# Buck Converter Design

Discussion in 'The Projects Forum' started by katam, Aug 12, 2012.

1. ### katam Thread Starter New Member

Aug 12, 2012
2
0
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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2. ### crutschow Expert

Mar 14, 2008
13,014
3,234
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.

3. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
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.

4. ### crutschow Expert

Mar 14, 2008
13,014
3,234
That is the bootstrap circuit I referred to.

5. ### katam Thread Starter New Member

Aug 12, 2012
2
0
Thank you guys. It helped me a lot. I will try p type mosfet first.