If the voltage is jumping up & down (oscillation), if it's solid as rock surely OK
Hi, it takes lab work to confirm what is happening but upping the gate resistors value indicates that more current setting increases transconductance and it takes more gate damping to stabilize.
In any case 360Ω R1 R2 is permissible. There are examples of 1kΩ even. The downside of using more than needed gate stoppers value is it unnecessarily truncates the cascode's PSRR curve in very high frequencies. But in this case it looks like that higher value is needed indeed.
In any case 360Ω R1 R2 is permissible. There are examples of 1kΩ even. The downside of using more than needed gate stoppers value is it unnecessarily truncates the cascode's PSRR curve in very high frequencies. But in this case it looks like that higher value is needed indeed.
Oscillation is confirmed if the oscilloscope can pick AC waveform riding the DC line. Sometimes jumps sometimes just brings heat sometimes hum and noise sometimes it does not respond to adjustment.
D2 is a Zener. Q2 you mean? Its Vds drain to source voltage times the mA it passes must mathematically explain the heat. Power=V*I. Else its oscillating. If Vds Q2 is high almost HV then Q1 does not work, its cold, and does not shield it.
Keep in mind that when you will put the Mosfets and gate resistors back into place without test pins the behavior will probably be more stable. Because gate resistors and gate pins work best at minimal distance.
Yesterday I had a solder bridge, on Q1 drain to source pins. My bad at placing the three pin row as a socket.
Since fixed is no more heat on Q2.
Now I am happy to get finally 172mA. @ 77Vdc.
That's only by using DN2540 both on Q1 Q2.
I must note that it is consistent no matter value is on R1 R2 gate stoppers. Same results 100R, 360R and 510R.
If I use Ixys on both, then can't reach the specifications Load. I am going to try different combinations, maybe one only Ixys is still possible.
EDIT:
did'nt mention, R4 is set to maximum strengh= 0ohm
Since fixed is no more heat on Q2.
Now I am happy to get finally 172mA. @ 77Vdc.
That's only by using DN2540 both on Q1 Q2.
I must note that it is consistent no matter value is on R1 R2 gate stoppers. Same results 100R, 360R and 510R.
If I use Ixys on both, then can't reach the specifications Load. I am going to try different combinations, maybe one only Ixys is still possible.
EDIT:
did'nt mention, R4 is set to maximum strengh= 0ohm
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Hola Felipe, no need to mod the schematic, for some reason now performs Ok. within my load specifications by just lowering R4 down to almost 0.
Another rule is mantain DN2540 as Q1, then I even can use IXTP02N50D as Q2 without any drawback.
I tried IXTP01N100D also as Q2 only, but fail to reach my load, by small margin.
Another rule is mantain DN2540 as Q1, then I even can use IXTP02N50D as Q2 without any drawback.
I tried IXTP01N100D also as Q2 only, but fail to reach my load, by small margin.
R4 could end as a fixed resistor, but not before replace the variac for a properly mains audio transformer. Current on R4 is a bit related to diferent Vin.
I wil replace now the R4 trim by a low ohm value rheostat.
I wil replace now the R4 trim by a low ohm value rheostat.
I will, thanks Felipe 👍hey, those gate stoppers R1 R1 did'nt deserve to upgrade to Carbon Film? R7also, as well?
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