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Old 18th August 2004, 11:56 AM   #1
Jay is offline Jay  Indonesia
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Default How to slow down MOSFET?

Anybody know how to slow down MOSFET's switching time? Will adding small capacitance between Gate and Drain do this?
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Old 18th August 2004, 12:02 PM   #2
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If it's only for a switching purpose then increasing the gate resistor until you are satisfied would do...
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Old 24th August 2004, 12:13 PM   #3
Jay is offline Jay  Indonesia
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It is for audio, for matching between positive and negative signals to be precise. I guess it is the same concept right?

I've heard people used that approach (the gate resistor) but I cannot understand the theory. I mean having extra capacitance to drive is more make sense to me. The capacitance will just delay the gate from reaching voltage above Vgs threshold.

The resistor may also slow down the rate at which the MOSFET turn on and off but I'm not sure if it is the same thing as what gate capacitance does to signal pulse at the gate. And I'm not sure either whether it is appropriate for matching purpose...

I think I have ever seen somewhere writing (from Nelson Pass?) regarding simulation of input, output and reverse transfer capacitance of a MOSFET but couldn't find it...
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Old 24th August 2004, 12:54 PM   #4
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Do you mean switch on or switch off time ?
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Old 24th August 2004, 01:05 PM   #5
Jay is offline Jay  Indonesia
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Dear Pavel, I mean both with the same amount of reduction. I wonder if the "zobel" in your DPA-330 might also do the trick?
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Old 24th August 2004, 01:23 PM   #6
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Default Re: How to slow down MOSFET?

Quote:
Originally posted by Jay
Anybody know how to slow down MOSFET's switching time? Will adding small capacitance between Gate and Drain do this?

I would think for audio you would want the fastest response time, even if it is not symmetrical. If the fast response time is (much) higher than the audio max rate of change (which occurs at full power 20kHz zero crossing, for instance), that would make sure that at least in the output stage you have no slew-rate limiting.

If you mean that the driver stage cannot source/sink enough current to turn the FETs on/off fast enough, than I would look for a solution in a modified driver stage rather than slowing down the fastest FET to be as slow as the slowest one. It may be symmetrical, but you would still have slew rate limiting due to the limited driver stage.

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Old 24th August 2004, 01:23 PM   #7
Mr Evil is offline Mr Evil  United Kingdom
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Increasing gate resistor value or adding extra gate capacitance will have virtually the same effect, since they both increase the time needed to charge the gate capacitance. I would suggest that increasing gate resistor value would be preferable to avoid any possible oscillation problems.
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Old 24th August 2004, 02:02 PM   #8
wenye is offline wenye  China
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Jay,

The most simply and efficiency way is add different resistor to N/P MOST's Gate....

Cheers
Wenye
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Old 24th August 2004, 02:36 PM   #9
zinsula is offline zinsula  Switzerland
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Default Nonlinear Gate Capacitance

I do not recall in which thread, but it was mentioned about higher distorsions if the gate resistor is too high, due to nonlinear gate capacitance. Therefore, isn't adding a cap the better solution?

BTW, Charles Hansen posted a schematic of an Ayre V-3 prototype in another thread some months ago, and there are different gate resistors between the N- and the complementary P-channel Mosfets. I believe they are in for the same reason you are looking for remedial.

Regards
Tino
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Old 24th August 2004, 03:02 PM   #10
Eva is offline Eva  Spain
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To slow down rise and fall times in switching applications and get flatter rise and fall slopes I recommend both adding external drain-gate capacitance and increasing gate resistance [gate clamping ie:zeners is also advidsed to prevent gate overvoltage]

Rise and fall times depend on gate resistance and on the relationship between Cd-g and Cg-s

Mosfets by nature show extremely non-linear capacitances so the rise and fall waveforms at constant current tend to be more exponential than linear

Class G amplifiers with switched supply rails are an example where controlled slew-rate is desirable for the rail-switching MOSFETs
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