Mixing output of two DC/DC converters??

Thread Starter

supermankid

Joined May 26, 2013
54
There must be some clever solution for this problem but I am finding it even
difficult to ask in proper order. Please read it all an help me find a clever solution.
I have two DC/DC converter
One is
Boost Vin(3-12V) and Vout(15V)
other is
Buck Vin(20-24V) and Vout(15V)
Now I want to combine both of these voltage(15V)
to provide more current for my load.
Now, I noticed one thing that if the Vin of boost is
higher than 15V it gives Vout of 16 V...etc...not 15v
also if Vin is less than 3V vout is about 5-6 volts.
Now what would be the clever way to give only 15V on the output
so that I can combine with the output of both buck and boost.
The main problem is as you may have already noticed by now that
at times i could be mixing two different voltage to cause
disaster.
I have attached a block schematic for further understanding.
adder circuit.jpg
 

MikeML

Joined Oct 2, 2009
5,444
A two-rectifer OR gate. Anode of first diode to first converter, cathode to output. Anode of second diode to second converter, cathode to output. Use Schottky rectifiers to minimize the forward drop.
 

ian field

Joined Oct 27, 2012
6,536
There must be some clever solution for this problem but I am finding it even
difficult to ask in proper order. Please read it all an help me find a clever solution.
I have two DC/DC converter
One is
Boost Vin(3-12V) and Vout(15V)
other is
Buck Vin(20-24V) and Vout(15V)
Now I want to combine both of these voltage(15V)
to provide more current for my load.
Now, I noticed one thing that if the Vin of boost is
higher than 15V it gives Vout of 16 V...etc...not 15v
also if Vin is less than 3V vout is about 5-6 volts.
Now what would be the clever way to give only 15V on the output
so that I can combine with the output of both buck and boost.
The main problem is as you may have already noticed by now that
at times i could be mixing two different voltage to cause
disaster.
I have attached a block schematic for further understanding.
View attachment 62326
Are you describing a buck/boost converter that can take a range of Vin both above and below the regulated Vo?

They go by various names like; CUK or SEPIC that I can think of off the cuff - once you start googling, some other names should surface.
 

ian field

Joined Oct 27, 2012
6,536
How can I limit the input within the specs 3-12v. I thought if Zener was not a help??
If you're struggling to get the current you need, a zener dropper that can handle the power might work out big and expensive - not to mention the heat sink! A better way is a chain of forward biased dioes so the sum of all the Vf adds up to the voltage you need to lose - but it still dissipates as much heat as the giant zener.

To get any decent current draw, what you really need is a 3rd step down switcher to provide an in-spec voltage for your existing PSU - by the time you've paid to buy that, you might just as well have bought a PSU that does the job you want done.
 
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