Boost Converter Duty Cycle

Thread Starter


Joined Mar 11, 2023

I have a project to design a high gain boost converter. I need a voltage gain of plus 20 and have decided to just keep it simple and use a cascaded boost converter. I understand a conventional boost converter can only achieve a maximum of approximately 10 which would be equivalent to a duty cycle of 90%. However I have never seen a satisfactory answer as to why 90% is considered a maximum value, would anyone be able to elaborate on this.

Many Thanks


Joined Aug 21, 2008
If you use a transformer instead of a choke you can out voltages hundreds of times the input voltage. Taped winding which makes it an auto transformer does the same thing.

I never asked that question.


Joined Oct 7, 2019
why 90% is considered a maximum value
There is not a wall at 90%. You can use 91%.
There is an "input power" time of 90% and an "output time" of 10%. As you approach 100%/0% the supply stops working. How close can you get to 0% and still work? It is like filling your coffee cup in 90 seconds but you must drink it all in 10 seconds.

DickC is right, use a transformer or tapped inductor.


Joined Feb 24, 2006
It has to do with the relationship between boost ratio as a function of dutycycle. It is nonlinear with a slope that is monotone increasing. At high dutycycles it becomes near vertical. There is a limit where the boost ratio becomes uncontrollable, in the sense that very small changes in the boost ratio result in large changes in dutycycle. This is a very difficult control problem. See the following clip:
As Vout gets much larger than Vin, the negative quantity approaches zero which means the duty cycle approaches 1, which it cannot exceed. Now look at the derivative of D with respect to the boost ratio (Vout/Vin) and the problem will become apparent.


Joined Jan 23, 2018
I see that actually the device to be created is a switching mode power supply. Stating the actual requirements rather than a guess at a possibility will allow others to provide much better advice.
In this forum you have access to a large number of folks with a whole lot of wisdom, as well as some clueless folks. So providing more information is a better way to ask for help.