Pages: [1]   Go Down
Author Topic: Daisy chaining buck converters = bad idea?  (Read 1104 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 38
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hi Arduino community!

Hope I'm posting this in the right section.

The background spiel

I'll be needing 3 different voltages in my project for various components; 5v for the AtMega328 chip, 3.3v for a display module and finally 6v for some servos.  I also want to run everything of 1 singular power supply so I plan to use a mains supply/wall wart as it's not something that requires mobility.

I plan to remove the ATMega328 chip from the board and have it permanently fixed into my project at the end (this way I can re-use the board for a future project), so I can't really rely on anything that's already built into the board to achieve this.

My plan was to have a 12V 3A AC-DC power supply (a laptop charger) connected to a buck converter to step the voltage down to 6V 3A.  This will provide power to the servos.  I would then daisy chain a second buck converter to this to bring the voltage down further to 5V which would then power the ATMega328 + it's components (crystal etc...).  From here I would then daisy chain a third buck converter to bring the 5V down to 3.3V for the display module (I may substitute this for a linear regulator as the current I need for the 3.3V part is very low; 20mA).

I was thinking of daisy chaining them in this manner, rather than having all 3 connected to the laptop chrager directly. I thought by doing this, the buck converters would have less voltage they need to drop when it gets to the 5V and 3.3V regulation (meaning less heat would be generated so I won't need to add any heat sinks or anything).  From what I understand buck converters are very efficient and will not generate lots of heat like linear regulators can, but I thought this still may be worth doing.

Questions
  • By daisy chaining buck converters, does this cause any problems?  For instance, current issues, heat issues or hazards? Is it better to have all 3 connected directly to the AC-DC plug, if so is there a reason?
  • This one may sound completely silly.  If the buck converter was rated for 3A max and the power supply says it can only provide 3A max, is there anyway to boost the current that is available after the buck converter?  For instance for powering the servos I have 6V and 3A max after it's gone through the converter, but ideally I would like 6A to be available if the servos need to lift any heavy loads as each servo can potentially try to draw up to 1A each on very heavy loads.
  • Slightly off the main topic here but, if I wanted to protect part of my circuit so it can never draw more than 2A, is there anything I can use to do this?  I'm just concerned in very heavy loads where the servos may potentially try and draw the full 3A, this leaves nothing for the rest of the circuit (ATMega328 for instance) and will cause resets or other issues.  So I was wondering if there was a way to limit this part of the circuit so it can draw between 0A-2A but no more than that, leaving 1A available for the rest of the circuit.

In case you need to know, the buck converter I planned on using has a LM2596 regulator.

Thanks you very much for your time and reading this, I look forward to your responses smiley

« Last Edit: November 18, 2012, 06:06:27 pm by Bobu » Logged

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17294
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Questions
By daisy chaining buck converters, does this cause any problems?  For instance, current issues, heat issues or hazards? Is it better to have all 3 connected directly to the AC-DC plug, if so is there a reason?

It would probably work either way, but I would hook them all up to the same DC input voltage. Switching regulators may even be more efficient with the higher input voltage, they don't regulate voltage via heat dissipation in either case like linear voltage regulators do.
This one may sound completely silly.  If the buck converter was rated for 3A max and the power supply says it can only provide 3A max, is there anyway to boost the current that is available after the buck converter?

No the converter cannot supply more current output then is available from it's input voltage source.

  For instance for powering the servos I have 6V and 3A max after it's gone through the converter, but ideally I would like 6A to be available if the servos need to lift any heavy loads as each servo can potentially try to draw up to 1A each on very heavy loads.

Then just add an additional 6vdc buck converter and split the servo loads between the two converters.

Slightly off the main topic here but, if I wanted to protect part of my circuit so it can never draw more than 2A, is there anything I can use to do this?  I'm just concerned in very heavy loads where the servos may potentially try and draw the full 3A, this leaves nothing for the rest of the circuit (ATMega328 for instance) and will cause resets or other issues.  So I was wondering if there was a way to limit this part of the circuit so it can draw between 0A-2A but no more than that, leaving 1A available for the rest of the circuit.

Not from the same converter. There are ways to add extra filtering caps and diode isolation if you end having this problem. These buck converter regulators are usually able to handle short 'overloading' situations as their chip usually has self protection functions inside them.
Lefty

 
Logged

Offline Offline
Faraday Member
**
Karma: 65
Posts: 2530
Now, More Than Ever
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Q) This one may sound completely silly.  If the buck converter was rated for 3A max and the power supply says it can only provide 3A max, is there anyway to boost the current that is available after the buck converter?

A) No the converter cannot supply more current output then is available from it's input voltage source.

That depends on what you really mean here.
If it's a switching supply, a power converter, it is possible.  
Say the input is spec'd "12V, 50W" and the output is spec'd "5V, 40W", "80% eff." roundabout, the input current would be 4A and the output current would be 8A.

PS - So, that ATX supply is looking better and better all the time, Bobu?  (:
« Last Edit: November 18, 2012, 09:23:29 pm by Runaway Pancake » Logged

"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

Offline Offline
Newbie
*
Karma: 0
Posts: 38
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Thanks for your input guys.  Looks like I'll go buy a batch of these buck converters (pretty cheap off ebay, they actually work out to be even cheaper than a linear regulator from a electronics store here in the UK!).

@Runaway Pancake
Quote
PS - So, that ATX supply is looking better and better all the time, Bobu?  (:
It certainly is.  The only reason I don't want to use an ATX power supply is because of the physical size of it.  It would be great to use as a lab power supply, but for the final project where I want to make everything permanent, it's just too big for my liking smiley-sad
« Last Edit: November 19, 2012, 06:09:40 am by Bobu » Logged

United Kingdom
Offline Offline
Tesla Member
***
Karma: 224
Posts: 6619
Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Don't use a buck converter to drop 6V to 5V, it won't be any more efficient than a linear regulator. Either use a 5V buck converter running from your 12V supply, or use a linear LDO regulator to drop the 6V to 5V.
Logged

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Pages: [1]   Go Up
Jump to: