21st Century Maida Regulator

[tomchr]

Originally Posted by grufti

I believe Paul was refering to C1b and C3 in your schematic.

Oh, my bad... I get tunnel vision some times...

C1 allows for the use of one 10 uF, 450 V electrolytic can for input voltages of 400 V or below OR one 1 uF, 875 V polypropylene cap for input voltages up to 800-ish V. I'm running over 400 V in, hence, have C1a populated. The footprint for C1b is actually covered by C1a to avoid confusion - or perhaps confuse everybody...

The 100 uF in the schematic (C3) does not need to be a high voltage cap, as it only "sees" 1.0 V in the circuit. In my prototype, I actually used a 10 uF, 25 V tantalum cap (near the edge of the board, by the regulator) as I didn't want excessive start-up time. With a 10 uF cap, the start-up time is supposed to be 10 seconds; a 100 uF cap extends this to 100 seconds. Sorry for not being clear on that.

~Tom

[tomchr]

Originally Posted by jackinnj

before you sell it, you'd best run a phase-gain plot to make sure it's stable -- this is, after, an LDO

I do agree that a gain/phase plot with phase margin, gain margin indicated would give me a warm and fuzzy feeling inside. However, the same information can be inferred from the transient response and output impedance plots.

Do you have any suggestions for how to take the gain/phase measurement on this regulator? I have the gear...

My idea is to use a small common mode transformer to inject a signal into the feedback loop and measure Vout. Do you have better suggestions?

~Tom

[tomchr]

Originally Posted by Globulator

Have you got dimensions of the populated final module?

Approx. 50x50x25 mm not including the heat sink.

~Tom

[tomchr]

Just found the answer to my own question about how to measure the phase margin in a non-invasive way.

Extracting Bode Plots from Output Impedance, Power Electronics Technology, March 2012.

~Tom

[pauldune]

Tom,
I'm a little bit worried about the buffer cap behind the regulator. Its an important part of the ac current loop, its the ac/signal return path to ground. If its not large enough, I expect a part of the signal current wil find its way through the regulator itself instead of the cap.
I dont have it really clear in my mind how the signal current loop goes with a regulator.

I read about maida regulators, that they dont do well with capacitive loads, but would this particular design work with say 10uF? I think that would be enough.

Or am I completely mistaken?

Greetings, Paul

[tomchr]

Originally Posted by pauldune

I'm a little bit worried about the buffer cap behind the regulator.

I don't know which cap you mean. Which part is it on the schematic?

Originally Posted by pauldune

I read about maida regulators, that they dont do well with capacitive loads, but would this particular design work with say 10uF? I think that would be enough.

I have had great trouble with my original Maida regulator (based on National Semiconductor LB-47 using an LM317) driving a capacitive load. That started my quest for a better regulator to begin with. With this 21st Century Maida Regulator, I have loaded it with up to 47 uF on its output without start-up issues as long as a resistive load was present. I need to ensure that the same is the case when starting up into a purely capacitive load.

My goal with this regulator is to get the best ripple rejection I can get. I would prefer that the regulator is also capable of starting up into a capacitive load (as you say, at least 10 uF would be nice). Ripple rejection is a "MUST HAVE". Start-up into a highly capacitive load is a "nice to have" in my mind.

~Tom

[pauldune]

I meant c3, 2,2uF in your schematic. I' d like it to be a bit bigger, thats the 10uF I was talking about.
Do i understand you correctly, as long as a resistive load is present, that would be ok?
How does this translate in a reallife situation, where for example a typical 300b se amplifier is powered through your regulator? During warmup the regulator sees only the buffer cap, because the tubes are not conducting. Or is this why you have a slowstart build-in?

[tomchr]

Originally Posted by pauldune

I meant c3, 2,2uF in your schematic. I' d like it to be a bit bigger, thats the 10uF I was talking about.

That makes sense. I think you mean C2 = 2.2 uF, though.

C2 is a critical component in the design. In order to ensure stability of the regulator, it must have low ESR - preferably well below 100 mOhm. This drove me to choose a polypropylene film cap. 2.2 uF was an acceptable trade-off between size, price, and performance.

I would like to be able to add a 10 uF electrolytic cap in parallel with C2. But I need to make sure the regulator can start up with such a cap in place. Stay tuned.

I don't think I would include a footprint for the electrolytic cap as the regulator actually works just fine without it. I'd rather suggest that people place a 10 uF electrolytic cap by the B+ connection in the circuit that's being powered by the regulator. This should result in the best performance.

Originally Posted by pauldune

Do i understand you correctly, as long as a resistive load is present, that would be ok?

That's the case presently. However, I'm working to characterize this fully. Stay tuned. The goal here is to have a regulator that can start up with at least 10 uF added on the output (in parallel with C2) without requiring any additional resistive load. 47 uF would be even better.

Originally Posted by pauldune

How does this translate in a reallife situation, where for example a typical 300b se amplifier is powered through your regulator? During warmup the regulator sees only the buffer cap, because the tubes are not conducting. Or is this why you have a slowstart build-in?

The slow start serves many purposes. Graceful start-up into a capacitive load being one of them. It is also required in order for the cascode device (Q1) to survive start-up.

I would model a typical 300B amplifier during start-up as an open circuit. This is why I would like the regulator to be able to start up into a purely capacitive load. My application happens to be a 300B amplifier, so that'll be my test bench (in addition to lab testing, of course).

Right now, I need to do more characterization before I can commit to start-up into a purely capacitive load.
However, I had no issues with startup into 47 uF in parallel with a 30 kOhm resistor (Iload = 14 mA, Cload = 47 uF).

~Tom

[tubelab.com]

Tom,

I have been experimenting with solid state regulators for use in tube amps for some time. I want to make one that's suitable for experimentors which imposes a new set of constraints. You need an adjustable output over a fairly wide range, and it needs to be stable for all loads including a short.

The one that I could never make right with the Maida design is the ability to handle an accidental (or in the Tubelab sense deliberate) short circuit on the output when the regulator is powered up and operational. Have you tried this. If not, and you do try it, shield yourself from mosfet schrapnel. The Fuji's that I have been using explode violently. The mosfet Maida with a few well placed zeners will start up into a short OK, but it self destructs when shorted

[jackinnj]

Originally Posted by tomchr

My goal with this regulator is to get the best ripple rejection I can get.

This is a fascinating topic -- I think that what you have to do is craft a board on which you can substitute a number of devices.

My guess is that below -80dB ripple rejection is going to become less relevant than Zout. (It's a somewhat educated guess based upon another project I've been working on). The corollary is that the amplifier in the regulator has to be pretty quick, high bandwidth. I would suggest looking at some of the fast transient response regulators from LLTC and not overly damping the device for low noise. Some of the LLTC regs allow for remote load sensing and this is a MAJOR IMPROVEMENT!