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I want to build a buck converter with a p type MOSFET.

Specification:

  • Input voltage: 24 V
  • Output voltage: 12 V

I have chosen frequency 20 kHz, and my inductor value and capacitor value is 1 mH and 20 uF.

The problem is that I can't drive the PMOSFET. I wanted to drive PMOSFET with an NPN transistor. The circuit diagram is given below. Is this okay? If this procedure is okay then what is the problem? Why it is not giving me 12 V output? If the circuit is not okay than what kind of circuit should I use to drive the PMOSFET?

enter image description here

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Please specify the part numbers you're using, especially for the transistors. Also, there's a built-in schematic editor on here that is much better than a photo of a hand-drawn schematic (although your hand drawing is better than most). Also also, have you evaluated your design already in a simulator such as LTSpice? \$\endgroup\$
    – Dan Laks
    Commented May 24, 2017 at 0:14
  • \$\begingroup\$ Thanks for comment!! Yes ,i have evaluated in PSIM with on off switch control and with the help of a comperator to create pwm signal.The result was satisfactory and there i used nmos.but i want to do it with a pmosfet and i want to implement it with hardwre level.so i need a way to drive the pmos.and actually i am seeking a circuit to drive the pmos. \$\endgroup\$ Commented May 24, 2017 at 2:59
  • \$\begingroup\$ i know ,to on the pmos gate voltage should be lower than source voltage at least vt(threshold voltage) and to off the pmos i assumed to have voltage of gate approximately equal to source voltage.So when arduino sends 5v signal,npn transistor is on and the gate of pmos find the ground right(PMOSFET ON)?(approximately though),and when arduino sends 0v signal then npn is off and gate and source voltage of p mosfet is close to each other (PMOSFET OFF), is this concept right? actually i want to learn how to drive this posfet. \$\endgroup\$ Commented May 24, 2017 at 3:00
  • \$\begingroup\$ It sounds like you have the theory pretty close to correct. But I have some concerns with your circuit that are highly dependent on which transistors you're actually using. So, again, please specify the part number of the actual parts in your circuit. \$\endgroup\$
    – Dan Laks
    Commented May 24, 2017 at 3:03
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    \$\begingroup\$ I think you will have a hard time finding a PMOS that can handle Vgs = -24V. There are some out there, but the supply may be very limited. I think it might be easier and better to just use NMOS with a high-side gate driver. If you want to persist in using PMOS, you may need to protect the gate with a Zener diode. \$\endgroup\$
    – user57037
    Commented May 24, 2017 at 3:52

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The IRF9530 is rated for a maximum of 20V between Gate and Source. You are applying 24V, which will probably blow it up. However your circuit should work fine with one small addition - a resistor in series between Q1 Collector and R3.

This resistor forms a voltage divider with R3. If you make it the same value then voltage will be divided by 2 and the Gate will get 12V. The total resistance load on Q1 will then be 200Ω, so when turned on it will draw 24V/200Ω = 120mA. This is bit much for a 2N3904, so I would increase the resistances to 220Ω each.

You should also have a low ESR 'bulk' capacitor across the 24V supply. Otherwise inductance in the wires will cause voltage spikes that may damage the MOSFET.

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