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Old 25th October 2006, 04:35 PM   #1
jkeny is offline jkeny  Ireland
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Default Using Mosfets as Diodes

Hi all,

What does you think of this http://peufeu.free.fr/audio/alim_mos_diode/

Would this work to reduce a trafo with a slightly higher voltage than what I require? Does anybody know the formula for calculating what drop in voltage this Mosfet usage will give? Is it only for low current usage & not suitable for power amp?

I want to use it on Greg Balls' SKA GB150D power amplifier using a trafo which has 32-0-32 output & combine both outputs to give about 55V DC after rectification - can this be done?

John
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Old 25th October 2006, 07:08 PM   #2
cerrem is offline cerrem  United States
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It appears as though using the body diode of the FET....
You need to watch your current to prevent blowing it...
You would be better off to synronous the FET for turn-ON durring foprward conduction.... It can be done passively by tying the gates to taps in the transformer....

Chris
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Old 25th October 2006, 07:51 PM   #3
Eva is offline Eva  Spain
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Yes, the circuit shown will drop almost 10V DC under heavy load and 6V DC or so under light load. Note that a center tapped transformer arrangement producing symmetric voltage rails would drop half the voltage in ecah rail. Concerning the body diodes, they are not used at all, furthermore, conventional diodes wired in series are required in order to bypass them as the schematic shows.
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Old 26th October 2006, 03:01 AM   #4
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Hi EVA,

I onced use IRFP150's Body Diode for normal 20KHZ rectification by shorting the gate and source terminals together and then using Drain and Source as Diode...isn't it right....

K a n w a r
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Old 26th October 2006, 07:12 AM   #5
Eva is offline Eva  Spain
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Yes, you get a diode that way, but it's not fast recovery at all. The body diode of a conventional MOSFET behaves much like a (damn expensive) standard recovery diode

Only some latest generation MOSFETs feature fast recovery body diodes (always at the expense of higher Rds-on), and they are really expensive.
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Old 26th October 2006, 09:15 AM   #6
jkeny is offline jkeny  Ireland
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Thanks all for the replies so I could drop 6-10V from the trafo using Mosfets.

The author says that the soft switching prevents ringing in the trafo. Any opinions as to the sonic advantages of this approach - has anybody tried it?

John
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