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how do vfets start?
how do vfets start?
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Old 8th March 2018, 05:14 PM   #1
arky is offline arky  Russian Federation
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Default how do vfets start?

Just a simple question, resulted from tan-5550 repair. Have both voltages fine rails +/-55 and driver +/-85, checked at gates/sources without vfets installed. Bias is adjustable, both channels OK. But when vfets are in place - they don't go off having tranny voltage drop down to almost zero (1 ohm source to drain). I suspect that bias voltage arrives some milliseconds later than rails, so vfets has no chance to go off. How that's managed? It should be some delay on rails? But no source voltage with full throttle gate will also ruins device. Can not understand this, all hints are highly appreciated.
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Old 8th March 2018, 05:32 PM   #2
nigelwright7557 is online now nigelwright7557  United Kingdom
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On power up and while running the gate voltage is usually set by a Vbe or Vgs multiplier. So the bias voltage should always be right on power up, power on and power down.

Perhaps your bias voltage is too high and the transistors are fully on shorting out the rails ?
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Old 8th March 2018, 05:49 PM   #3
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Quote:
Originally Posted by nigelwright7557 View Post
On power up and while running the gate voltage is usually set by a Vbe or Vgs multiplier. So the bias voltage should always be right on power up, power on and power down.

Perhaps your bias voltage is too high and the transistors are fully on shorting out the rails ?
this particular amp is using two secondary winding pairs - one for rails, second for driver. Driver's power is actually +/-82, however it goes down to about +/-66v on gates after several regulating cascades. So its just usual 12-20V Vgs (adjustable). Just wondering how it can work if Vgs arrives a bit later after rails power...
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Old 8th March 2018, 05:53 PM   #4
nigelwright7557 is online now nigelwright7557  United Kingdom
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Its ok if no volts on mosfets on power up, this means they are turned off.
So long as the voltages are correct when it settles down that is fine.
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Old 8th March 2018, 05:58 PM   #5
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how do vfets start?
Doesn't this type of design rely on the time constants of the rails ? The higher voltages needed to reverse bias the gate are derived from a small auxiliary supply with small reservoir cap. That will allow the gate cut off voltage to develop faster than the main rails can rise.

I would imagine we are talking milliseconds differences but that is all that is needed.

Remember these are depletion mode FET's and so are normally on... and they have to be actively turned off.

If you are managing to see them short the rails out then that doesn't bode well for the FET's.

I would guess that if the main reservoir caps were in poor shape and with high E.S.R. then that could cause a problem in allowing the main rail to build to quickly.
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Old 8th March 2018, 06:01 PM   #6
Mooly is offline Mooly  United Kingdom
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how do vfets start?
These are depletion mode FET's Nigel. Normally on at zero volts Vgs, similar to a triode valve or JFET, they need to be deliberately biased off to prevent current flowing.
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Old 8th March 2018, 06:02 PM   #7
nigelwright7557 is online now nigelwright7557  United Kingdom
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I saw vfets in the header and immediately thought of vertical mosfets.
The gate power supply would need to come up first in that case.
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Old 8th March 2018, 06:03 PM   #8
arky is offline arky  Russian Federation
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Quote:
Originally Posted by Mooly View Post

I would guess that if the main reservoir caps were in poor shape and with high E.S.R. then that could cause a problem in allowing the main rail to build to quickly.
yep, also thought this. Just checking caps... the problem is that PSU mechanically sits on cans' screw terminals, so its either to have exactly same size replacements or to manage this somehow ... just a mess. Thanks Mooly.
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Old 8th March 2018, 06:05 PM   #9
arky is offline arky  Russian Federation
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Quote:
Originally Posted by nigelwright7557 View Post
I saw vfets in the header and immediately thought of vertical mosfets.
The gate power supply would need to come up first in that case.
they are vertical jfets, actually, so yes... gate should go first or at least simultaneously.
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Old 8th March 2018, 06:13 PM   #10
Mooly is offline Mooly  United Kingdom
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how do vfets start?
If the caps are in poor shape then there is no alternative but to replace.

I would also make sure no one has upped the value of the auxiliary rail caps in a misguided attempt at improving things.

Hmmm

Good luck.
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