Article - Sony VFETs part 1

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D

Dennis Hui

Member
Joined 2002
Paid Member
dantwomey said:
Father Pass ;)

I believe the 19.8v I'm sending to the bias circuit is to high to allow it to get below 6VDC. I'm going to try rewiring the secondaries to send 15vdc and see where that gets me.

Regards,
Dan :)
Click to expand...

Hi Dan, I think you need to lower the R7 resistor value (or increase P1 value)
if you want it to go lower than 6V.

If I'm reading the TL431 datasheet correctly, the voltage you expect
to get is:

Vout = Rref (1 + R7/P1) (Rref~2.5V). For R7=7.5K and with P1 value <=5K,
you'll always have Vout >=2.5*(1+7.5/5) ~ 6.25V

Cheers,
Dennis
 
D

dantwomey

Member
Joined 2002
Paid Member
Dennis Hui said:
Hi Dan, I think you need to lower the R7 resistor value (or increase P1 value)
if you want it to go lower than 6V.

If I'm reading the TL431 datasheet correctly, the voltage you expect
to get is:

Vout = Rref (1 + R7/P1) (Rref~2.5V). For R7=7.5K and with P1 value <=5K,
you'll always have Vout >=2.5*(1+7.5/5) ~ 6.25V

Cheers,
Dennis
Click to expand...

Dennis,
I used two of these for my bias supply. One for each channel.
3FD-324 Tamura | MT2114-ND | DigiKey
I connected a small bridge rectifier to each separate secondary as seen here.
An externally hosted image should be here but it was not working when we last tested it.

This produced an output close to 20vdc.
I'm going to try and wire the secondaries in parallel as seen on page 2 here.
http://www.tamuracorp.com/clientuploads/pdfs/engineeringdocs/3FD-3XX.pdf

I'm going to put 5K across one transformer I've wired in parallel and 5K across the old way to see if it makes a difference in output. If not we'll either go to a transformer with less secondary voltage or change R7.

Regards,
Dan :)
 
D

dantwomey

Member
Joined 2002
Paid Member
Dennis Hui said:
Hi Dan, I think you need to lower the R7 resistor value (or increase P1 value)
if you want it to go lower than 6V.

If I'm reading the TL431 datasheet correctly, the voltage you expect
to get is:

Vout = Rref (1 + R7/P1) (Rref~2.5V). For R7=7.5K and with P1 value <=5K,
you'll always have Vout >=2.5*(1+7.5/5) ~ 6.25V

Cheers,
Dennis
Click to expand...

Also, I thought by following the supplied schematic I would get the 4-12vdc range mentioned in Nelson's article. From what I can tell I'll need to get around 5vdc to get correct bias. The other channel gets up to 100mV with 6.25vdc. As mentioned previously the second channel will only get up to 75mv across R11/R12 before it stops. Either way I have to find a way to lower the range of my bias supply to get the VFETs to draw enough current.

Regards,
Dan :confused:
 
D

dantwomey

Member
Joined 2002
Paid Member
Nelson Pass said:
Yes, it does seem you have a problem, although I don't know what it is.

:cool:
Click to expand...

I've checked everything and it all looks good except for being unable to get my bias voltages low enough to get 1.5amps across the VFETs. It's easy to get 1 amp on one side with zero offset and .75 amps on the other side with zero offset. More soon.........:)

Regards,
Dan
 
D

dantwomey

Member
Joined 2002
Paid Member
Dennis Hui said:
Hi Dan, I think you need to lower the R7 resistor value (or increase P1 value)
if you want it to go lower than 6V.

If I'm reading the TL431 datasheet correctly, the voltage you expect
to get is:

Vout = Rref (1 + R7/P1) (Rref~2.5V). For R7=7.5K and with P1 value <=5K,
you'll always have Vout >=2.5*(1+7.5/5) ~ 6.25V

Cheers,
Dennis
Click to expand...

If I follow Nelson's suggestion and lower R7/R8 to 5K that would only get me down to 5.5vdc. How does the article calculate a 4-12vdc range?

Regards,
Dan :)
 
D

dantwomey

Member
Joined 2002
Paid Member
Nelson Pass said:
The LT431 goes down to 2.5V

:cool:
Click to expand...

Thanks Father Pass. It's two possibilities. Either I missed something in the assembly or using the KD-33 instead of the KE-33 (2SK82) is throwing off my results. I'm building a standalone version of both TL431 REF circuits to accomplish two things. Test my construction skills and give me somewhat of a reference point to learn about this circuit.

More soon.........

Regards,
Dan ;)
 
Nelson Pass

Nelson Pass

The one and only
Joined 2001
Paid Member
dantwomey said:
Thanks Father Pass. It's two possibilities. Either I missed something in the assembly or using the KD-33 instead of the KE-33 (2SK82) is throwing off my results.
Click to expand...

I have tested quantities of both, and there was little difference between the
KE and KD.

The Vgs distribution for 0.5A at 20V Vds ranges from 6V to 10V for the 2SK82
and 8 to 13V for the 2SJ28, the most common figure being in the middle for
each.

The lowest figure for N's at 1A were 5.5V, and I think that's about the bottom,
so I think you probably have an error somewhere.

:cool:
 
F

flocchini

Member
Joined 2004
Help please

I am trying to bring up the CSX1 (permaneder boards)

Regulated voltages about 23V at both boards
I set VRef1 and 2 to be 8V

On one of my boards I obtain about 10mV DC at the output with 1.25A max current

On the other board I can alter the current through VFET between 1 and 2 amps but DC at output stays at -19V VREF2 can only vary between 4.5 and 4.9V when this happens.

Any suggestions accepted

Thank you

Bob
 
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