PIC and Buck Converter

[pjohn]

Dear All,

I have been working on a prototype of Buck converter, I have been using PIC18F25k80 in order to generate Complementary PWM using ECCP, I am only using one of the PWM pin at the moment for switching Logic MOSFET, as in the future I would like to make the Synchronous buck conveter hence Complementary PWM.

Here are my System Parameters and components:
Vin = 12v
Vout = 5v
Duty Cycle = 41.5
Iout = 2A
Switching Frequency = 7.5 Khz
Inductor = 1000uH
Diode = Schottky 3A
Cin = 47uF
Cout = 1000uF

I get the Complementary PWM on both the pins (Confirmed using Oscilloscope), I also Get the required output Voltage i.e. 5v, but as soon as I put any load on it (LED, 10mA) my Voltage gets reduced to 1v for some reason.

I have tried changing the Duty Cycle when drawing 10mA but it does not effect the output, when Duty Cycle is 90%, the maximum I get is 3v at 10mA load. Please help me, as I am lost what I am doind wrong.

The way I calculate my PWM is, I Measure the output Voltage, using ADC, and incease or decrease the duty cycle untill Output Voltage is 5v, is this how PWM Duty cycle is adjusted using Feedback?


I have also attached my circuit, Any Help or direction will be highly appreciated.

Best Regards

 

[drmorbius]

You will need to use a MOSFET high side gate driver IC (lots to chose from, google...) in between the PIC output and the MOSFET gate (or use a homebrew discrete solution, would probably work fine at 7.5KHz)

 

[Taner]

Hi,
You should use a P-channel MOSFET instead of N-channel.

 

[Tahmid]

If you're using an N-channel MOSFET (as you are), you need to use a high side driver. This can either be an IC or you can make your own discrete driver.

When you are driving an N-channel MOSFET, the gate needs to be at least above the threshold voltage for the MOSFET to be driven and above 8-12V with reference to the source for completely turning it on (for POWER MOSFETs). When the source is connected to ground, it's simple since the reference (source) is ground. However, in high-side operation (as in your case), you need to drive it higher than ground. This can be done via a bootstrapped driver or pulse transformer. You can google for more details on this method.

If you want to avoid this, you can simply use a P-channel MOSFET and turn it on by pulling it to ground. This works for upto 20v, above which you need to have a sort of level shifting, otherwise pulling it to ground will destroy the MOSFET.

Hope this helps.
Tahmid.