# Voltage not varying with duty cycle

I want to make a circuit that changes voltages with the duty cycle as well as doubles that for me. I therefore made a voltage doubler circuit along with alternating PWM wave. But as I changed the duty cycle my voltage at the output didn't change. I don’t know what's the error as I have seen in the buck converter that with a capacitor in the path of the PWM, as the duty cycle changes the voltage changes. Using the concept of buck converter and voltage doubler I made the following circuit but this circuit isn't working accordingly.

Can anyone help me to make a circuit for this use? Also, what is the error in this?

With a duty cycle of 5:

With a duty cycle of 50:

I used the load such that my RC time is less than the time of charge and discharge so that my capacitor voltage drops with the alternating PWM. But nothing helped at all. When I used a small value of capacitor/resistor to decrease the RC time my wave became rippled and I failed to get DC voltage and still I was not able to achieve my result of changing PWM to give changing voltage.

Kindly help and give some formulas to get the desired capacitor for 25kHz so that I get a proper DC voltage and also a PWM-based voltage.

## 2 Answers

I don’t know what's the error as I have seen in buck converter that with a capacitor in path of the PWM, as the duty cycle changes the voltage changes. Using that concept of buck converter and voltage doubler I made the following circuit but this circuit isn't working accordingly.

A capacitor charge pump circuit must leave the capacitors charged or nearly charged throughout a cycle in order to be efficient. This has two consequences.

1. To be efficient, the input to output voltage ratio must be some ratio of small whole numbers, like 2:1, 1:2, 2:3 etc., and that ratio will be fixed by the topology of the circuit

2. To get any input to output voltage ratio other than the above, such as by using pulse-width modulation, will inevitably result in the circuit being inefficient if you stray very far the small whole number ratio.

You state that you want to vary the voltage with PWM, as in a buck converter, and ALSO double it.

Assuming you want to be efficient, you need a buck converter WITH AN INDUCTOR, (to get your voltage to vary with duty ratio) AND a voltage doubler. There is no efficient way to do this without an inductor (or what amounts to the same thing, a transformer).

• If you can live with inefficiency, you can effectively regulate a charge pump with a combination of PFM and PWM. I used that for the Sunslicer (sunslicer.com/about.html) instrument to get the ~130V detector bias. But Sunslicer only needs a few microamps at that voltage. Commented Sep 17, 2023 at 16:16
• Thank you Sir I understood the importance of the inductor in this case. Basically without an inductor even if the voltage duty cycle is 5, the capacitor immediately charges and gives the desired output. But with an inductor the current is limited and the capacitor doesn't take charge instantly, the inductor opposes it and hence due to this reason, I was not getting a propper voltage. After Adding an inductor my problem is solved. Commented Sep 17, 2023 at 17:32

You have a load of 1.6 MΩ on a capacitance of 5 nF. The time constant of this is $$\ \tau = RC = 5 \times 10^{-9} \times 1.6 \times 10^6 = 0.008 = 8 \ \text{ms} \$$ but you're recharging at 25 kHz (every 0.04 ms) so the capacitors are just holding their peak voltage.

Reduce the value of the load resistor. (For practice, do some mental arithmetic first to see if you can estimate the expected results before you experiment.)

• even after reducing the frequency to 2k i am getting the same results. Commented Sep 17, 2023 at 9:45
• kindly explain how should I choose the value of capacitor Commented Sep 17, 2023 at 12:51
• Did you calculate the time period between recharging the capacitor at 2 kHz? What did you get? Why did you not reduce the load resistance value? Commented Sep 17, 2023 at 13:16
• I have tried all possible values but am not able to get the results, making the load small gives s sawtooth type of wave instead of a straight line. Kindly help me find a suitable value Commented Sep 17, 2023 at 13:59