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Old 4th February 2009, 12:30 PM   #1
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Default MOSFET bias current

Just ask in normaly for power mosfet, how many mA of bias to set? I have adjusted at 150mA, but it seem too hot.

Can anybody help me, thank in advance.
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Old 4th February 2009, 01:48 PM   #2
h_a is offline h_a  Europe
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Mosfets? Bias them as high as you can afford.

These parts do not have a current at which crossover distortion is minimized (as it is the case for BJTs).

Have fun, Hannes
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Old 4th February 2009, 02:43 PM   #3
lineup is offline lineup  Sweden
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150mA - 200mA is quite normal for standard power amp output.

If 'only' 150mA makes too hot,
then you have either some other problem (like instability/oscillation)
or your heatsink is way too small.
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Old 4th February 2009, 04:24 PM   #4
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Quote:
Originally posted by lineup
150mA - 200mA is quite normal for standard power amp output.

If 'only' 150mA makes too hot,
then you have either some other problem (like instability/oscillation)
or your heatsink is way too small.
ditto
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Old 4th February 2009, 04:59 PM   #5
AndrewT is offline AndrewT  Scotland
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Borbely recommends a minimum 150mA through each Output FET and a minimum of a total 500mA through all the parallel output FETs.

He only designs and builds FET amplifiers and ancilliary gear.
Some good design papers on his website.
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Old 5th February 2009, 12:59 AM   #6
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Thank above, I will check and try.
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Old 5th February 2009, 06:01 AM   #7
mtlin12 is offline mtlin12  Taiwan
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Quote:
Originally posted by pengpeng
Just ask in normaly for power mosfet, how many mA of bias to set? I have adjusted at 150mA, but it seem too hot.

Can anybody help me, thank in advance.
I had same problem once, at +/-60V 150mA*4 (2 pairs power MOS)

the temperature of a normal 140mm width heat sink will probably up to 130~140°¶F

and it is too hot.

So I ...
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Old 5th February 2009, 06:10 AM   #8
mtlin12 is offline mtlin12  Taiwan
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Use two heat sinks for 2 pairs power mosfet, and hope can get >250mA

bias current for each at normal temperature.

(+/-32.5V 500mA is just for testing, other 70mA is for the Relays.)
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Old 5th February 2009, 11:30 AM   #9
darian is offline darian  France
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It doesn't make sense : if you use a heatsink the size of the one in the picture, at idling the temperature should be max 100įF with 250mA.

What does your dissipation calculation says? If it's way too hot, there must be some HF ringing or so. It's just calculations, they don't lie.

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