Calling all builders of the Swenson/Pimm HV Reg

[riotubes]

I need your help. I've built the original Swenson design of this HV reg multiple times attempting to target 70V out. Each time, under load, the input voltage sags as well as the output voltage. Then, I ordered Gary's revised board that he optimized for stability and measurement...and I get the same results...sagging voltage. I'm at a total loss and could really use some guidance.

Power supply:
AC in: 122VAC
Triad N68X Isolation Traffo rated 115V @ 0.435A = 50VA (spec'd by JS)
Schottky bridge followed by 1uF film with 100K bleeder R strapped across it (spec'd by JS). This feeds the reg

Measurements with NO load:
182VDC at 1uF cap (sags to 147VDC under load)
70VDC out of reg (sags to 58VDC under load)

The reg powers a DAC analog stage that requires 70V with 33mA running through it. I've been using a resistor strapped across the reg's output as a proxy load (70V / 33mA = 2,121 ohm) to take measurement under load conditions. I'm actually using a 2.25K resistor, and have used multiple values ranging from 2k to 68k...the voltage starts to sag under 69.5VDC when < 3900 ohms

The Pimm schematic only requires the user to change 3 part values to conform it to one's specific circuit. I've attached Gary's circuit with my part replacements in red font. Maybe I've messed up on a part choice for the mosfets or transistors?

I'm hoping that someone can find my error. I've been at this several months and have exhausted what little knowledge I have.

Thanks for your help

[DigitalJunkie]

1uf after the rectifier seems awfully small to me.. maybe you could give it a try with 10-20uf,and see how it behaves?

[riotubes]

Thanks for the feedback. I just ordered a range of replacement caps to see if this is the issue. Yes, it is unconventional to use that small of cap. John's early feedback on this was that the reg would take care of the ripple, and that with the small 1uF cap the peak currents through the diodes would be fairly low causing a lot less high frequency crud floating around. He added that the problem with the small cap is a very large ripple after it. So the transformer voltage has to be high enough that the bottom of the ripple is sufficiently high above the dropout voltage of the regulator that you always stay above regulation no matter what the load is. Maybe the 1uF is not accomplishing this, and by increasing it to 10-20uF my problems will be solved.

Parts choice. I'd appreciate any other feedback on the replacement parts for the mosfets and 2NNNA transistor if anyone sees a problem with my choices here. There's a pdf on the 1st post with the schematic and my part choices in red font....there was some posts by Gary about the differences between the IRFS820A and IRFS820B that required use of different resistor values. I'm using IRFI820Gs which are the insulated version. I'm hoping that these are OK and not causing the problem. His choice for 2N2222A has higher power dissipation (1200mW vs 500mW) and I"m hoping the ST Micro 2NNNA is satisfactory for the task at hand.

[Gary P]

Hi Riotubes,

Looks like the issue is caused by the 1uf filter cap. If you have a scope you can look at the voltage before the regulator to see the problem. I used PSUD2 to simulate what is going on.

The unregulated voltage is dropping to low between pulses of current from the rectifier bridge. If you increase the capacitor to 4.7uf things should start working. The regulator likes to see a minimum of 25 volts to regulate, 50 volts for optimum performance.

With the 1uf capacitor the unregulated voltage drops way too low at 33ma.


With a 4.7uf capacitor the unregulated voltage holds above 125 volts at 33ma.

[riotubes]

Thanks Gary for going to the trouble of modeling this out for me. My embarrassment in not having learned how to model in PSUD2 is overshadowed by my elation of understanding the problem (hopefully the only one!). I was even able to adjust my parts order to include a 4.7uF value based on your timely feedback. I'll report back this weekend after the caps arrive. Wow, I really need to learn how to use PSUD2...lesson learned. Thanks again DigitalJunkie for casting the spotlight on this issue. Much appreciated!

[riotubes]

Well, the 4.7uF cap arrived and it worked immediately. I can dial in exactly 70V on the reg's output. I do measure 175 VDC on the reg's input. So it's dropping 105V...I have a honkin' heatsink on Q2 and a clipon on Q1. Yes, this is unconventional, but is it too much voltage drop? ''