Author Topic: Is it ok to use two step downs in parallel?  (Read 696 times)

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Offline Sway07

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Is it ok to use two step downs in parallel?
« on: October 14, 2021, 01:04:16 pm »
Hello,
my end goal is to power a RPI 400 and a old laptop screen with controller board from a single old laptop charger. Is it ok if I use the charger to convert mains into 19.5V and then two constant voltage step down modules wired in parallel to power the rpi and the screen? Here is a very crude but hopefully understandable paint drawing of what I mean.
Also I've measured the power draw of both the rpi and the screen. The rpi is drawing tops 2.3A ish at 5V and the screen is drawing 0.3A at 12V so I have plenty of head room in terms of power.
Thank you for any answers in advance.
 

Offline tooki

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Re: Is it ok to use two step downs in parallel?
« Reply #1 on: October 14, 2021, 01:18:20 pm »
If you mean whether it’s OK to wire their inputs in parallel to the 19.5V, sure, absolutely fine.

Just don’t connect the outputs of two DC-DC converters in parallel (to increase the current) or in series (to achieve higher voltage), without checking the specs carefully first. Often that isn’t possible.
 
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Offline Sway07

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Re: Is it ok to use two step downs in parallel?
« Reply #2 on: October 14, 2021, 02:44:38 pm »
If you mean whether it’s OK to wire their inputs in parallel to the 19.5V, sure, absolutely fine.
Yes, this is exactly what I mean. Thank you!
 

Online Brumby

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Re: Is it ok to use two step downs in parallel?
« Reply #3 on: October 15, 2021, 01:16:19 am »
You need to be careful if there is going to be a connection between the RPi and the screen, though!

This usually concerns the 0V reference point of the two - but there is the possibility that other connection issues may exist.  These mostly tend to be non-issues, but just be aware.
« Last Edit: October 15, 2021, 01:18:59 am by Brumby »
 
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Offline Sway07

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Re: Is it ok to use two step downs in parallel?
« Reply #4 on: October 16, 2021, 04:47:11 pm »
You need to be careful if there is going to be a connection between the RPi and the screen, though!

This usually concerns the 0V reference point of the two - but there is the possibility that other connection issues may exist.  These mostly tend to be non-issues, but just be aware.
Hmm, I tried it out and it seems to be working fine. Would this kinda problem be there from the moment I would power it on, or is it a thing that could down the line mess something up? My setup atm is I split the laptop adapter in to two wires with a wago connector. Those split wires go into the two step down modules, one for rpi, one for the screen. Obviously the outputs of the step downs go into the rpi or the screen respectively.
« Last Edit: October 16, 2021, 04:50:04 pm by Sway07 »
 

Offline tooki

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Re: Is it ok to use two step downs in parallel?
« Reply #5 on: October 16, 2021, 04:58:50 pm »
Whatever issues he’s referring to aren’t obvious at all; given the question you asked, I don’t see how he could expect to7 to figure out what he means. I can’t.

For regular, non-inverting (positive in, positive out) DC-DC converters, ground is the 0V reference. As long as your grounds all end up together you’re fine.
 
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Offline Sway07

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Re: Is it ok to use two step downs in parallel?
« Reply #6 on: October 17, 2021, 06:58:50 am »
Whatever issues he’s referring to aren’t obvious at all; given the question you asked, I don’t see how he could expect to7 to figure out what he means. I can’t.

For regular, non-inverting (positive in, positive out) DC-DC converters, ground is the 0V reference. As long as your grounds all end up together you’re fine.
Hmm, does that mean I should connect grounds together after the converters? Or can outputs of each of the converters stay kinda isolated - one goes to the RPI and one goes to the screen. (This question might seem very dumb, but I rather ask it in this way to be 100% sure I understood it. Also thank you for answering my questions, I really appreciate it)
Would this kind of problem arise if I used two power banks in the same configuration? One powering one, thing one powering the other?
Update: I just measured voltage between the outputs of the converters and I could measure either 12V or 5V depending on which + terminal I probed. So this means that there is a potential for short circuit between those two? Or is the issue another thing entirely?
« Last Edit: October 17, 2021, 07:16:46 am by Sway07 »
 

Offline jeroen79

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Re: Is it ok to use two step downs in parallel?
« Reply #7 on: October 17, 2021, 10:01:34 am »
If the convertors have a common ground on the input and output then you can tie all grounds together.

If the convertor's grounds are not common, for example due to a low side current sense,  then you cannot tie them together.
 
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Offline Sway07

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Re: Is it ok to use two step downs in parallel?
« Reply #8 on: October 17, 2021, 12:34:06 pm »
If the convertors have a common ground on the input and output then you can tie all grounds together.

If the convertor's grounds are not common, for example due to a low side current sense,  then you cannot tie them together.
Again I'm really sorry for asking stupid questions, but is there a benefit to having all grounds tied together? Or is it fine to have the input grounds tied together and have the outputs grounds separated
 

Offline Just_another_Dave

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Re: Is it ok to use two step downs in parallel?
« Reply #9 on: October 17, 2021, 01:13:15 pm »
If the convertors have a common ground on the input and output then you can tie all grounds together.

If the convertor's grounds are not common, for example due to a low side current sense,  then you cannot tie them together.

Additionally, a current loop is required in order to connect switched converters in parallel. Without a current loop in each converter (normally a peak current one is used) nothing guarantees that all of them deliver the same amount of power
 

Offline jeroen79

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Re: Is it ok to use two step downs in parallel?
« Reply #10 on: October 17, 2021, 01:13:28 pm »
Again I'm really sorry for asking stupid questions, but is there a benefit to having all grounds tied together? Or is it fine to have the input grounds tied together and have the outputs grounds separated
Depends on what exactly you want to make.
Often when parts of a circuit communicate with each other it will by something referenced to ground.
 

Offline Sway07

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Re: Is it ok to use two step downs in parallel?
« Reply #11 on: October 17, 2021, 01:39:15 pm »
Again I'm really sorry for asking stupid questions, but is there a benefit to having all grounds tied together? Or is it fine to have the input grounds tied together and have the outputs grounds separated
Depends on what exactly you want to make.
Often when parts of a circuit communicate with each other it will by something referenced to ground.

The only communication between the two will be with a HDMI cable. I want to make a sort of a laptop powered by a RPI 400. So my goal is to power both the RPI 400 and the repurposed screens controller board from the 16 or so volts the charger provides.
I used to use two power adapters and I wanted to improve it by only using one. I also want to add two power banks in the future to make it fully portable. But that is a job for future me to handle :D
 


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