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Old 4th January 2006, 05:20 AM   #1
lurker is offline lurker  Brazil
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Default Replace IRF by Lateral Mosfet

Is possible to change the fosgate output IRF9540 / IRF540 with 2SK1058 / 2SJ162?

If yes, how to do? I have the schematics.
p.s. excuse my poor english
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Old 4th January 2006, 06:50 AM   #2
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Laterals have very different characteristics, with much higher rds and much lower gm. You would not normally expect to see more than a few volts Vgs on a vertical fet in a linear audio application.
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Old 4th January 2006, 08:15 AM   #3
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Lateral mosfets have also quite different biasing voltage, so direct replacing isn't possible....
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Old 4th January 2006, 09:16 AM   #4
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And the pinout is different, requiring some innovative soldering and rewiring.

You would also have to disable the amp temperature compensation.

I really wouldn't bother.
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Old 4th January 2006, 09:45 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi Richie,
do you need to disconnect the bias temp comp?

What are the disadvantages of leaving the comp connected?
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Old 4th January 2006, 09:48 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi David,
Quote:
You would not normally expect to see more than a few volts Vgs on a vertical fet in a linear audio application
Have you got this the right way round?
The lateral FET has a low voltage bias requirement and the vert has a high voltage requirement.
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Old 4th January 2006, 10:10 AM   #7
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by AndrewT
do you need to disconnect the bias temp comp?
What are the disadvantages of leaving the comp connected?
Yes, and if you don't the bias current will drop well into crossover distortion when the outputs heat up. This is assuming the existing compa can actually generate a bias voltage for the gates that is low enough (~2V instead of ~6V).

Quote:
Originally posted by AndrewT
Have you got this the right way round?
The lateral FET has a low voltage bias requirement and the vert has a high voltage requirement.
Low BIAS voltage requirement, but not low voltage requirement. The threshold voltage is low (~1V) but the gm is also low so more Vgs is needed compared to verticals, at the same output current. The lower threshold voltage advantage is lost very soon as output current increases. Unless the driving section has separate higher voltage power rails, as a concequence of using laterals, this exact higher Vgs presents an increased rail losss, and a net decrease of maximum power, compared to verticals.
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Old 4th January 2006, 11:03 AM   #8
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Some VFETs have low threshold too ("logic level")

The IRF540 is fully turned on at Vgs=5V, the lateral requires Vgs=10V just to get above 10A
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Old 5th January 2006, 08:34 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
thanks gentlemen for another angle on Gm. It helps me get the difference between LFET and VFET into my mind.

Biasing the push pull LFET output stage with a BJT driver pair.
Should the bias temp compensator be attached to the driver heatsink? to keep the bias voltage to the gates substantially constant.

David,
most LFETs have an Id max = 7A. Only >=10A when driven in one shot short term peak. So you should never see Vgs=10Vpk when on any music signal.
After allowing for temperature derating your current limit is even lower and if it is a short circuit current then the SOAR limit brings permissible Id much lower still.
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Old 5th January 2006, 08:42 AM   #10
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Yes you could include thermal compensation on the drivers, but as their temperature remains reasonably constant and not that hot, in practice it can be skimped on.

I think davidsrsb was just making a point about the big differences in transconductance between lateral and vertical devices with his comment, rather than stating a true 'rule' that is physically practicable.
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