Switch mode power supply filter

[paulca]

On language I also understand that the language of electronics is effectively mathematics.  Mathematics can be used to describe the relationship between things, how they interact and if you put values in, calculate cause and effects.

Things like Ohms law etc. I am fine with, it's when you get into capacitors, inductors, and anything that involves a frequency that trigonometry raises it's head.  Putting PI, sin(), cos() etc, into an equation I can probably keep up with, they are just functions and constants after all.  But when it get's into things like simultaneous equations to get values out or quadratics I just switch off.

[MrAl]

Been through a bunch of articles and you tube videos and I'm still a bit lost.

I have a PSU which produces two rails, +15v and -15v.  However both have 50mVpp pulsing ripple.  On a scope there are repeatative ringing pulses which I assume are inductor spike from the switching.

I'd like to at least have a crack at removing them.

So far I tried placing capacitors between a rail and ground, a 470uF + 100nF from each rail to "common/ground".  This had virtually no effect which is a bit bizarre.  Should the caps be across both rails, ie, the full 30V span?  I would have thought this would be pointless as both + and - rails have the same noise, making it common mode noise.  The GND/common rail is quite quiet (or so it seems on a scope, but really I am just connecting the scope GND and probe to the same circuit, so I won't see noise).  Actually the fact the caps did nothing might suggest this is indeed common mode noise.

One solution I found on the web was to place small ferrite ring inductors before the decoupling caps.  Is this likely to work?  I don't want to order them, even though they are about £1.50 for 10, if they aren't going to work.

The project they power will consume around 100-200mA, the PSU is rated for 200mA.  The project invovles lots of opamps and while I'm sure the ripple artifacts will probably not upset the audio signal, they make my scope look a mess when debugging and it's urk'ing me.

Hello there,

Did you try a post LC filter yet?  That's the typical way to get rid of small ripple out of a switcher.

A small inductor with large capacitor and ceramic capacitor takes the spikes down quite a bit.  The inductor can be low inductance so you can use an air core device which you can wind yourself.  Even 2uH and 100uf in parallel with 0.1uf will take the spikes down.

An important point though is to NOT connect the feedback of the power supply to the output of the filter.  The filter goes after the feedback.  If you put the feedback after the filter you can get all kinds of oscillation so dont do that.
So dont change the feedback at all, just add the L nad C and C to the output and see the nice results :-)

I've seen this work on numerous power supplies, but one i remember is when myself and some other online users did a 300kHz power supply that used a Zetex chip made for use as an LED driver.  Adding a small inductor and some capacitance took the 300kHz spikes down low.

The other thing to watch out for is fake readings.  If the scope picks up the noise from the inductor in the switcher it could look like ripple on the output.  You may have to make your own low impedance probe, bu try the 1x setting first.

[paulca]

Nothing wrong with Tims posts that I can see (Sometimes I learn things from them).

No, there is nothing wrong with them at all.  It's me who is at fault.  I think me and Tim are on different sides of the room regarding understanding of electronics so I maybe lack the understanding to fully understand what he is telling me.

As to whether I want to become an electrical engineer.  Probably not.  What I do want is to continue a progression as a hobby which involves challenges, solving them and producing practical, functional electronics.  This I find fun.

I can certainly do "Maker" style stuff of buying ebay boards and hooking them together and maybe introducing a micro-controller for logic and orchestration.  However projects like this often throw me into the deeper darker worlds of analogue filters and I'm up to my neck in the intricate details and maths.  Without the mathematic's background I struggle as I cannot "see" or "hold" the problem in my head.

At the same time I can't expect to come in here and ask someone to design it for me.  However, what I often hope for is that there is a "good enough" generic solution that can be explained fairly easily that will get me by the problem for now and provide me with a little understanding I can build on later.

Back to context, trying not to forget the problem I'm trying to solve and the scope I'm solving it in.  I am not going to spend months learning everything about SMPS filters, LC / LRC filters and iterating to get a perfectly quiet supply.  The circuit will not effected that much by the noise anyway.  I just wanted to make a "best effort" attempt at reducing it somewhat, without... and this is key... without actually introducing more noise, such as resonance from the LC circuit and making things worse.

I might go with something like the Spice I posted, redo the PCB layout, and get the PCB made and built and see how it does.  If it gets a net reduction in noise I will consider it a success and move on for now.  I'm sure I'll be back to filters again, many times and hopefully each time I learn more and get better and ask less stupid questions.

[TimNJ]

Thanks.  The thing that puzzled me more was that the voltage is attenuated to nearly 0, but the current remains quite high related to the input.  I figure there is an obvious reason for this I haven't quite spotted yet.

On the sub 1 Ohm resistors, I can always stack 1 Ohm resistors on top of each other.  A little bit Magiver, but hey.  Might not do noise any favours.

Do you think the noise on the output voltage is attenuated to nearly zero just because it looks like it's zero? I think the auto-scaling of LTSPICE is fooling you. You have an input signal with very high peak-to-peak amplitude, and so your output voltage looks nearly flat in comparison. In the same vein, your current signal is auto-scaled such that you can see it, so it appears pretty large.

Let's do a quick sanity test.

By my estimate, the peak-to-peak amplitude at n001 (blue) is approx 0.02V / ~8 = 0.025V. (Not a lot of resolution on the y-axis, so it's hard to tell.) With a 75ohm load, the peak-to-peak ripple current through the load should 0.025V/75 = 0.033mA. Now let's take a look at the peak-to-peak ripple current waveform through the 75 ohm resistor.  My estimate is 199.365 - 199.343 = 0.022mA.

0.033mA vs 0.022mA. Pretty close for an eyeball estimate.