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Buck conver using PMOS gate driving problem

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zarzisur

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I want to build a buck converter with p type mosfet. Specification: input voltage:24v Output voltage:12v I have chosen frequency 20khz,and my inductor value and capacitor value is 1mH and 20uF.

Now all seems to be okay.the problem is i cant drive the pmosfet.I wanted to drive Pmosfet with a 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 output 12 vol? If the circuit is not okay than what kind of circuit should i use to drive Pmos? View attachment 138916
 

Hi,

A schematic without values.
A bjt without resistor to limit base current.
And a vague error description:
why it is not giving me output 12 vo
What does this mean? Is it 12.1V, or 0V, or 24V?

How did you try to achieve 12V?

Klaus
 

You need to make sure both NPN and mosfet shut completely off when they are supposed to be off.
And when the NPN turns on you need to make sure it pulls the mosfet gate low.
 
I want to implement this schematics.
eda.JPG
I am not sure about R3 and R4 though
 

HI BradtheRad,
Thanks!!!
When NPN is on doesnt this circuit pulling gate to ground?eda.JPG


Rony
 

Hi,

When the NPN is ON.....
* then there is about 24V across gate-to-source.
FET datasheet says max V_gs is +/-20V...so you are off specification.

* Then there is 24V across the 100R, too. Generating more than 5.5W of heat...
is it specified for this?

* Then there is 2.4A through the NPN
Is it specified for this?

Klaus
 
As already explained, the discussed driver circuit exceeds the MOSFET Vgs,max rating and also involves large power dissipation.

On the other hand, driving the gate of a 20 kHz switcher with 100 ohm driver impedance brings up some switching losses due to slow switching. The driver topology is simply unsuitable for a MOSFET switcher. You want a driver impedance below 10 ohm, reasonable gate voltage level of 10 to 12 V and low static power dissipation.

If using discrete transistor drivers, a NPN and PNP transistor for a complementary voltage follower should be afforded, also gate voltage adjustment respectively limitation.
 

HI BradtheRad,
Thanks!!!
When NPN is on doesnt this circuit pulling gate to ground?

Rony

Looking at your schematic, the NPN must attain a resistance that is much less than the value of R3( 100 ohms). Then it pulls the mosfet gate low.

Try a higher value for R3, say between 2k and 20k. Then the NPN is more likely to create proper operation.

If it were me I would reduce clock freq to 1 Hz, then watch my meters for a couple cycles, to make sure I'm getting complete turn-Off, and sufficient turn-On. This is similar to a short circuit, therefore disconnect output components if you are concerned.
 

If you just need a PMOS type buck switcher, maybe you can look at this thread
https://www.edaboard.com/showthread.php?t=362915 where I posted a design that is around 90% efficient. The circuit shows values for 48 V input but can easily be adapted for 24 V. I have tested the design intensively, and it works very well.
 

Why you are using the old buck converter design by using expensive and bulky inductor. Unless you don't want much power from the buck converter and you have the arduino why not use the inductor less design with feedback for your constant 12Volts output.
Please specify the output power and input power. For low power consumption we can use the inductorless convert of constant voltage, constant current or both.
 

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