LM2596 - Buck Converter in series

Thread Starter

SamarthGohil

Joined May 11, 2021
6
Hi Everyone,

Is it possible to have 2 LM2596, buck converters' output voltage in series to add them? I am not too sure if these are isolated kinds or not.

I am trying to connect (4) 9V batteries in series to provide input to the LM2596 buck converter. I have two of such configurations. Each one is regulating output at 24VDC. Now is it possible that i connect the (2) outputs from the (2) LM2596 buck converters in series to get 48VDC?

Buck converter:
https://www.amazon.ca/Zixtec-Conver...9Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

Thank you.
Regards,
 

Attachments

Danko

Joined Nov 22, 2017
1,079
I am not too sure if these are isolated kinds or not.
Seems they are not isolated.
1620769928300.png
You can use them in series, as shown on your circuit, but each output should be shunted by diode, anode to -Out, cathode to +Out.
ADDED:
Not sure about what the diode is doing in a series connection with each output having a shunt diode.
Diode D1 or D2 decreases current through inductor and diode of converter in cases, when SW3 is ON, but SW1 or SW2 is OFF:
1620773333416.png
 
Last edited:

Papabravo

Joined Feb 24, 2006
16,091
It is probably not a good idea. Especially if you do not understand the details of the LM2956 operation. One of the problems you might have is the upper converter might be depending on a ground reference for regulation. If what it thinks is ground is something else you might have an unstable mess on your hands.
 

Papabravo

Joined Feb 24, 2006
16,091
Seems they are not isolated.
View attachment 238054
You can use them in series, as shown on your circuit, but each output should be shunted by diode, anode to -Out, cathode to +Out.
The TS did say buck converter but he describes using it in a Boost configuration. Not sure about what the diode is doing in a series connection with each output having a shunt diode. Doesn't that just shor each supply? As is typically the case words fail to accurately convey the proper meaning.
 

Thread Starter

SamarthGohil

Joined May 11, 2021
6
Seems they are not isolated.
View attachment 238054
You can use them in series, as shown on your circuit, but each output should be shunted by diode, anode to -Out, cathode to +Out.
ADDED:

Diode D1 or D2 decreases current through inductor and diode of converter in case, when SW3 is ON, but SW1 or SW2 is OFF:
View attachment 238055
So i did a test where in the same circuit instead of 24VDC at each input i connected about 8VDC and so far the output is reading a proper addition of the series. The only concern is if it would work fine at proper voltage i need. I
 

Thread Starter

SamarthGohil

Joined May 11, 2021
6
It is probably not a good idea. Especially if you do not understand the details of the LM2956 operation. One of the problems you might have is the upper converter might be depending on a ground reference for regulation. If what it thinks is ground is something else you might have an unstable mess on your hands.
Thank you. I will be careful. Thats why i am here seeking some advise. Although experimenting is a key to keep learning right? Perhaps you can guide me in the right direction i want to progress to achieve my goal. 48VDC as output using a few 9V batteries..
 

Papabravo

Joined Feb 24, 2006
16,091
First we should get our DC-DC converter terminology correct.
  1. When your input voltage is strictly greater than your output voltage, that is a Buck converter.
  2. When your output voltage is greater than you input voltage, that is a Boost converter.
There are other types, but we should concentrate on these two for the moment. The design of any DC-DC conversion scheme should begin with the power inequality. It states succinctly, that: "Power out will always be less than Power in". I should add that: "Sometimes it will be much less".

To compute either power in or power out we need a voltage and a current. Batteries supplying a DC-DC converter is tricky because as the batteries discharge they try to compensate for the drop in voltage by delivering more current which causes them to discharge faster. Depending on what type of batteries you have you really don't want to let that happen.
 

Papabravo

Joined Feb 24, 2006
16,091
Seems they are not isolated.
View attachment 238054
You can use them in series, as shown on your circuit, but each output should be shunted by diode, anode to -Out, cathode to +Out.
ADDED:

Diode D1 or D2 decreases current through inductor and diode of converter in cases, when SW3 is ON, but SW1 or SW2 is OFF:
View attachment 238055
Thanks for the drawing. Does this work for Boost converters as well as Buck converters, and can the LM2596 do either topology?
I'm not familiar with that particular chip.
 

