Electronics > Beginners

Power supply multi-rail noise separation?

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paulca:
I am looking for a simple, robust noise suppression circuit.

My input power for this project is a 30-36V LFP (with BMS/balancer).  I will have, 0V 15V, 30V taps.  Allowing me to provide -15V,0V,+15V for amps and also low power linear 12V and higher power switched 12V + USB.

Loads are heavy weight MCUs, 4" color TFT LCD screens and a headphone amp.    I have no worries about the source supplying enough power, at a stretch (not planned, but considered), the pack could supply the desktop speakers, replacing it's internal SWPSU.

I am considering using a series of LPF circuits to supply bulk caps, creating high impedance in both directions, leaving the spikey demand power up to the cap.

I know these are limited as they significantly limit the average current capacity and efficiency.   "Capacitor" multiplier circuits are a limited option.

I also expect having some induction in there and having a pair of LRC circuits might be wise.  I just have no idea where to start calculating.

On the RC circuit....  If I decide the most I want to lose is 1V, then I need to select the resistor that will allow the capacitor to charge fast enough that the opamps can never drain it more than 1V.

Actually.  Induction.  Induction will dynamically limit current, so if a chunk of power gets sucked out of the cap by the opamp and the supply rushes to replace it the inductor will resist the current spike and smooth it out.

Is it a "PI" circuit when you put a LC and RC circuit is series?  Is this even what I'm looking for?

Basically a narrow path for current to trickle smoothly while either side can be spiking and noisey.  Hoping to prevent parasitic noise on rails.

aliarifat794:
Pi can provide effective noise filtering across a broader frequency range. This option sounds good to me.

Terry Bites:
Before considering filter requirements plan the wiring. Don't add too many filter components unitll you know how big your noise problem really is.

Keep analog supply and digital supply wiring separate. The only place where analog and digital power and ground wiring should meet is at the battery or battery switch.
Use CM chokes/ filters on all digital power lines to contain the nasties at source. ie on the display and micro pcbs or as close as you can physically get.
Keep wiring loop areas small, use twisted groups (pairs)wires.

Capacitance multipliers (gyrators) made from MOSFETs introduce unacceptably high voltage drops. If you want to use them, try Darlingtons or medium power types instead.
The headphone amp will only need to supply a few 100mW max so a ZTX651 will likely be good enough. Power supply regulation for audio amps can be very relaxed.
The LDOs in fig 1 could equally be capacitance multipliers. RFI filtering of the incomming audio is wise.

Voltage regulators will help keep the spikes out. If you can afford a bit of voltage drop on your small signal supply then consider using them.
An LDO will be a good choice. Adjustable regulators can reduce noise still further by bypassing the adj pin to ground.

The batteries are huge capacitors in their own right. Adding energy storage and bypass caps may be ineffective if not located at the point of load.
Low ESR types are a must. If not, they won't act as capacitors at high frequencies. Regular eclos may not work well.

https://www.omnicalculator.com/physics/lc-filter

jonpaul:
EMI needs to be analyzed , see the many books and seminars on EMI filter design and DM, CM noise measurement.

Considet both input and load side filters.

Off the shelf   EMI filters  are reccomend.

See coilcraft,

j

paulca:
The main noise that plagues me in the audio band is USB/PC/Digital chatter.  It may even be from the HDMI cables.  The kind of noise that changes as you move the mouse around on the computer.  It seems that anything which connects to either the USB, or connects to the same power supply which is powering the USB hub... suffers this noise.

It's the age old USB ground noise.  The noise IS in the audio band, so it's not like I can make a filter to remove it without removing the audio within that range as well (hi hats etc.).

Additionally there is buck converter whine in there somewhere.  I expect it's the USB hub stepping the 12V down to 5V.

In an ideal world I would remove all USB power connections and only use optical links to PCs.  The world is not ideal though and there will inevitably be a USB audio source needed.  In an ideal world I would pull all my cabling out of the lab/office and reroute noisy signal cables like HMDI far away from DC power lines and USB cables.  The world is not ideal and my lab/office is a noisy environment for DC power.  The whole point of this device is to try and decouple me from it.

I suppose there "is" the option of just making a new amp with an Optical-In (TOS), a DAC and my OPAs.  Run that and that alone on the battery.  The screen showing the battery status etc can be put on a push button, when it's pressed the MCU and screen are powered the MCU boots and shows the data.  Let go and it dies, disconnects, goes away alone with it's noise.

It's an expensive battery for a DAC amp, but it would be damn silent.  My only risk then becomes RF.

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