Thanks Zen, I will hope it drops the voltage after the VFETS are installed once I am done with the initial setup of biasing and offset.
Can anyone please look into my above issues with one of the board, how do I rectify or where do I need to look at as I am unable to set the bias around R5 (0.00v) and R6 (254mv) with R34/35 being at 562R.
Bad channel board
T6-T7 = 0.00V
T8-T9 = 0.229V
T6-T11 = 8.20V and
T9-T12 = 8.05V
when P3 and P4 are at their maximal values.
Good channel board
T6-T7 = 1.35V
T8-T9 = 1.35V
T6-T11 = 8.40V and
T9-T12 = 5.45V
when P3 and P4 are at their maximal values.
This is the P3 pot orientation pic which I replaced with multi-turn and I hope the orientation is correct!
T6-T7 = 0.00V
T8-T9 = 0.229V
T6-T11 = 8.20V and
T9-T12 = 8.05V
when P3 and P4 are at their maximal values.
Good channel board
T6-T7 = 1.35V
T8-T9 = 1.35V
T6-T11 = 8.40V and
T9-T12 = 5.45V
when P3 and P4 are at their maximal values.
This is the P3 pot orientation pic which I replaced with multi-turn and I hope the orientation is correct!
An externally hosted image should be here but it was not working when we last tested it.
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Looking at your picture, I notice a few of the solder joints are very dry, especially around Q11. Might be a good idea to touch up here and there.
I did reflow of the solder for all the joints on the board. Did the test again and its the same result no voltage across T6-7 and around 0.245v across T8-9 with 562R on R34/35. So I changed R34 to 1k and retained R35 at 562R which gives me only 0.221v across R8-9 and does not change with P4 turning
Also with 1k on R34 the voltage across G-T18 shows 32.1v with P1/P2 at center .
Hmmm...I don't think I would have gone with 1K on R34 on your 'bad board'. It could have made Vgs too high, stressing/damaging your mosfet.
I suggest taking a step back. Unpower your 'bad board', and go back to your good board to make sure that it is biased correctly.
(It seems you may have played around with the pots on the good board per post #323). Once things are ok (ie, low offset at
G-T18, and ~1.5V across R5 and R6) on your good board, try measuring T7-T10 and T9-T12 Vgs values.
On the bad board I think you will need to change R34 back to something lower.
I suggest taking a step back. Unpower your 'bad board', and go back to your good board to make sure that it is biased correctly.
(It seems you may have played around with the pots on the good board per post #323). Once things are ok (ie, low offset at
G-T18, and ~1.5V across R5 and R6) on your good board, try measuring T7-T10 and T9-T12 Vgs values.
On the bad board I think you will need to change R34 back to something lower.
On the good board everything is set as per the guide and works perfectly without any VFETS installed.
I am powering and checking only one board at a time. So currently the good board is not being powered and post #323 I just powered both to check the comparison values between the good/bad boards and listed the values. Regarding the 1k on R34 I will remove it and reset everything back with P1/P2/P3/P4 and start once again. And why do I need to check the Vgs values on the good board?
Hmm only the P channel or do I need to replace the N channel Fairchild q5/6? I do not have those readily in stock need to order them on element14 or mouser India. I got these from the Mouser US couple of months back.
Also I am curious that all the initial voltage readings are very close to the published figures in the NP guide, so not sure how the bias is not coming up.
You mentioned measuring the good board with P3 and P4 set at maximal
value, so I thought you might have played around with them.
As for measuring the Vgs, it was
just to get an idea what you may expect the corresponding values to be on the
'bad board'.
Have you tried measuring T7-T10 and T9-T12 on the bad board?
Do their values change when you adjust P3 and P4 (should start with
them low and not overdo it.)?
value, so I thought you might have played around with them.
As for measuring the Vgs, it was
just to get an idea what you may expect the corresponding values to be on the
'bad board'.
Have you tried measuring T7-T10 and T9-T12 on the bad board?
Do their values change when you adjust P3 and P4 (should start with
them low and not overdo it.)?
Thanks wdecho
Curiously on the first board also I have the same Fairchild FQP and I used 562 ohm, easily was able to get the bias of 1.5v both sides and working stable with offset close to 0.08v.
I tried 1k only on the R34 but not on R35 and could not get the bias properly on the non-working board. Should I change to 1K on both and then try?
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Note that for your 'good board', 221R + 500R pot already got you resonably
close to the required Vgs so bumping things up a bit was all that was needed.
I'm surprised by your ~22V measurement for T7-T10 and T9-T12 that are unaffected by P3 and P4 settings.
Hope you get this resolved soon.