# Creating a circuit to linearly increase the input voltage as the output voltage increases.

#### Bod

Joined Sep 18, 2016
311
I bought this: https://www.amazon.com/KKmoon-Digital-Programmable-Constant-Step-down/dp/B0745C58S4
and this to go with it: https://www.amazon.co.uk/KKmoon-Communication-Interface-Constant-Converter/dp/B077NYLPNH
I have to say, they are great but have one major downside - you can't have an output voltage more that your input as there is no boost circuit. Take this: if I were to provide it with 12.22V exactly, I could pull out 11.22V at the max.
With this, I decided I should buy a boost converter and power it with 50V (the max) so I can have full use. However it recommends not to power it with that much as it can be damaged. Although this sounds like a strech, I don't want to risk it (and this also sounds like a fun project!).
My main idea for this is that:
I will provide it the boost converter with 12V (possibly 5 if I wanted to add another small fan.)
That means I can pull out 11V. When I am pulling out 11V, the circuit activates the boost converter and boosts it to 24V.
If I pull out 23V it will boost it to 48V - just under the max. This means it will only provide the right amount of voltage when needed.

That was my main idea, although, as I am writing this, it occured that I could be a linear change and that might be easier. As the output voltage increases by 1V / 2V , the input voltage increases by 1V / 2V.

Hope this makes sense, feel free to ask questions.

Bod

EDIT: I want to follow up on my second idea of the linear increase. I think it might be slightly easier.

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#### crutschow

Joined Mar 14, 2008
32,861
I don't understand what you plan to use for an input power source, why you want a boost circuit, and why you want it variable. (?)
Unfortunately what you said, doesn't make much sense as you don't seem to understand how the module works.

The input voltage should be a near constant DC at slightly above the maximum output voltage you will ever want (but below the max rating, of course).
The module will convert that to whatever output voltage you want by adjusting the output voltage setting.
It should do that with good efficiency, as it's a switching type supply.

Joined Feb 20, 2016
4,314
First, what are you going to power it from? If the mains, just get a transformer , bridge rectifier and filter cap to supply the voltage you need.
I don't see much advantage to adding a tracking boost converter to the front end. Although it would be an interesting project.
Sometimes a tracking pre regulator is used to limit the dissipation on linear regs but this is a switching one so that is not really needed.
A 12V 10A boost converter set to 32V could be used but just remember, the power loss and extra current the circuit will draw from the 12V voltage supply. Maybe an opto coupler with series resistor and zener wired across the KKMoon power supply could be used to detect the drop across it and the output of the opto coupler be used to set the voltage on the boost converter. But make sure you do not go over the max power supply input voltage. In fact, I would stay a few volts lower.

How does the 50V option work? Is is a board build option or a user selectable one? Or another model of the power supply?

#### Bod

Joined Sep 18, 2016
311
To clear this up. I am powering this with 12V. As I said in my post, that means I can get 11V out. However, I would like at least upto a 45V output, which means I’ll need an 45V power supply. Of course, I could take this 12V and boost it to 45V with a boost converter but as it states on the Amazon page, it isn’t recommended to provide it with this voltage as you are at risk of damaging it. This is where my idea of so called “tracking boost converter” came from. If it increases the input as the output voltage increases, it will only be at 45V for the time that you have the output set to 45V.

How does the 50V option work? Is is a board build option or a user selectable one? Or another model of the power supply?
I don’t really get your question. There is no “50V option” - that is the maximum voltage you can provide on the input and the max you can get on the output.

Hopefully I cleared some confusion,
Bod

Joined Feb 20, 2016
4,314
I don’t really get your question. There is no “50V option” - that is the maximum voltage you can provide on the input and the max you can get on the output.

I was just going by that on the web site.