Danko

Joined Nov 22, 2017
1,079
Thanks for the drawing. Does this work for Boost converters as well as Buck converters, and can the LM2596 do either topology?
I'm not familiar with that particular chip.
"The LM2596 series of regulators are monolithic integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of driving a 3-A load with excellent line and load regulation.
These devices are available in fixed output voltages of 3.3 V, 5 V, 12 V, and an adjustable output version."
"• Adjustable version output voltage range: 1.2-V to 37-V ±4% maximum over line and load conditions
• Available in TO-220 and TO-263 packages
• 3-A output load current
• Input voltage range up to 40 V"

https://www.ti.com/lit/ds/symlink/lm2596.pdf?ts=1620734398714&ref_url=https%3A%2F%2Fwww.google.com%2F
TS wants to use 8x9V batteries (36V+36V=72V in summary) as input and desires to get 24V+24V=48V from out.
 

Papabravo

Joined Feb 24, 2006
16,091
Got it. Let us know how it works out.
I'm especially interested to know how long his 9V batteries will last and if he intends to run them down completely.
 

Thread Starter

SamarthGohil

Joined May 11, 2021
6
First we should get our DC-DC converter terminology correct.
  1. When your input voltage is strictly greater than your output voltage, that is a Buck converter.
  2. When your output voltage is greater than you input voltage, that is a Boost converter.
There are other types, but we should concentrate on these two for the moment. The design of any DC-DC conversion scheme should begin with the power inequality. It states succinctly, that: "Power out will always be less than Power in". I should add that: "Sometimes it will be much less".

To compute either power in or power out we need a voltage and a current. Batteries supplying a DC-DC converter is tricky because as the batteries discharge they try to compensate for the drop in voltage by delivering more current which causes them to discharge faster. Depending on what type of batteries you have you really don't want to let that happen.
Great! So the configuration we are working with is a buck converter. As (2) X 30 voltage downs to (2) X 24 voltage. When the two 24VDC outputs are put together in series, they are expected to be 48VDC. which still is a buck converter overall as total input voltage is about 60VDC.

In the circuit diagram attached to my original post, instead of connecting 4 batteries in series as an input to each buck converter i tried with just one (9v) at each input. Each output was configured for 7.5VDC. Then connected both outputs in series, now total i was able to produce with that configuration was about 15VDC. So my curiosity is in the explanation you might have, if i can or cannot use 30VDC input at each buck converter. Both inputs are separate from each other. Output from both buck converter is set to 24VDC each. And i would like to wire both 24VDC outputs into a series to supply my load with 48VDC. what potential issue do you foresee?
 

Thread Starter

SamarthGohil

Joined May 11, 2021
6
"The LM2596 series of regulators are monolithic integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of driving a 3-A load with excellent line and load regulation.
These devices are available in fixed output voltages of 3.3 V, 5 V, 12 V, and an adjustable output version."
"• Adjustable version output voltage range: 1.2-V to 37-V ±4% maximum over line and load conditions
• Available in TO-220 and TO-263 packages
• 3-A output load current
• Input voltage range up to 40 V"

https://www.ti.com/lit/ds/symlink/lm2596.pdf?ts=1620734398714&ref_url=https%3A%2F%2Fwww.google.com%2F
TS wants to use 8x9V batteries (36V+36V=72V in summary) as input and desires to get 24V+24V=48V from out.
To confirm, do you see any challenges with my configuration? or you see it working fine?
 

Danko

Joined Nov 22, 2017
1,079
To confirm, do you see any challenges with my configuration? or you see it working fine?
It should work without any problem.
In case, when I need 48V 3A_max power source, but have only two independent 30V batteries,
I could be use your solution, as cheap, good quality and efficient.
 
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