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Parallel Switch Mode Supplies

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StudentSA

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Hi,

Can anyone please help me understand any complications that might arise in connecting two switch mode (buck) power supplies (LM2576) in parallel? I would connect the ground directly together.
The Vout will be connected together using diodes (1N4001,1N4148 etc) to prevent supply from the one to the other?

Thanks
StudentSA
 
I prefer not to explain the complications that MIGHT arise for all the mistakes I can imagine. I would prefer to explain how to do it well. Life is so much simpler that way!
Your idea is good. I would only warn that a common diode like the 1n4000 series can only handle 1 amp. Most switchmode supplies provide more than that. You did not tell the current of your supplies, so I am only guessing.
 
Diodes aren't good enough. THey will prevent converter backfeeding, however, they will have slightly different forward voltages and only one will turn on (not both) so only one supply will be providing all the current. What you need are low resistance high power current balance resistors instead of diodes. These will eat up the voltage difference between the outputs of the two converters, but will also reduce your load regulation and efficiency.

Nasty problems can also arise from two parallel converters have unsynchronized clocks causing interference, noise, and beat frequencies. THis can be a problem even when converters are running off the same input supply with different outputs (let alone being in parallel).

It'd be better (but not great) if you could connect various devices to one and only one converter rather than combine both outputs and connect everything to that. It's still shabby, but it's better than connecting them in parallel.

You'd be best to pick another switching converter IC where you can choose your power transistors and inductor so you only have one switching supply instead of two parallel ones.
 
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Hi there,


I've used the LM2576 in numerous power supply related projects, and yes it can put out at least 3 amps with the correct related parts (inductor, catch diode, etc.)
The most common method of paralleling supplies is to use a small series resistor in series with both outputs, but i've never tried this with these switchers.
The idea is to provide a small voltage drop so the two regulators dont fight for control. You probably dont need diodes however, because these regulators can take whatever voltage is presented on it's output without a problem, as the control circuit will just cut back the output current if the voltage is already up to par. They are made that way already. As far as voltage matching, you may want to make one adjustable so you can adjust the output current to match the other one...that will ensure both supplies share the current pretty well. Another idea would be to add an extra ripple filter to each output, which would come in the form of a small inductor. That would help the current sharing too.

I would not suggest a target current level as high as 6 amps though, even though each one can put out 3 amps each, because that would imply perfect matching which usually doesnt work out in real life. 5 amps might be ok though, or maybe a little higher.
 
so what is the solution of this problem. i want a switching power supply/charger of at least 20A.

That's an interesting idea, what is you power supply, can it deliver the 20 amps, what voltage are you starting with, what out put voltage are you looking for at 20 amps? What are you going supply this power to.?
 
You might be able to get that from a pair of lm25xx devices however you'd have to do some carefull design or use series resistors and waste some energy.
It'll probably be a lot better to design a supply that will provide 20a from the start, I've never seen it done but you might find some info on connecting an outboard switching tranny to the lm25xx to increase its output if you really want to use one of those.

Nice mini, you got a valver?
 
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Qaisar, for that kind of current you really want to use a separate PWM controller that drives power mosfets, rather than trying to get multiple integrated regulators to play well together.

What are your input and output voltages?
 
That's an interesting idea, what is you power supply, can it deliver the 20 amps, what voltage are you starting with, what out put voltage are you looking for at 20 amps? What are you going supply this power to.?

acctually i started designing the linear power supplies and chargers for pb-acid batteries for my companey and design all these using lm338 linear regulators. i used 3 regulators to make 15A power supplies and 15A chargers using approx: same design as given in application notes. the problem with these LDO regulators was that:
1) they got very heatup while passing large currents.
2)they can't conduct large current at high voltage difference between input and output. i.e; the voltage difference decrease their maximum current capability.
3) their thermal shut down property also decrease the out put voltage and current as well on heatup.
4) i had to design large PCB layouts to accomodate bulky components i.e; many 5w resistors, more than 3 large capacitors etc.

now i wana use SMPS designs to overcome all these setbacks of LDO regulators.
so i have designed a 3A power supply using lm2576 regulator and drive a 2A load successfully. now i wana proceed my work to develop 20A and greater SMPS chargers and supplies.

i have a large 34A bulky transformer with rms output equals to 36V that on rectification provide 50V across capacitors to be fed in to the regulators. and since the regulator is adjustable so it will give the outputs from nearly i think 2v to a maximum of 47V. and if i want to make a charger for 6v, 12v and 24v lead acid batteries i will have to get 7.9V, 15V and 28V outputs from the regulator (here 0.7V drop across diodes at the o/p of the charger is also take into account) thus a difference of input and output voltages will be 39.8V, 32V and 19V across the regulator.

so thats the whole story.
 
You might be able to get that from a pair of lm25xx devices however you'd have to do some carefull design or use series resistors and waste some energy.
It'll probably be a lot better to design a supply that will provide 20a from the start, I've never seen it done but you might find some info on connecting an outboard switching tranny to the lm25xx to increase its output if you really want to use one of those.

Nice mini, you got a valver?

how should i connect them in parallel? do you have any idea? kindly upload any schemetic layout if you know. or any application note you have. i have this
https://www.electro-tech-online.com/custompdfs/2012/10/snoa327a.pdf

but can't find anything for large currrents.
 
Qaisar, for that kind of current you really want to use a separate PWM controller that drives power mosfets, rather than trying to get multiple integrated regulators to play well together.

What are your input and output voltages?

that is a good idea, can you please give me any circuit diagram for that. can i use IRF3205 power mosfets and attany13 controller. another problem which is a great problem for for me is to design high frequency transformers for that, and such transformers are also not available in our country too. can you please tell me from where i get these transformers and what should be there specifications.
i wana use multiple regulators for my ease. or wana get any high current regulator.

for my input and output voltages please see my post # 9 in the same thread.
 
When I said controller, I meant a dedicated PWM power controller, not a microcontroller.

I would start on this page.

For that much power, you will probably want a half or full bridge. And you may want to do a PFC front end.
 
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