It all depends on where the MOSFET gate driver is referenced. In the usual case of high-side switching the driver is referenced to the high-side mosfet source and there indeed is "Miller effect". This is
*not* a follower configuration.-- Thanks, Fred.
High-side switching is *not* a follower configuration.
-- Thanks, Fred.
I think Jim's talking about "Miller effect," (gain*Cdg) whereas I'm talking about "Miller capacitance" (Cdg).
G=1, so the effective capacitance isn't multiplied, true, but it still matters.
Cheers, James Arthur
Even a follower has a change of Vdg when it switches. It may not look like a follower has voltage gain, but from the fet's perspective it sure does.
If Vgs changes 4 volts but Vdg changes 20 volts, there is effective voltage gain and "Miller effect", namely lots of gate current caused by the change in Vdg into Cdg.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Lots of people call it that. Technically, "Miller effect" is the apparent increase of plate-grid capacitance that results from the negative voltage gain of a tube.
It's certainly there in a follower. Plate-grid or (drain-gate) capacitance gets charged by the change in plate-grid voltage.
Right. Miller effect.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
The fet doesn't know that the gain, as you define it, is 1. If, relative to the source, the gate swings +5 volts and the drain swings -50, the gain is -10. And the gate current will correspond. The drain-gate capacitance has to be charged.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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Vdg
I stand corrected. In Jim's circuit Vgd goes from -12 to +12v, while Vs goes from 0 to 12v. So, Cgd sees 2x the voltage swing of Cgs. Cdg is effectively doubled.
So you're right, there is effective gain, and Miller effect too.
James
I think we're saying the same thing.
-- Thanks, Fred.
Miller implies gain from gate-to-drain. However the Si4666DY I'm using doesn't have the QG spread that appears in some of the devices Spehro referenced.
Yep. That's why QG is a better measure of gate movement than the various capacitances given in the tabulated data. ...Jim Thompson
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There _is_ change in the gate-drain voltage, but there's no _gain_ term as in "Miller". ...Jim Thompson
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Of course, but there's no "gain". AND CDG, CGS, CDS aren't real lumps... that's why better results are obtained from QG. ...Jim Thompson
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Until the gate is "charged" the source doesn't move. ...Jim Thompson
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There is _no_gain_ from gate-to-drain. So I think "Miller" is a mis-used term here. ...Jim Thompson
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Of course there is. Just for a thought experiment, change the circuit reference to the switching mosfet source (the gate driver being referenced from this point) and all that becomes obvious. A big Vds change for a small Vgs change looks like voltage gain to me.
-- Thanks, Fred.
Tried to post a PNG here and in A.B.S.E, but it wouldn't take, so go here....
...Jim Thompson
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So, still no evidence that you calculated the values that you claimed to have calculated.
Because you didn't. And apparently still can't.
You fiddled it in Spice, which was the sensible thing to do. So why lie about having calculated it? Bizarre.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
If you think in terms of measuring small-signal gate capacitance, a linear follower doesn't show multiplication of Cdg. But the high-side switch isn't a linear follower. If you think in terms of how much gate charge it takes the high-side driver to switch, Vcc matters, and more Vcc requires more gate drive.
Since it's nonlinear too, Spicing makes much more sense than an analytic solution based on "capacitance."
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Right. The words that we use don't change what the circuit does.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
You can certainly define gain relative to the mosfet itself, deltaVds over deltaVgs. Or you can define gain of the overall circuit as being somewhat below
The circuit doesn't care how we think about it. The voltages and currents vs time are what matter.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Mr. Miller probably knew that before he discovered that his common-cathode amp was anomalously slow. ;0
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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