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Mr Evil 12th March 2005 07:01 PM

Finished capacitance multiplier
 
2 Attachment(s)
I've completed the capacitance multiplier first proposed in a previous thread. The final design is pretty much the same as the original. It's intended to supply the output stage of the subwoofer amplifier I'm currently working on.

Information and pictures concerning the capacitance multiplier can be found on my website.

Additionally, there is a simple regulated supply to go with it, also described on my website.

It works rather well. It's certainly tougher than my poor abused dummy load resistors:hot:

For those too lazy to click a link I've attached a photo of it to this post.

Mr Evil 2nd April 2005 05:04 PM

No interest? I thought it was a pretty neat design myself. I know it's only +/-15V, but it can easily be scaled to any voltage by choosing transistors with appropriate voltage ratings. It is good for lower voltages though, where the low dropout gives the biggest efficiency advantage.

2pist 3rd April 2005 04:13 AM

um, I like it..... what does it do?

padamiecki 4th April 2005 06:01 AM

hello!
are you realy evil?
by the way, some people say that non global nfb psu is the best,
did you try it?

Yoghourt 4th April 2005 06:54 AM

Hello mr evil,

Why 4700uF at supply output? What does this PSU typically drives? (type of load, typical current ratings, decoupling/bypassing).

Mr Evil 6th April 2005 02:51 PM

Quote:

Originally posted by 2pist
um, I like it..... what does it do?
A capacitance multiplier is similar to a voltage regulator, but instead of maintaining the output at a fixed voltage, it keeps it at a proportion of the input voltage, similar to how a very large capacitor would work.



Quote:

Originally posted by padamiecki
hello!
are you realy evil?
by the way, some people say that non global nfb psu is the best,
did you try it?

Am I really evil? Opinions differ. Some would say yes, some would think it impossible ;)

The usual, simple capacitance multiplier has no global feedback. It's not as good as this more complex circuit, which needs NFB to keep output impedance low.



Quote:

Originally posted by Yoghourt
Hello mr evil,

Why 4700uF at supply output? What does this PSU typically drives? (type of load, typical current ratings, decoupling/bypassing).

The schematic of the prototype had 4700uF, the final version (in the third link, which could probably have been better marked as the 'main' link) has 100u at the output. I experimented with various values and found that high values improve impulse response but do nothing for ripple. This PSU is used to power a low-power amplifier which has its own 1000u local bypassing. Load current would be about 3A rms at most. The purpose of the PSU is low ripple, since the speakers are high sensitivity. It's overpowered for this, but as such it provides good experience for future applications.

padamiecki 7th April 2005 05:45 AM

Quote:

Originally posted by Mr Evil

The usual, simple capacitance multiplier has no global feedback. It's not as good as this more complex circuit, which needs NFB to keep output impedance low.


so, it is better than other, because of lower impedance and more complexity?
Do you have any comparsion?

Dark Harroth 7th April 2005 06:54 PM

Impressive PS design, and even the PCB topology is present... :) :up:

But if I for exapmle need it to work at +/- 60 Volts, must I replace only the pass transistors or the whole circuit is to be recalculated (I mean the resistors, capacitors etc...)? What maximum on-RDS for the pass FET would you recommend?

Mr Evil 8th April 2005 02:21 AM

Quote:

Originally posted by padamiecki
so, it is better than other, because of lower impedance and more complexity?
Do you have any comparsion?

I didn't mean it had lower output impedance than the simple design, I meant that it has to have NFB to avoid the output impedance being too high. I don't remember which had the lowest impedance, but both are low enough. The advantages of this design are lower dropout voltage and lower ripple for a given RC filter capacitance or smaller capacitance for the same ripple (by about 1000x).



Quote:

Originally posted by Dark Harroth
Impressive PS design, and even the PCB topology is present... :) :up:

But if I for exapmle need it to work at +/- 60 Volts, must I replace only the pass transistors or the whole circuit is to be recalculated (I mean the resistors, capacitors etc...)? What maximum on-RDS for the pass FET would you recommend?

The pass transistors must withstand full input voltage, so they will need to be replaced. Choose ones with the lowest Rdson you can find as it limits dropout voltage/efficiency. The LTP MOSFETs and BJTs likewise need to withstand full input voltage. The CCS JFETs have 1.5x full input voltage across them. I don't know if you'll find ones that can take 90V+, so you'll need another type of CCS (a simple resistor will do). The rest of the circuit should be suitable without changes.

Dark Harroth 9th April 2005 02:05 PM

Quote:

Originally posted by Mr Evil
The CCS JFETs have 1.5x full input voltage across them. I don't know if you'll find ones that can take 90V+, so you'll need another type of CCS (a simple resistor will do).
Somehow I feel that a JFET would be better, but the only ones that can handle such a high voltage are Soviet-made (and these ones have to be properly matched, because their parameters can differ from each other for more than 20%).
So I have an another question (a stupid one I should say), can I put three JFET CCS in series to obtain a 150-volt limit?
If not, what resistor should I use for +/- 60 Volts as a replacement, a 60/0.0053=11,5K one?


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