Simplistic Mosfet HV Shunt Regs - Page 256

Salas · 14th January 2012, 06:12 PM

We have one running at 60mA among the proto testers (with no problems), and I will ask at what value the trimmer and resistor combination measures. Its an area I am expecting some practical feedback due to possible differences to datasheet and IDSS spread of the Supertex. In the end we will designate the most ''safe'' values. But I would like to see a fixed resistor replacing the CCS trimmer in permanent installations after initial fixing. Its good for PPM & long term ease of mind.

Salas · 14th January 2012, 07:01 PM

Quote:

Originally Posted by Lazybutt

* Note: my calculations accuracy is still to be proven, I haven't built SSHV2 yet to verify my formulas work in real life..

[/I]

In that 60mA one the trimmer is at 21.5 Ohm, got a check. So Mosfet parts will inevitably deviate Vgs, but we are in ballpark.

Lazybutt · 02:44 AM

Quote:

Originally Posted by Salas

In that 60mA one the trimmer is at 21.5 Ohm, got a check. So Mosfet parts will inevitably deviate Vgs, but we are in ballpark.

Could he measure Q2 Vgs and Vds please ?

Edit: bugger me ! I can calculate Vgs by myself :P 1.21V . But I'm still interested to know Vds of Q2.

Salas · 15th January 2012, 02:44 AM

Can you calculate Q1 Vgs?

Lazybutt · 15th January 2012, 02:56 AM

Quote:

Originally Posted by Salas

Can you calculate Q1 Vgs?

A little bit difficult because Q2 Vds enters into play, so it starts to be tough in Excel.

I'll give it a try...
My Calc just neglects Q1 altogether. I suspect the difference between my value and the real life may come from this calculation approximation ( although my gut feeling would have been that R4 would be lower than what I calculate.. not higher....

)

bigpandahk · 15th January 2012, 11:51 PM

Quote:

Originally Posted by bigpandahk

Thanks Salas. Will do the modification in this week and let you know the result.

Dear Salas

I've reduced R1 to 14ohm and mount the Q1 & Q3 on larger heatsink add heatsink to Q2. I connected a 5.6K 20W as dummy load and test the regulator. With input 300V, I adjust the VR to have 250V output but it drift up slowly to the same value of the input. Please advise the possible problem.

Salas · 16th January 2012, 12:27 AM

Possibilities: 2SK147? Very drifty especially if heated up enough. Try BF245A. Cable connections to the Mosfets? Possible oscillation. Bad trimmer? Can drift. Did you check there is VGS voltage on Mosfets and Vbe voltage on the transistor? All semiconductor parts should be checking OK first.

bigpandahk · 16th January 2012, 05:07 AM

Checked the voltage at R1 drift from 1.54 down to 1.4x. Vgd a Q1 is 4.066, Q3 is 2.958. I believe is the 2sk147 problem. Can I use J201 to replace it?

Salas · 16th January 2012, 09:18 AM

J201 will have low IDSS, maybe less than 1mA sometimes so you will need double the R4 value plus maybe. Its also noisier. Use it if having nothing else. For drift is stable enough.

vgeorge · 16th January 2012, 12:28 PM

Felipe,

got my caps too.
Thanks a lot.

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14th January 2012, 06:12 PM	#2551
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	We have one running at 60mA among the proto testers (with no problems), and I will ask at what value the trimmer and resistor combination measures. Its an area I am expecting some practical feedback due to possible differences to datasheet and IDSS spread of the Supertex. In the end we will designate the most ''safe'' values. But I would like to see a fixed resistor replacing the CCS trimmer in permanent installations after initial fixing. Its good for PPM & long term ease of mind.

15th January 2012, 02:44 AM	#2554
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Can you calculate Q1 Vgs?

16th January 2012, 12:27 AM	#2557
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Possibilities: 2SK147? Very drifty especially if heated up enough. Try BF245A. Cable connections to the Mosfets? Possible oscillation. Bad trimmer? Can drift. Did you check there is VGS voltage on Mosfets and Vbe voltage on the transistor? All semiconductor parts should be checking OK first.

16th January 2012, 05:07 AM	#2558
bigpandahk diyAudio Member Join Date: Jul 2009	Checked the voltage at R1 drift from 1.54 down to 1.4x. Vgd a Q1 is 4.066, Q3 is 2.958. I believe is the 2sk147 problem. Can I use J201 to replace it?

16th January 2012, 09:18 AM	#2559
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	J201 will have low IDSS, maybe less than 1mA sometimes so you will need double the R4 value plus maybe. Its also noisier. Use it if having nothing else. For drift is stable enough.

16th January 2012, 12:28 PM	#2560
vgeorge diyAudio Member Join Date: Mar 2004 Location: poros island	Felipe, got my caps too. Thanks a lot.