Boost Converter Duty Cycle

andyguyy

Joined Mar 11, 2023
3
Hi,

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

DickCappels

Joined Aug 21, 2008
9,877
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.

ronsimpson

Joined Oct 7, 2019
2,712
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.

Papabravo

Joined Feb 24, 2006
20,169
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.

MisterBill2

Joined Jan 23, 2018
14,761
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.