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Newton's third law problem.
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electrodacus:

--- Quote from: Kleinstein on November 28, 2022, 05:14:55 pm ---The effective input voltage see by the DC/DC converter is the sum of input voltage + output voltage (not considering a sign). So the available output current / load current between the DCDC converters outputs is the difference of the input current minus the output current.  So if the DCDC converter is something like a 24 V input to 6 V output type  with 75% efficiency one would have 3 times the ouput courrent at the DCDC converter an 2 times the output current from the input current. So one could get something like 18V 1A in and -6 V 2 A out.

There is nothing magic with the circuit - depending one the DCDC is may just need some help on start up.

--- End quote ---

Maybe I need to be more clear as I do not think you understood the setup.
Get a plastic box and put in it whatever you want as long as it is not an energy source like a battery. So you can put a DC-DC converter inside.
The box will have just two wires one on each end and nothing else.
Now can you have a difference in current between the two wires ?  Have you ever seen a two pin IC that could have a current input at one of the pins and a different current on the other pin.
cbutlera:

--- Quote from: electrodacus on November 28, 2022, 05:01:10 pm ---
--- Quote from: cbutlera on November 28, 2022, 09:15:30 am ---
I just tried it with an old Traco TEF2011 module that I found in my junk box, 10-30V in 5V out.  It needed an additional 5V power supply connected to the output to get it started, but once started the additional supply could be disconnected.  As expected it worked just fine, and even continued to run with the input power supply reduced to 2V, giving 7V across the converter input terminals.

--- End quote ---

Sorry but can you be more exact in describing your setup ?

--- End quote ---

The setup was exactly and precisely the one that you described, other than the temporary connection of a second power supply across the output terminals of the DC/DC converter to get the thing started.  Once started the second power supply was disconnected.


--- Quote ---You can of course have a voltage drop on the converter if you have a current flowing through the converter depending on the type of converter.
What you can not have is a higher voltage at output than input, a negative voltage or different current at output than at the input.

--- End quote ---

The output voltage was not higher than the input.  The output of the DC/DC converter was 5V.  The external power supply, set to 2V, was connected between the input and output of the DC/DC converter just as you described, which boosted this voltage to 7V. So there was 7V at the input to the DC/DC converter.

The input current and output current are necessarily equal.  With no load, the circuit naturally settles at an operating current such that the 2V boost to the output voltage exactly compensates for the losses in the DC/DC converter.

This experiment took less than 10 minutes to perform.  Rather than arguing about it, please just try it for yourself.
james_s:

--- Quote from: cbutlera on November 28, 2022, 05:36:12 pm ---This experiment took less than 10 minutes to perform.  Rather than arguing about it, please just try it for yourself.

--- End quote ---

You expect a troll to try an experiment? He just got you to waste your time setting one up, and he's going to continue to argue that it can't work despite the fact that it does.
electrodacus:

--- Quote from: cbutlera on November 28, 2022, 05:36:12 pm ---
The setup was exactly and precisely the one that you described, other than the temporary connection of a second power supply across the output terminals of the DC/DC converter to get the thing started.  Once started the second power supply was disconnected.


The output voltage was not higher than the input.  The output of the DC/DC converter was 5V.  The external power supply, set to 2V, was connected between the input and output of the DC/DC converter just as you described, which boosted this voltage to 7V. So there was 7V at the input to the DC/DC converter.

The input current and output current are necessarily equal.  With no load, the circuit naturally settles at an operating current such that the 2V boost to the output voltage exactly compensates for the losses in the DC/DC converter.

This experiment took less than 10 minutes to perform.  Rather than arguing about it, please just try it for yourself.

--- End quote ---

The setup was surely not as I asked.
But let me simplify things and make it even simpler to understand.
You get a plastic box and put whatever you want in the box as long as there is no energy source like a battery.
And out of that box you have two wires and nothing else.

Now what you say is that you will put a DC-DC converter in that box and get the positive input and positive output out of that box.
My understanding will be that you connected the positive input to your lab power supply positive output and the positive output of the DC-DC converter to the negative of your lab power supply.
But then it will not make sense to say you measured 7V if the lab power supply was set to 5V. Where will you measure 7V ?
I may guess you charged some large capacitor on the DC-DC converter and then you measured that in series with your lab power supply voltage.

So the plastic box with just two wires coming out seems a better simplification and you can get inside whatever you want.  You will not be able to have a different current through one of the two wires.
fourfathom:
I think you guys should get back to wheels, gears, and surfaces.  Misunderstood switching regulator topology when the model doesn't even represent the thing being analyzed is getting you nowhere.

Or, DDWFTTW: The cart wheels drive a generator, the generator powers a fan blowing backwards.  To start, the wind pressure gives the idle fan a little push.  The cart starts rolling downwind, spinning the generator which powers the fan.  Take it from there.  But remember the wheels rolling on the ground.  For starters, assume no slip, no energy storage, no friction, no loss.

And while interesting, I realize that this discussion is a lost cause.  Fortunately for me, I enjoy some lost causes.
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