Buck Converter Design

Thread Starter

katam

Joined Aug 12, 2012
2
Hello guys,

I am working on an buck converter project. I realized something weird while doing this project.

My input voltage Vin is 15V and PWM is 0-15V. I am using IRF520N mosfet. On the 100% pwm signal, I expect Vd(voltage accross diode) to be approximately 15V. But I am getting 10.8V. I found the reason. It is because of the threshold voltage of the mosfet which is 4V. Output of the mosfet goes up until 10.8V and then stops there because after that voltage Vgs becomes less then threshold voltage. (Let me be more clear. Vgate-ground =15V, Vsource-ground =10.8V. Vgs=15-10.8=4.2V. )

I thing it is weird. Isn't it? If I want 30V buck converter, I have to build a 0-34V PWM and find a mosfet which can be max Vgs 34V (Because first time when we fire the system Vsource-ground=0V).

I am wondering if I do something wrong. I attached a general buck converter schematic, in case. i appreciate if anybody could help.

Thank you.
 

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crutschow

Joined Mar 14, 2008
23,351
You are seeing the normal result of using an N-MOSFET for a positive high-side switch.

One way around the problem is to use a bootstrap boost circuit to increase the gate voltage above the supply voltage.

A more common way is to use a P-MOSFET for the task. The source is connected to the plus voltage supply, the gate is grounded to turn on the transistor, and the drain is the switched output. As long as the plus voltage is greater than the required full-on Vgs voltage for the MOSFET, it will fully turn on when the gate is grounded and the source-drain voltage drop is determined solely by the transistor ON resistance.

Edit: Note that the gate voltage needs to be raised to the power supply voltage to turn off the P-MOSFET.
 

bountyhunter

Joined Sep 7, 2009
2,498
That's why buck designs using an N switch have to have a floating drive stage raised up a little to get the drive voltage high enough.
 

Thread Starter

katam

Joined Aug 12, 2012
2
Thank you guys. It helped me a lot. I will try p type mosfet first.
 
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