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Boost converter PWM frequency and time period calculation

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Advanced Member level 3
Oct 31, 2009
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I am simply trying to make a high current capable boost converter. I am giving input from an SMPS which can give a max 35A at 5V. I am making a digital variable boost converter which can give any voltage from 5 to 30V. I am using a 4.75mH torroidal inductor with around 10A DC current capacity but not sure about saturation current, it may be less. I am using IRF540N power mosfet for the switching, also I am using a 500mA gate driver IC from microchip to ensure the mosfet is getting switched with proper square wave.


Now my main question is, what should be the PWM time period for maximum load regulation?
By experimenting with the dutycycle and time period, I kept 2uS ON and 1uS OFF (T = 3uS) and I am getting a DC output of around 14.5V and I observed a very good load regulation at this duty cycle and time period for 15v. I connected heavy load and still the voltage is getting maintained around 14 to 13.9 and this is without any feedback control loop, which I will implement later.

If I am keeping ON time as 10uS and OFF time as 5uS keeping the same above ratio, then I am getting the same output voltage of 14.5V but the load regulation seems poor.

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Actually I got the torroidal core of 1.5cm diameter from a common mode choke. It seems like my inductor is having less saturation current even though the wires are very thick but the inductance is limited by core, may be because of that it is going to saturation with in 30uS.

My Questions:

1> If I am decreasing the coil resistance beyond a limit, it doesn't improve the boost regulator current capacity because the core may be a bottle neck for saturation current. Am I right?

2>How will I find the optimum PWM frequency for best load regulation?

3> Also, what is normally followed, is it fixed time period and variable ON time or fixed ON time and variable OFF time for better performance and efficiency?

4> If I am using an air cored inductor with litz wire, then the saturation current is mostly dependent on the coil resistance. Am I right?

5> Also, what is normally followed, is it fixed time period and variable ON time or fixed ON time and variable OFF time?
Last edited:

Load regulation is purely impedance ratio of source impedance to step load change, during any flow of current.

Since PWM modulates switch RdsOn average impedance by duty cycle where 0% =∞, 50% = 2x and 100%=1x you can estimate load regulation and ripple due to RdsOn and impedance of coil with average Frequency of ripple current. Both affect source impedance.

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