the info
 

WBahn

Joined Mar 31, 2012
26,074
Google logic gate propagation delay and logic gate rise fall time and read up on it -- there's quite a bit you'll learn in the process.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
156
Thanks for all the inputs. Have read a bit more and now have a handle on the propogation delay vs rise and fall time. Those ones I knew anyway.

Anyways... So no one knows of any good books around that explain drive circuitry for symmetry hu?
Wonder why no one has written about this since it seems to be an important topic?
 
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