Electronic – Best way to filter DC-DC output noise

dc/dc converternoisepower supply

This question has arisen due to the below issue, where a analog design board was scrapped due to too much noise from switching regulators.


The design which caused issue is as below:

LT8471 based dual supply

My complete power budget is like below:
– Input : 12V
– Ouput
Rail Load(mA)
5 1040
-5 544
2.5 800
-2.5 800
3.3 16

This kind of power supply distribution and design can be done with a bunch of switching regulators, but my question is how to completely eliminate the switching noise of DC-DC switching regulators.

In my previous design, I tried using output filter caps of 0.1F, 0.01uF, 47uF, 4.7uF as suggested, but it did not help me to the extent needed.

I am thinking of putting an linear regulator at the output of DCDC to reduce the noise. Is that proper solution, or should I use a pi filte? But they can create EMI issues to me (really not an expert in EMI avoidance. My senior suggested me to go for an inductor less module which helps in avoiding EMI issues.)

As my application is an electro-optic one, which involves analog opamps, I tried to have an output ripple voltage not more than few mV, so that it will not be difficult for me pick signals of low amplitudes around 30mV.

I agree to moderators suggesting to take care in PCB design, but primarily I want to take care schematic wise as much as possible and then want to take care in PCB design.

Please suggest techniques/tutorial to tackle noise in very low noise floor analog circuits, stressing noise filters to be adopted at output of DC-DC converters, or should I have to scrap the idea of using a DC-DC itself and go for using linear regulators.

EDIT 1 : Addition of a CLC filter at DC-DC output:

Eith some suggestions of pi filters, I have tried to create a CLC filter using components at my desk.

L = 10uH and C being 4.7uF, 47uF, 0.1uF and 0.01uF(all 0603 SMD.)

I did not get a 1nF but I was able to see noise suppressed to an extent, this set up is bare soldered and checked whether filter output is proper or not, I did not solder this on actual board, instead I took +/-5V from board and checked the filter output.

Please find the images below

Without CLC filter:

With Out Filter

With filter:

With Filter

Please help me in reducing it much further. Will a common mode choke or adding a 1nF can help me more?

Will an output LDO reduce it much further?

Best Answer

Generally speaking, high-frequency noise will go straight through a linear regulator. There's a temptation to think of them slicing off everything above a certain voltage but they don't operate like that.

Inductors can be used to block the propagation of noise waveforms but the resultant load-side rail needs to be very well decoupled. This is because a transient demand for current by the load will be blocked instantaneously by the inductor (to view it simply) so the capacitance must supply this transient current in the very short term. I put a good amount of bulk and high-frequency capacitors across the load rail to effect this.

I can't say this is the definitive solution but this approach has worked very well for me on many boards.

I had to produce quiet analogue supply rails from a DC-DC converter output. I did so by putting a pi filter (C-L-C) between my +15/-15 V DC-DC and +12/-12 linear regulator and then another pi filter on the output of the linears leading to the analogue signal conditioning load circuitry and a generous number of 10 uF and 100 nF capacitors distributed well across the board, close to each load.

In my pi filters, the C was an assortment of parallel capacitor values (1 nF, 100 nF, 47 uF). But this also ensured enough capacitor sites on the board for me to experiment with other values during operational testing and EMC testing. The L was 10 uH. I also had 100 MHz bead ferrites between the DC-DC output and the first pi filter. (I prefer not to paste/redraw the circuit for confidentiality reasons but this describes it well enough.)

Again, I can't say this is the definitive solution but this worked very well for me on this board.

Related Topic