pwm

[danilorj]

Guys,

          this circuit in anex is about a buck converter. The circuit in the left is the pwm generator and the circuit in the right is the buck converter, I'm using a MOSFET as a switch, the potenciometer is the D parameter. But I don't know what is going wrong when I set the source of the buck converter to 20 V or anyother, the load tension is not 10 V as D=0.5. 

I would appreciate any help.

[OStep]

I am not sure how you plan on turning on the mosfet using 10V at the gate as the buck is operated from 20V. Also, I would suggest that you use a simple clock source to get the main power circuit to work and then proceed to more elaborate ways of generating the PWM.

[danilorj]

The way of generating the PWM I think it's correct. I don't know about the MOSFET used in the buck converter, I think the mistake is in it.  

[danilorj]

The problem is that the mosfet I'm using is not switching anyway.

[OStep]

If you want a mosfet to turn ON, your gate voltage must exceed the source voltage by an amount specified in the datasheet (typically ~2V). You are driving the gate with 10V while expecting to keep ON a mosfet that would have a source of voltage of 20V. This is not a regime a mosfet can operate in. You need to either increase the gate voltage or drive it as a differential voltage across gate and source directly.

[danilorj]

I increased the Vcc voltage but it's not switching anyway.

[OStep]

your mosfet has on Rds of 5ohms which is twice as large as your load. You're getting a huge drop across it. This mosfet is not meant for this application; you need to use a power transistor and one with a much smaller Rds (e.g <100mOhm).

alternatively you can use an ideal voltage controlled switch (component SBREAK from the switch family).