Hello everybody I'm an Italian student of electronic engineering, i joined this website because i found it on google and i thought that i could find a lot of qualified people, who talk to.
Now i have a strange issue whit a pass transistor, which i have to use in a DC-DC buck converter(step down). The issue is that: trying the NMOS pass transistor i'm applying 8V on its drain, 5V on its gate and i attached a capacitor of 1uF on its source. From the teory and, having done also some simulations in ORCAD PSPICE i expect 8V in output on the capacitor, but when i read with a multimeter the voltage on the capacitor, it is only 2,45V. Could anyone help me to understand what is going wrong? The device is an IRF520 NMOS, i thought first that it is an issue of the device but trying whith a 70N10 NMOS, the result is close to the previous, and the voltage read is 3 V.Thank you all in advance.

 

Please post a circuit diagram.

 

Here's a simple schematic

 

That looks like a source follower. Similarly to a emitter follower, the mosfet will bias itself so that it has ~Vgs threshold between source and gate. To turn the mosfet fully on, you need ~10V between source and gate.

I am not entirely sure what you're trying to achieve, but relying on Vgs is a bad approach, because it varies widely not just between different types, but also in the same batch, also it is dependent on die temperature. It is similar to relying purely on gain of bipolar trasistor.

 

Ok I need a Pmos i didn't mean that whit NMOS i cannot use this configuration because Vgs would become negative and the then the transistor switch off. What i was trying to do is a simple trial of a switch mosfet to use in a buck converter. I need the buck converter(Step down) to charge a LI_Ion battery, using as feedback a control in an ATmega mcu, driving the ON/OFF state of the pass mosfet via PWM and then the duty cycle of the buck converter.
Thank you

 

If you need an N fet for high-side switching, you can try mosfet drivers like IR2110 and similar.
Anyway your application seems very typical, so maybe have a look at ti.com and linear.com and see if you can get samples of some ready-made switcher that will do what you need.

 

Thank you Kubek you were very helpful to me. I'll look at this two webisites now.