Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
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 19th October 2017, 09:36 PM #21 2 picoDumbs   diyAudio Member     Join Date: Sep 2013 The jfets aren't that sensitive to Vds. Your choice. I would be happy at anything between 13V and 20V on the jfets. Usually a good rule of thumb is maximum peak input signal + 10V. Assuming 2.8V peak input then 13V is quite good. __________________ "If you leave this point unattached to some circuitry, an ideal constant current source will emit a small lightning bolt which will travel until it connects to something." Nelson Pass Last edited by 2 picoDumbs; 19th October 2017 at 09:40 PM.
gionag
diyAudio Member

Join Date: Jun 2007
Quote:
 Originally Posted by 2 picoDumbs The jfets aren't that sensitive to Vds. Your choice. I would be happy at anything between 13V and 20V on the jfets. Usually a good rule of thumb is maximum peak input signal + 10V. Assuming 2.8V peak input then 13V is quite good.
Good to know. for 13v is 4k

gionag
diyAudio Member

Join Date: Jun 2007
In the calculation, i have to account the dissipation of the 8 mur3020 ?
from my point of view, they have a very low resistance, so the heat dissipated is quite low... but maybe i am wrong.
From before i have estimate a total power dissipation from the mosfets, but how i need to weight the diodes ? (from a thermal perspective)

Every channel has 8 mosfets and 8 diodes, in the attached simulation i have calculated with a total power dissipation of 140w (and an ambient temp of 30degC).
I think i can push a little bit more on this chunk of aluminium.
Attached Images
 smpl_dissipatore_5314-HGS_5314-Thermal-Thermal1.jpg (298.7 KB, 246 views)

 20th October 2017, 12:48 AM #24 Zen Mod   Official Court Jester diyAudio Member     Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain in moments of peak currents , current is practically subtracted from mosfets , and given to speaker so , sum dissipation on heatsink is declining ,so you can ignore dissipation on/through diodes if you're going full bang , counting on heavy AB class work of F5T , then you really need to carefully calculate everything ..... __________________ my Papa is smarter than your Nelson ! clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
AndrewT
diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by 2 picoDumbs The jfets aren't that sensitive to Vds. Your choice. I would be happy at anything between 13V and 20V on the jfets. Usually a good rule of thumb is maximum peak input signal + 10V. Assuming 2.8V peak input then 13V is quite good.
the input jFET is a voltage to current conversion stage.
You input a voltage and the jFET output is a current.

The input jFET can operate with a Vds of 2 volts even though the input voltage can swing to over 2Vpk.
If you add on a cascode you can run with a Vds anywhere from ~2Vds to the maximum where base current leakage starts to become unmanagable.

The drain resistor converts the output current back into a voltage read by the next stage.

This drain resistor having a voltage output is what needs voltage to allow that swing.
The drain resistor is above the cascode.
The jFET does NOT need an extra 10V above the input signal.
The purpose of the cascode is to separate the jFET from the effects of the high supply rail voltage.
__________________
regards Andrew T.

gionag
diyAudio Member

Join Date: Jun 2007
Quote:
 Originally Posted by AndrewT the input jFET is a voltage to current conversion stage. You input a voltage and the jFET output is a current. The input jFET can operate with a Vds of 2 volts even though the input voltage can swing to over 2Vpk. If you add on a cascode you can run with a Vds anywhere from ~2Vds to the maximum where base current leakage starts to become unmanagable. The drain resistor converts the output current back into a voltage read by the next stage. This drain resistor having a voltage output is what needs voltage to allow that swing. The drain resistor is above the cascode. The jFET does NOT need an extra 10V above the input signal. The purpose of the cascode is to separate the jFET from the effects of the high supply rail voltage.
said that, there is a particular sweet spot for the sk170/sj74 ?
i also imagine that if the cascode device needs to regulate more and more the voltage down then its dissipation figure start to increase. not so much because the current involved are very little...

 20th October 2017, 08:44 PM #27 Zen Mod   Official Court Jester diyAudio Member     Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain one can have different understanding , when looking at device load lines graph in other words - I personally don't like having device starving , neither in voltage nor current take care to have 10-12V as D-S voltage for JFet , around 90% of Idss and you're there __________________ my Papa is smarter than your Nelson ! clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
2 picoDumbs
diyAudio Member

Join Date: Sep 2013
Quote:
 Originally Posted by gionag said that, there is a particular sweet spot for the sk170/sj74 ? i also imagine that if the cascode device needs to regulate more and more the voltage down then its dissipation figure start to increase. not so much because the current involved are very little...
Don't get confused, just copy Nelson if you don't understand.
I notice that it is usually between 12V and 14V when cascoding.
__________________
"If you leave this point unattached to some circuitry, an ideal constant current source will emit a small lightning bolt which will travel until it connects to something." Nelson Pass

 21st October 2017, 02:03 AM #29 gionag   diyAudio Member   Join Date: Jun 2007 Nelson seems to use @ 10v on the v3 schematic with 32v rail so i will go with them. In your opinion, there's a practical way to guestimate the power dissipation that comes from each of 8 diodes (mur3020). I want to optimize a little bit the placement of the whole board assembly on the heatsink and i am thinking to shift a little bit the board to get the most "hot" components (mosfets) in the center.
gionag
diyAudio Member

Join Date: Jun 2007
I was looking into power supply options...
i will start with relatively low bias setting, but i plan to rise as much as possible to extend the class-A region.

Obviously the ripple will rise with it and for limiting the input bulk-capacitance i am start thinking about something like the zenV4 capacitance multiplier / regulator.

I made a sim in spice and it seems to works quite OK. i see that i will loose some voltage and the dissipation is around 25w for 6amp bias (too much... i know).

For sure i need to swap the irfp240 with something more beefier because the power dissipation at startup exceed specs.

What do you think about ?
Attached Images
 euvl_reg.PNG (47.5 KB, 178 views)
Attached Files
 zen_v4_exp.asc (2.0 KB, 2 views)

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