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SMPS - Capacitive loading
SMPS - Capacitive loading
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Old 3rd August 2019, 08:33 PM   #11
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by knutn View Post
Please clarify.
Nynorsk or bokmål?

A capacitive multiplier is in principle an emitter (or source) follower. For an emitter/source follower, the base/gate needs to have a bias voltage AND the collector/drain voltage needs to be such that the NPN transistor/N-channel FET is in the active area and not saturated. If saturation is entered, the capacitance multiplication is gone with the wind.

Let's try to image the capacitance multiplier circuit with the values you indicate. We assume you have a FET with a Vgs of 4V (active) . If you use a FET, you only need a simple RC circuit to the gate to have 20V out. As long as the drain voltage is above some 20.5V, the FET is in the active area and the capacitance multiplication works. If the drain voltage drops below some 20.5V, the FET saturates and the load sees only the actual capacitance at the drain without multiplication. Why could the drain voltage drop below 20.5V? Because an amplifier with music is a very dynamic load perhaps demanding 100mA for a moment and short after several amperes. The FET can only get this energy from the SMPS or the capacitance at the output of the SMPS. Far the majority of SMPS are designed for pretty static loads and when the amplifier, through the FET, suddenly demands a high current, the SMPS cannot supply such a current imediately. Unless there is good capacitance at the output of the SMPS, the drain voltage will zag quite rapidly and the drain voltage may pass below 20.5V. We balance the step response of the SMPS with extra output capacitance at the SMPS in order to improve the dynamic response. I guess you will need some 2200uf-10000uF for a good step response.

With a bipolar NPN transistor, the saturation voltage will be in the same order (but of a different nature) while you need two resistors (and a capacitor) to have 20.7V at the base from a 24V supply. The base current puts a limit to the value of the time-constant resistors.

Was this reasonably clear or do you want me to make my explanation more dodgy in bokmål?
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Old 4th August 2019, 08:36 AM   #12
knutn is offline knutn  Norway
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Quote:
Originally Posted by FauxFrench View Post
Was this reasonably clear or do you want me to make my explanation more dodgy in bokmål?
He, he , thank you very much!
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Old 10th August 2019, 01:01 AM   #13
warrjon is offline warrjon  Australia
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Large capacitance on the output of a SMPS is next to useless. All the HF switching noise will go straight through it.

I have repaired and tested a lot of SMPS power supplies and most of the noise components are HF at the switching frequency or harmonics of the switching Hz. I have seen low frequency noise but this is usually induced.

I have 2 SMPS's on the bench now, both are 24V 1.5amp supplies, the first is an eBay cheapie and the other is from medical equipment I scrapped and whipped the SMPS out.

The cheap one has 38mVrms ripple at 166KHz the medical supply has 2.3mVrms ripple at 100KHz, I removed the output filter from the medical supply so both are unfiltered.

There are 2 ways you can go about filtering SMPS noise.

Firstly you can determine the switching frequency and build a LCR lowpass filter at say 1/4 of the switching frequency.

The best way IMO is to use a filtered buck converter then a LDO after the buck to filter any low frequency noise.

Last edited by warrjon; 10th August 2019 at 01:11 AM. Reason: typo
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Old 10th August 2019, 01:34 AM   #14
warrjon is offline warrjon  Australia
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Here are a couple of photos of the output noise of the 2 supplies. Supply 1 = eBay supply and S2 = good medical supply

Pics in order,

S1 no filtering, S1 100nf to mains ground, S2 no filtering
Attached Images
File Type: jpg P_20190810_111015.jpg (982.8 KB, 34 views)
File Type: jpg P_20190810_111031.jpg (934.0 KB, 33 views)
File Type: jpg P_20190810_101326.jpg (968.8 KB, 33 views)
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Old 10th August 2019, 09:29 AM   #15
knutn is offline knutn  Norway
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I found another thread:

GB for Simple Cap-Mx Regulated Low-Noise PSU

Could I use something like this (post 1)?
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Old 10th August 2019, 09:30 AM   #16
DF96 is offline DF96  England
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I don't understand the original question. First, a 'capacitance multiplier' does not multiply capacitance; it is merely a CR low pass filter followed by a simple power buffer.

The SMPS will not see any extra capacitance, as it just sees the R of the CR.

If the SMPS can handle the peak current then all will be well. A 5A PSU feeding two Class A amps with 1A quiescent current drain should be OK. The 'cap multipler' will merely filter out some SMPS noise. If the SMPS cannot handle the peak current then no amount of 'cap multiplication' will make any difference.
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Old 10th August 2019, 09:37 AM   #17
knutn is offline knutn  Norway
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Referring to post #15:
It is the solution with the CLC-filter and capacitance multiplier I was thinking of.
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