You maybe able to use something like this..
https://www.amazon.com/Regulator-Ye...ost+converter&qid=1551738252&s=gateway&sr=8-6
And modify the voltage feedback sampling by adding the output transistor of an opoisolator as I mentioned before, or even a LDR driven by an LED with appropriate resistor and zener or series voltage reference chip so the LED starts to light as the volts across the main power supply gets above maybe 3 V.
Another way is to just have 2 steps of voltage. Boost the 12V to say 28V so yo can go from 0-24V in one range, then boost to 48V for above 24V.
That could be done with a voltage comparator on the main output that switches another resistor into the boos converter feedback look.
My analog power supply has a relay that switches the transformer secondary tap for more volts on the high part of the output range.

#### Bod

Joined Sep 18, 2016
311
View attachment 171568
I was just going by that on the web site.
Oh right, I missed that, sorry. I do recall there being 30V option as well.

View attachment 171568
Another way is to just have 2 steps of voltage. Boost the 12V to say 28V so yo can go from 0-24V in one range, then boost to 48V for above 24V.
That could be done with a voltage comparator on the main output that switches another resistor into the boost converter feedback look.
My analog power supply has a relay that switches the transformer secondary tap for more volts on the high part of the output range.
I will try that option out.
Also, what is the 'boost converter feedback loop' and is there one on the converter you provided? If not, will I have to make my own converter? (
)

Bod

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#### crutschow

Joined Mar 14, 2008
32,861
What is the max current of the 12v supply?

#### Bod

Joined Sep 18, 2016
311

#### crutschow

Joined Mar 14, 2008
32,861
So the maximum power is 24W, thus the maximum current at 45V will be about 0.4A (due to the inefficiencies of the two converters).

#### Bod

Joined Sep 18, 2016
311

#### bertus

Joined Apr 5, 2008
22,217
Hello,

Did you read the document that goes with the schematic?
I have converted it to PDF and attached it here.

Bertus

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#### crutschow

Joined Mar 14, 2008
32,861
Would that work?
Possibly.
But it would likely be better and simpler to use a chip dedicated to a boost regulator operation, such as the LT1070.
The LTspice simulation below shows the chip with an added op amp configured as a differential amp to provide for the external voltage input control from the module output.
Note that the LT1070 output (green trace) stays about 5V above the module output (Vadj, yellow trace) starting above about 9V for Vadj,
Is that what you wanted?

#### Bod

Joined Sep 18, 2016
311
I will copy this into LTspice now and try it out.

It's my first time using it but hopefully it wont take too long.

#### Bod

Joined Sep 18, 2016
311
I have successfully built it and it seems to work like the one shown:

There are a couple things I still dont get though, would the output be Vadj? And also, what is making the voltage increase on LTspice?

Bod

EDIT: can I use a PWM signal to controll the output of this?

#### Bod

Joined Sep 18, 2016
311
Ok, I keep changing it but I could use a 555 timer boost converter as that would use PWM right?

Bod

#### crutschow

Joined Mar 14, 2008
32,861
what is making the voltage increase on LTspice?
The simulated module voltage from V2.
I could use a 555 timer boost converter as that would use PWM right?
I don't understand.
The LT1070 already is generating a PWM signal to control its output voltage.
To do the same function with a 555 requires a bunch of additional circuitry. Its PWM duty-cycle is not readily adjustable over a wide range using a voltage control, as is required here.

#### Bod

Joined Sep 18, 2016
311
The simulated module voltage from V2.
Oh, right! That makes sense now. So I just provide it with a 12V supply, take the output of my module into 'Vadj' and use the wire labled 'OUT' to go into the input of my module.
If that is right, then that works just like I needed it!

The LT1070 already is generating a PWM signal to control its output voltage.
To do the same function with a 555 requires a bunch of additional circuitry. Its PWM duty-cycle is not readily adjustable over a wide range using a voltage control, as is required here.
It was just an idea.

Bod

#### Bod

Joined Sep 18, 2016
311
Hello,

Did you read the document that goes with the schematic?
I have converted it to PDF and attached it here.

Bertus
I had not, and will do. Thanks.

Bod

#### crutschow

Joined Mar 14, 2008
32,861
If that is right, then that works just like I needed it!
That is right.
But I still see no significant advantage in doing that over just providing a fixed input voltage to the module, since the module already is a switching supply that uses PWM to efficiently control the output voltage.

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