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Old 8th August 2011, 01:21 AM   #1
GregH2 is offline GregH2  Australia
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Default Signal Fet dissipation

Hi All,

I have a dissipation question I'm hoping you can help with. I know you Pass guys like pushing your fets so I thought I would ask this here.

I am currently building a mosfet amplifier which has a VAS using ZVP3310A (650mW) and DN2530 (750mW) fets. They are both TO92 devices.

I am interested to know how much dissipation I can comfortably push out of these while still maintaining a long lifespan. The thing I understand about the TO92 package is that it can't dissipate heat very well at high ambient temperatures and needs very severe derating. Let's assume an ambient temperature of 45 degrees C inside the enclosure.

I currently have about 25v across each and am running them at 6mA, but I would like to use a higher current if possible.

Any insight into this would be most appreciated!

Cheers,

Greg.
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Old 8th August 2011, 06:57 AM   #2
h_a is offline h_a  Europe
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The general rule of thumb is not to exceed half of the power rating, for jfets I would even go lower than that for best reliability.

If you need more current, parallel 'em!

Hannes
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Old 8th August 2011, 07:05 AM   #3
GregH2 is offline GregH2  Australia
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Thanks h_a,

Would you really go that high with an ambient temperature of 45 deg C?

This is great news!
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Old 8th August 2011, 08:07 PM   #4
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If the max chip temperature is 150, then you can figure that you don't
want to run it past 100 ever, and with a 50 deg ambient and a 5 mw
derating per degree, we get about 250 mW rating figure that I would
consider the maximum just sitting inside an amplifier.

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Old 8th August 2011, 09:58 PM   #5
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Nelson, Does this also apply to the Simisouth jfets ? I see on the data sheet they are rated at 150c max. but didn't you mention the chips fail at about 800c ?
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Old 8th August 2011, 10:09 PM   #6
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I think it is the packaging and die bonding which fails above 175 deg or so.

I feel free to run them up to 100 deg case temperature, no higher.

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Old 9th August 2011, 12:44 AM   #7
GregH2 is offline GregH2  Australia
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Thanks Nelson, this is the kind of info I wanted.

Greg.
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Old 9th August 2011, 07:40 AM   #8
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Thanks Nelson I was a little concerned about that heet issue.
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