digital modules and a/symmetric power draw

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Kurt Girdle
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digital modules and a/symmetric power draw

Post by Kurt Girdle » Sat Feb 20, 2021 1:29 am

Hi all

for my diy stuff i use a few meanwell switchers to get +/-12V and they give me equal current capacities for each rail.
previously i had just taken it as a given that my digital modules internally deriving 5V/3V3 from the +12V rail would have an asymmetric current draw and i would thus hit the current limit on the positive rail before getting close on the negative.

however, i was recently looking at the specs of the newish noise engineering modules using the daisy seed (desmodus versio and imitor versio) and noticed something odd... symmetric power draw! even though they are using a pretty beefy STM32H7 MCU.
is this a standard thing and i've just been oblivious to it? know of any other modules doing this?
looking at the rear pic of imitor versio on schneidersladen it looks like they are using a CUI DC-DC module to step down to either 5V or 3V3 (daisy seed might have an LDO for 5V input, not sure).

so i'm wondering if anyone knows how they've gone about this? it would really help me get the most out of my power situation going forward.
my guess is they have stuck it across the 12V rails(i.e. Vin:+12V, gnd:-12V), so presumably the part would need to handle a 24V input. would the converter in this configuration need to be isolated? hmm
anyway, any ideas are much appreciated!
stay safe!

edit:
did a bit more thinking... i forgot that isolated modules don't share a gnd from input to output (isolated...duh...)
so it seems like using a 24V input isolated module hooked across the rails and connecting 0V(output side) to gnd might work
something still seems a bit off here... not sure how the switcher's input filtering/conditioning would need to change

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Grumble
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Re: digital modules and a/symmetric power draw

Post by Grumble » Sat Feb 20, 2021 3:34 am

so it seems like using a 24V input isolated module hooked across the rails and connecting 0V(output side) to gnd might work
That’s how I would do it if I needed balanced power drainage. Filtering can stay the same, like a small inductance and some caps to gnd where the power supply enters the module.

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MikeDB
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Re: digital modules and a/symmetric power draw

Post by MikeDB » Sat Feb 20, 2021 8:50 am

Yes a SEPIC convertor can be referenced to almost any point you choose. As well as a symmetric load the HUGE advantage of doing it this way is that any noise going back into the source goes nowhere near the ground wires. Opamps are very good at rejecting power supply noise, but as we've all experienced, when it gets on your ground you're in trouble.

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EATyourGUITAR
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Re: digital modules and a/symmetric power draw

Post by EATyourGUITAR » Sat Feb 20, 2021 10:00 am

inverting opamps are very good at rejecting common mode noise on +12v, -12v. if you power your module from an isolated DC DC converter running on 24v vin, any noise going back up into your bus board is differential mode obviously because you are already floating the ground at Vin. on your output of your DC DC converter is where you actually reference ground with a voltage divider.
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