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Old 23rd February 2014, 06:10 PM   #21
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If you apply a small sine signal to the ouptut node, look at the FFT simulation of non-linearities with various values of C4:
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Old 24th February 2014, 12:30 AM   #22
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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 stacked-film polyesters come to mind). Easy to simulate, anyway.
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Old 24th February 2014, 01:18 AM   #23
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The circuit is the one you proposed in post 17. It appeared to me that value of C4 significantly over-damped the response, so I used a sine wave to look at the non-linearity - in simulation, of course. The idea wasn't original to me, but suggested by one of the folks corresponding to the regulator shoot-out article in Linear Audio.
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Old 24th February 2014, 06:37 PM   #24
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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 series-pass regulator. It looks like next time around I could use just the sine-wave 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.
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Old 24th February 2014, 06:46 PM   #25
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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 time-domain transient response. I have a bunch of 2 uF Evox polypropylene caps that may be just the ticket.
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Old 25th February 2014, 08:02 PM   #26
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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..
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Old 26th February 2014, 07:55 PM   #27
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I brought in one of my Evox 2.2 uF PMR series caps to measure it on the HP4194A impedance/G-P 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...
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Old 26th February 2014, 08:19 PM   #28
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Yee-haw...
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 well-damped. 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.
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Old 27th February 2014, 06:25 PM   #29
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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.
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Old 28th February 2014, 06:09 PM   #30
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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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