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Old 29th May 2008, 08:25 AM   #1
jkeny is offline jkeny  Ireland
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Default Capacitance Multiplier on SMPS

I posted this in the Class-D area but no replies - it's probably more suited to this forum!

Has anybody tried this to reduce HF noise on SMPS. AFAIK cap mult acts as an inductor & so HF is attenuated more. Would this be a good way of removing the HF spikes on these PS?

I'm thinking of improving my CD/DVD players as well as a digital amp that use SMPS

The idea and some oscilloscope shots are here as part of an interesting discussion on how to improve 3 pin voltage regulators: http://www.acoustica.org.uk/t/3pin_reg_notes2.html
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Old 29th May 2008, 01:48 PM   #2
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Why not? On certain audio circuits like preamps or opamps in general, it's good to have a lot of filtering. IMO you can never have too much filtering.
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Old 29th May 2008, 02:08 PM   #3
jkeny is offline jkeny  Ireland
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Yea, my simplistic thinking on this was that most, if not all, of the noise on the supply of a SMPS tends to be well outside the audio band up around the switching freq & this is where a gyrator or cap multiplier has it's biggest attenuation.

I just wanted to check out with some of the SMPS experts here if there is any gotchas before I try it out on real equipment!
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Old 31st May 2008, 11:49 PM   #4
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I wouldn't expect a capacitance multiplier to have very good attenuation at the >1MHz frequencies of interest. It should have the same problem as the 3 terminal regulators tested in that link.

And why would you need it? The cap values required are small. An RC or LC filter will work much better.
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Old 10th June 2008, 09:03 PM   #5
Tolik is offline Tolik  Israel
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Else check AVX multilayer ceramic caps 20..50..60uf, specially designed for noise attenuation in SMPS. These can be found at Digy Key.
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Old 11th June 2008, 08:36 AM   #6
cerrem is offline cerrem  United States
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Default Re: Capacitance Multiplier on SMPS

Quote:
Originally posted by jkeny
I posted this in the Class-D area but no replies - it's probably more suited to this forum!

Has anybody tried this to reduce HF noise on SMPS. AFAIK cap mult acts as an inductor & so HF is attenuated more. Would this be a good way of removing the HF spikes on these PS?

I'm thinking of improving my CD/DVD players as well as a digital amp that use SMPS

The idea and some oscilloscope shots are here as part of an interesting discussion on how to improve 3 pin voltage regulators: http://www.acoustica.org.uk/t/3pin_reg_notes2.html

EMC radiated emissions is a royal pain to tackle with SMPS...
I have designed PWM chips with dither in the clock to spread spectrum the noise, thus lowering the amplitudes of the noise...
The other option is to use quasi resonant circuit...

cerrem
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Old 11th June 2008, 08:41 AM   #7
cerrem is offline cerrem  United States
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Default Re: Capacitance Multiplier on SMPS

Quote:
Originally posted by jkeny
I posted this in the Class-D area but no replies - it's probably more suited to this forum!

Has anybody tried this to reduce HF noise on SMPS. AFAIK cap mult acts as an inductor & so HF is attenuated more. Would this be a good way of removing the HF spikes on these PS?

I'm thinking of improving my CD/DVD players as well as a digital amp that use SMPS

The idea and some oscilloscope shots are here as part of an interesting discussion on how to improve 3 pin voltage regulators: http://www.acoustica.org.uk/t/3pin_reg_notes2.html

EMC radiated emissions is a royal pain to tackle with SMPS...
I have designed PWM chips with dither in the clock to spread spectrum the noise, thus lowering the amplitudes of the noise...
The other option is to use quasi resonant circuit...

cerrem
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Old 11th June 2008, 01:22 PM   #8
star882 is offline star882  United States
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Default Re: Re: Capacitance Multiplier on SMPS

Quote:
Originally posted by cerrem



EMC radiated emissions is a royal pain to tackle with SMPS...
I have designed PWM chips with dither in the clock to spread spectrum the noise, thus lowering the amplitudes of the noise...
The other option is to use quasi resonant circuit...

cerrem
Isn't the simplest solution just enclosing the power supply in a metal box?
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Old 11th June 2008, 02:59 PM   #9
Eva is offline Eva  Spain
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EMI is mostly radiated by external wiring so metal enclosures are not of great help.

The most useful filtering is common-mode and its purpose is to keep input and output grounds at the same RF potential. Then differential-mode filtering attenuates the remaining RF components before they reach the wiring.

A capacitance multiplier is mostly useless in comparison with a LC filter.
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