Recently I facing a problem on constructing the DC-DC buck convert, at beginning i just using n-channel mosfet to work as switch. i found out that
if wanted for the n mosfet to fully conduct, the gate voltage must more than Vin +5V , but the requirement of the project just can use only "ONE" Voltage source which are 12V for overall circuit. And because of this, the Output voltage from my testing result i get only 7V something. I have search for many information
to overcome this problem, first method is using charge pump , second is using
p channel mosfet. it seen like p channel mosfet is more easier, so i start to make the research on it and testing out the operation by using p channel mosfet as the switch. output load to the drain, +Vin to the source, square pulse waveform to the gate. just like figure 1.1, at Vg = 0V the pmosfet should get fully conduction. but is not in my testing, it get approximate 8V with 12V use as the input Votage.

figure 1.1 Picture: http://postimage.org/image/ib1351tw/


The main problem which i need the suggest of you all is, if you are try using p-channel mosfet to constuct a buck converter (12 Vin , Vout can adjust 4 ~ 8, current limit at 1A). How do you construct. and the circuit you have construct have the ability to overcome the problem which stated above.


Very Apperiate the suggestion and recomendation you provide.

 

Picture figure1.1 (resize): http://postimage.org/image/ib1351tw/

 

To make a buck converter, you need a diode from node 4 (the MOSFET drain) to node 0 (ground). Cathode to node 4.

For best efficiency this should be a schottky diode. High speed, low forward voltage drop.

If you use a P-channel MOSFET, your drive voltage should be OK.