# Variable input, adjustable output DC DC conversion

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#### dzu

Hello,

I'm looking for a solution for a project of mine, but I'm pretty stuck.

Can you guys help me about a circuit solution, so the case is:
I've got a Li-ion 18650 high current battery cell, which has an output of eg.: 3.4 to 4.2 volts.
This is the voltage that I want to convert to the range of 2.7 to 12V.

The output load would be in the range of 0.2 to 3 Ohms, so the output current is pretty high.
The adjusting should be real time, it would be managed or adjusted by a microcontroller. I thought about using a synchronous buck/boost converter, but I'm stuck with the scaling of the inductivity, and the output capacitor. And I really cannot find anything about the PWM signals that control the switches(in this case, high power MOSFETs), what should be the duty cycle of the signals and stuff like that.

The other question of mine is that is there any solution that can manage this problem?
To be clear I'm not asking you for a solution, and for you guys to completely do my project. I'm just desperately asking for a starting point.

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#### steve869

So the maximum output current you want is 60 amps when the output is set to 12 volts. This would mean that the required input current from the 18650 cell would be about 212 amps. That calculation is based on the converter being 100% efficient. In practice it would be less than 100 % so the required input current would be even higher. I do not believe that any 18560 cell can provide that current.

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#### dzu

Yes, you are right I didn't mean it that way. I would limit the output drain, by measuring the resistance of the load. Technically my main goal is to give out a constant wattage(performance) on a constantly changing load. The load's resistance is changing due to heating of the resistor.

So basically I want to build a simple variable wattage vape or e-cig. When the heating wire(load)'s resistance is only 0.2 Ohms, the output voltage would not be 12V, only like ~3.8V, to output ~75W. I don't want any firework to happen because of an exploding cell.

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#### vmast

you said ... "Variable Wattage" ?

So, do you want to regulate / limit the output Current ( amps ) initially into the "Cold" Low Ohms load
and then as the load warms and the Voltage rises, then you want to regulate / limit the Max Voltage ?

There are PWM circuits that can limit Max Amps and then Max Volts

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#### dzu

So I want the wattage to be set to a constant value, I said variable, because I want to make the circuit( and the software) the way, that the user can set the wattage the way he wants(1-75W for example).

So the user sets the performance as desired, after that a button is pressed, so the wire starts heating up, and because of that the resistance starts rising. The MCU measures the load’s resistance constantly and sets the output voltage the way that the wattage remains the same preset value.

The question is that how can I adjust the output voltage in ~real time, so the wattage can stay at that preset value.

The buck-boost seems to be the easyest solution, but I’m still stuck.