Combing two outputs of a DC/DC converter into a single equivalent output

Thread Starter

SiCEngineer

Joined May 22, 2019
320
Hello.

What would be the process for combining two outputs from a DC/DC converter into a single output? I am trying to design a resonant converter, which requires information regarding the load resistance. However, there are two load resistances in my application. I have seen something similar done but the two outputs were not connected to each other - they were two separate outputs and therefore current did not flow between them. In my application, I have one main output, which has two current flows - one to ground (which represents losses to the body of the device) and one to the other output (which represents the useful current transfer in the device).

I have attached a schematic for illustration. I have been scratching my head at this for a while and it makes circuit analysis and converter design all the more difficult.

If someone could help that would be appreciated.

Thanks, J

output_SiCEngineer.jpg
 

Thread Starter

SiCEngineer

Joined May 22, 2019
320
If you would like to mix 2x DC power supply into 1x power supply but more powerful, you should use 2x "ideal diode" with forward voltage = 0 [VDC], like this :

Please look at the schematic and the datasheet attached.
Thanks. However I don’t want to combine two converters, rather I have one converter which has two outputs. For analysis purposes I need to know a single, equivalent load value. However they aren’t in parallel nor are they in series (?) therefore I am unsure how to combine them.
 

Tonyr1084

Joined Sep 24, 2015
6,365
I understand @crutschow's confusion, I'm a little confused also. Given this is somewhat my natural state - please bear with me. You say your two outputs are totally isolated from each other. Neither in parallel or in series.
I don’t want to combine two converters, • • • therefore I am unsure how to combine them.
Can you understand our confusion?

From your drawing I've redrawn it in a way that is clearer to me. After editing (and @kubeek) I now see a current path for Sb (Secondary B) side. Current will flow through V01 and V02 only from Sb. Sa (Secondary A) does nothing without modification.

[edit - partial comment removed due to being in error on my drawing] I don't know what your goal is either. I know you said you're having trouble coming up with a circuit. I'm just trying to establish a common starting point.

Z Combined.png

output.jpg
 
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kubeek

Joined Sep 20, 2005
5,765
Tony I think your drawing is missing a connection from D4/C4 to ground. If you imagine the same grounding on the top half, then it starts to resemble voltage doublers.
 

Thread Starter

SiCEngineer

Joined May 22, 2019
320
Tony I think your drawing is missing a connection from D4/C4 to ground. If you imagine the same grounding on the top half, then it starts to resemble voltage doublers.
Yes, they are voltage doubler rectifiers, connected in series afterwards to get more high voltage. I then have two loads which represent two flows of current - one of which flows to ground, whereas the other flows to another component of the output. I have attached an image which shows how this type of load is typically connected, only difference is it doesn't use multipliers.
 

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Tonyr1084

Joined Sep 24, 2015
6,365
What would be the process for combining two outputs from a DC/DC converter into a single output?
In a DC/DC converter, there's more circuitry than you've illustrated. I'm not here to find fault with your post, I'm trying to understand it better. Transformers (as you have drawn them) do not work with DC. If your DC/DC converter converts DC into AC then back to DC - I can wrap my head around that somewhat. But I'm by no means an expert here. Please don't take me spouting off facts I know little or nothing about.

Can you clarify how you want your two outputs to form a single output? Instead of drawing circuitry try block diagrams. It might help us figure out what you're trying to do.
 

Thread Starter

SiCEngineer

Joined May 22, 2019
320
In a DC/DC converter, there's more circuitry than you've illustrated. I'm not here to find fault with your post, I'm trying to understand it better. Transformers (as you have drawn them) do not work with DC. If your DC/DC converter converts DC into AC then back to DC - I can wrap my head around that somewhat. But I'm by no means an expert here. Please don't take me spouting off facts I know little or nothing about.\

Can you clarify how you want your two outputs to form a single output? Instead of drawing circuitry try block diagrams. It might help us figure out what you're trying to do.
This is a DC/AC/DC converter, but to design the inverter correctly I need to know the load resistance value. However, my converter has two loads, and therefore I need to reduce it into one, single representative load so that it can be used to represent the entire output for analysis purposes. I'll attach a photo of something similar done in a textbook I have, Maniktala's Switching Power Supply Desing & Optimization, however I don't believe I am able to use this method:
 

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kubeek

Joined Sep 20, 2005
5,765
I somehow still don´t understand why you need voltage doublers, when you can much more easily just use the correct transformer turns ratio in the first place.
 

Thread Starter

SiCEngineer

Joined May 22, 2019
320
I somehow still don´t understand why you need voltage doublers, when you can much more easily just use the correct transformer turns ratio in the first place.
Mainly because using voltage doublers will reduce the amount of step up required from the transformer. Also, it allows the use of lower voltage rated components in each of the windings, and the diode voltage is clamped to the voltage across each of the doubler capacitors.
 

MisterBill2

Joined Jan 23, 2018
9,493
There is nothing wrong with the initial circuit, which seems to be two voltage doublers in parallel, sort of, feeding one load, with the lower doubler having an additional load in series with it's output.
Now for determining the power, the approach is to first know the power required by each load connected to the supply. You need to also know the voltage required by each load. The voltage will allow you to know what the input to each doubler circuit must be, the total power will let you know both how much power must be supplied to the transformer, and also how much power the transformer core must be able to handle. These are the easy calculations. The hard part is designing the circuit to drive the transformer with a bit more power than is needed, because none of the circuit is 100% efficient.
 

MisterBill2

Joined Jan 23, 2018
9,493
@MisterBill2: Secondary A (Sa) is missing a current path for the loads.
Quite true. What ever was I thinking.
BUT two supplies in series is a well known and totally valid way to achieve the goals of needing lower voltage parts.Plus it can provide a lower voltage source without the inefficiency of any dropping resistor. So it is a good option. And my comments in post #15 still hold.
 

MisterBill2

Joined Jan 23, 2018
9,493
@MisterBill2 OK, so what's missing?
The missing connection in the drawing in post #1 is from the lower side of the lower capacitor in the top voltage doubler either to the bottom line shown with a ground symbol, as the bottom of the resistor tagged as "VO1", if the two doublers are to be sort of in parallel, or, if the two doublers are to be sort of in series, that same point at the negative side of the upper doubler would have a short connection to the positive side of the bottom voltage double immediately below. That connection would put the sum of the two doubler voltages across the resistor tagged as "VO1", and have only the voltage from the upper voltage double across the resistor tagged as "VO2". Sorry about not being clear enough.
I may not be responding for a few days because of being involved in activities in an area without phone or internet access. available. I hope to be able to return.
 

Thread Starter

SiCEngineer

Joined May 22, 2019
320
The missing connection in the drawing in post #1 is from the lower side of the lower capacitor in the top voltage doubler either to the bottom line shown with a ground symbol, as the bottom of the resistor tagged as "VO1", if the two doublers are to be sort of in parallel, or, if the two doublers are to be sort of in series, that same point at the negative side of the upper doubler would have a short connection to the positive side of the bottom voltage double immediately below. That connection would put the sum of the two doubler voltages across the resistor tagged as "VO1", and have only the voltage from the upper voltage double across the resistor tagged as "VO2". Sorry about not being clear enough.
I may not be responding for a few days because of being involved in activities in an area without phone or internet access. available. I hope to be able to return.
Yes, this is exactly how it should be. The voltage multipliers are to be connected in series. I attached an updated schematic of the configuration.
 

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