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-   -   Crossover distortion in push-pull (http://www.diyaudio.com/forums/solid-state/201907-crossover-distortion-push-pull.html)

LoveVirus 6th December 2011 12:38 AM

Crossover distortion in push-pull
 
I would like to use irfp140 and irfp9140 build a push-pull circuit, but I found very different Vgs, one is 4v, one is 5V, even at a bias circuit, how to ensure the two can also micro-conduction?

nigelwright7557 6th December 2011 12:45 AM

Use a Vbe multiplier for the bias.
I have designed a few class AB amps using IRFP240/9240 and they sound very good.

I usually use an LTP with current mirror (with dc offset pot) and constant current source.
Into a VAS with Vbe multiplier with constant current source.
This will drive 2 or 4 mosfets.

LoveVirus 6th December 2011 01:13 AM

If I use the regulator(eg 1n4730a) is feasible?

nigelwright7557 6th December 2011 01:16 AM

A regulator keeps the voltage the same so could cause thermal runaway.

A Vbe multiplier with the transistor on the heatsink is a better solution.

LoveVirus 6th December 2011 03:05 AM

I found
irfp140N Qgs = 94nc @ Id = 16A, Vds = 80V, Vgs = 10V
irfp240 Qgs = 70nc @ Id = 18A, Vds = 160V, Vgs = 10V
If I was Vds = 20V case, is not irfp240 of Qg will be more smaller

AndrewT 6th December 2011 11:17 AM

Vgs values of 10Vgs to 20Vgs is near saturation level and near destruction level.

Note also the junction peak dissipation 1280Wpk for the 140N and 2880Wpk for the 240.
How fast will the junction heat up?
How long can that power pulse last before you damage the device?

I don't think you are looking at the correct data for your application.

LoveVirus 6th December 2011 01:38 PM

I said Vds, rather than the Vgs, you sure did it wrong!

AndrewT 6th December 2011 02:01 PM

Quote:

Originally Posted by AndrewT (Post 2808706)
Vgs values of 10Vgs to 20Vgs is near saturation level and near destruction level.

you clearly show Vgs = 10Vgs
Quote:

irfp140N Qgs = 94nc @ Id = 16A, Vds = 80V, Vgs = 10V
What have I surely got wrong?

LoveVirus 6th December 2011 02:26 PM

Quote:

Originally Posted by LoveVirus (Post 2808413)
I found
irfp140N Qgs = 94nc @ Id = 16A, Vds = 80V, Vgs = 10V
irfp240 Qgs = 70nc @ Id = 18A, Vds = 160V, Vgs = 10V
If I was Vds = 20V case, is not irfp240 of Qg will be more smaller

irfp140N Qgs=94nc@Vds=80V
irfp240 Qgs=70nc@Vds=160

irfp240 Qgs=?@Vds=80V

What I mean is the parameter manual to different conditions, may mislead some of our judgments。

AndrewT 6th December 2011 02:34 PM

if that data is important to your use, then look up the graph that shows how the lead capacitances change with voltage.


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