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Old 21st September 2012, 03:32 AM   #91
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I think you've got it.

My suggestion of a 27v zener was based on belief that at 0v input, gate is at +4v and drain is at +12v. Zener would not conduct. With a negative input of -4v, gate would be at 0v and drain would go to +24v. Zener would just start to conduct.

At power-on, power supply starts at +55v, zener would force gate up to +28v, which turns on device hard which then reduces drain voltage.
Initial gate-drain stress would be 27v, gate-source stress would be 28v.

I would also predict that a 040 would be the best for surviving the voltage stress (I think the gate oxide thickness for the oldest device would also be the thickest as the way to reduce cost is to make area smaller for more devices per wafer, but smaller area requires thinner gate oxide).

Glad to help. Have fun.
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Old 21st September 2012, 11:23 AM   #92
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I will have to order some 27v zeners. 15 was as close as I had on hand.

Zc
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Old 21st September 2012, 05:35 PM   #93
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Two 15v zeners wired in series = 30v. Give them a try before ordering anything.
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Old 23rd September 2012, 03:30 AM   #94
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I found I had one more 044N left and so I tried two 15V zeners in series....dead mosfet!
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