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23rd February 2014, 07:10 PM  #21 
diyAudio Member

If you apply a small sine signal to the ouptut node, look at the FFT simulation of nonlinearities with various values of C4:

24th February 2014, 01:30 AM  #22 
diyAudio Member
Join Date: Feb 2004
Location: Silicon Valley

It would help if you described your test setup a little better, along with the circuit you tested. I might try a scan with a modulated current source to look at output impedance vs. frequency. I'd like to keep the low ESR caps at the regulator output, but no one says they have to have a huge capacitance associated with them (good high current polypropylenes or stackedfilm polyesters come to mind). Easy to simulate, anyway.

24th February 2014, 02:18 AM  #23 
diyAudio Member

The circuit is the one you proposed in post 17. It appeared to me that value of C4 significantly overdamped the response, so I used a sine wave to look at the nonlinearity  in simulation, of course. The idea wasn't original to me, but suggested by one of the folks corresponding to the regulator shootout article in Linear Audio.

24th February 2014, 07:37 PM  #24 
diyAudio Member
Join Date: Feb 2004
Location: Silicon Valley

Here's an output impedance scan taken by placing both a 0.1 A static load and a 0.02 A sine wave load on the output of the seriespass regulator. It looks like next time around I could use just the sinewave current source, as it has provision for DC bias. I kept the 1300uF output capacitor with 5 milliohm impedance.
The schematic is attached for reference. I used a 1uF coupling capacitor with 100k (1M) to probe the output voltage excursion. 
24th February 2014, 07:46 PM  #25 
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Join Date: Feb 2004
Location: Silicon Valley

Here's the output scan  divide the results by 0.02 to get the effective output impedance. The loop is still working to reduce the output impedance at 100 kHz, as you still don't get the full 0.02A X 0.005 ESR X Current signal at 100kHz. Next up would be to try this with some nice simulated film caps (Lower ESR) to look at both this plot and the timedomain transient response. I have a bunch of 2 uF Evox polypropylene caps that may be just the ticket.

25th February 2014, 09:02 PM  #26 
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Join Date: Feb 2004
Location: Silicon Valley

I've tried several frequency scans using equivalents of 4 X 47 uF 50V polymer coas (22 milliohms ESR) and 4 X 2uF film caps ~15 milliohms as measured by an impedance analyzer). They all loko about the same as the plot shown here, wth the polymer caps being marginally better (at 2 bucks per bitty cap, they'd better be). Next I have to look at transient response, and maybe try looking at harmonic products..

26th February 2014, 08:55 PM  #27 
diyAudio Member
Join Date: Feb 2004
Location: Silicon Valley

I brought in one of my Evox 2.2 uF PMR series caps to measure it on the HP4194A impedance/GP analyzer today at work. These are metallized polypropylene capacitors designed for high frequency, high current duty in demanding apps like switching power supplies. The effective resistance clocked in at ~7.5milliohms, lower than the calibration short for the instrument (which probably needs to be cleaned and polished). I'm going to be conservative and say the cap has an ESR of ~ 5 mohm for purposes of simulation. More later...

26th February 2014, 09:19 PM  #28 
diyAudio Member
Join Date: Feb 2004
Location: Silicon Valley

Yeehaw...
The frequency domain plots don't tell everything. With 4 of the Evox film cpas in place, the time domain simulation shows oscillation. With a cap of 190uF, 5 milliohms (representing 4 X Nichicon 47 uF, 50V polymer caps), the response is crisp and welldamped. Incidentally, this combination with the ploymer caps is also the combination that fared the best in the output impedance scan, though only by a whisker. 
27th February 2014, 07:25 PM  #29 
diyAudio Member
Join Date: Feb 2004
Location: Silicon Valley

I found a sine current source in my PSpice suite that will be suitalble for doing Fourier plots of power supply response, so I'm going to do the time domain, frequency domain, and Fourier plots for the series pass regulator with simulated 4 X 47uF, 50V polymer caps at the output. I might possibly be able to stabilize the simulation with film cap loading, but I don't want to go to the trouble at this moment. I'll then switch to regulator #2, which is essentially an open loop boosted zener diode. I'm curious how it will look with various capacitive loads.

28th February 2014, 07:09 PM  #30 
diyAudio Member
Join Date: Feb 2004
Location: Silicon Valley

Here is the variant of the series pass regulator used for time domain pulse response, output impedance vs. frequency, and fourier /THD analysis. The output capacitace represents 4 pieces of a Nichicon 47uF, 50V polymer capacitor, 22 milliohms ESR, so the composite capacitor is 190 uF, 5 miliohms. A 1uF coupling cap in series with a 1M (1000k) resistor is used as an output probe.

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