SMPS vs 50Hz Transformer noise

Audible noise? Electrical noise? Conducted? Radiated?

Yes you can post-filter the output of a SMPS, to good effect. But before you do that you have to pay careful attention to component parasitics and circuit layout -- capacitors that don't act enough like capacitors won't help, and trying to short out noise that has already escaped the boundaries of your supply is futile.

I'd like to see that 1 ohm resistor in a 20 amp 3.3V supply...

Mostly I mean radiated(Electromagnetic) and Electrical(Ripple)

you mean parasitic cap parallel to resistor of the filter and parasitic inductor in series with capacitor of the filter, right?

I want to conclude a SMPS can more easily generate a ripple free output than a 50Hz transformer.

Well then it should have multiple filters for each let's say 1A and SMPS Voltage should be higher to compensate drop.

"Ebrahim"

SMPS produce regulated, nealyr pure DC while 50Hz supplies do not.

Look it up.

Learn something.

..... Phil

Not really. Power supply filtering is a case by case issue. You don't want to add resistance when you have manufacturers outdoing each other to provide the lowest ESR caps possible. You also need to look at stability issues with such a large resistance on the output. It depends on the regulator design, the load type and what kind of noise you are looking to get rid of. Read the data sheet of the supply.

Which for ICs is usually dramatically better at 100 Hz than at 100 kHz.

If one needs quiet power, a SMPS followed by a two-pole capacitance multiplier is an excellent place to start. (An op amp loop around the CM will improve the regulation if it's really needed, but it usually isn't.)b

Cheers

Phil Hobbs

What PSRR has to do with subject of this thread? I'm comparing noise(especially ripple) of SMPS and 50Hz transformers(After bridge). different components have like OpAmps have different PSRR that has nothing do with subject of this thread.

Define 'practical'. Classical (transformer, bridge, capacitor) power supplies seem to do pretty well with a BFC as a ripple filter. Depending on how you define 'pretty well', of course.

Sorry, I didn't understand what BFC is. I think you are right and it can't be generalized to all modes and as indicated before there are situations like low voltage high current SMPS that a filter can't be practical. I usually have a lot of problem with 50Hz ripple. In my current designs output current may change anywhere from 1nA to 10A and voltage has to remain constant within 10uV, previously I've built a device that it's output current was 10uA-10A and voltage was within 100uV. It was challenging and I used SMPS there and it worked but now for 1nA-10A I'm also considering 50Hz transformer. If I use a 50Hz transformer + bridge, capacitors will be extremely big >0.1F(in total).

PS: about building bridges, I like to build bridges but in a peaceful territory not in a place constantly bombarded ;)

Big F*cking Capacitor.

Cheers! Rich

LOL :D !

A 100 uF electrolytic cap will have lots of inductance and maybe 0.1 ohms of ESR. So you won't get wonderful filtering working against 1 ohm. Ceramic caps are better here, 10 uF maybe. Ferrite beads and ceramic caps work nicely, too.

Switcher spikes can crawl all over the place through paths that are not visible on a schematic. We had one National synchronous switcher making such huge spikes that it messed up opamps clear across a board, with no obvious coupling mechanism.

John

"Ebrahim"

F*ck off and DROP DEAD !!!

"John Larkin"

.... Phil

I sure hope you are being spoofed and haven't completely lost it.

Look at the archives. Phil A has been doing this for years--sometimes more, sometimes less. It's a pity, because he's actually pretty smart underneath.

Cheers

Phil Hobbs

Yeah I thought that too and tried it. You only get about 20dB reduction. If you look at cap spec sheets you=92ll find that most caps have a minimum impedance of only about 0.1 ohms. (Except for a bit of resonant peaking (really dipping)). But use an inductor instead and you=92re in business.

George Herold

g on

Woah, 10A and 1 ohm.... I don't think that is going to work.... and you want the voltage not to change by more than 10uV. Those look like tough specs to me. (But I'm far from a power supply expert).

George Herold