What is the PWM frequency that you are using, or the time between the ON pulses?

I don't understand what you are trying to say here:

Once i increase the Duty Cycle beyond 85%, the transistor driver circuit (Which converts the 5V PWM to an inverted 14V PWM) at the front part of the Buck Converter, went to flat 0% On Duty cycle

Are you trying to say that the Vgs stays at 0v when you try to increase the PWM duty cycle beyond 85%?

Did you verify that the output from the PIC pin was showing the proper signal?

[eta]
Post a photo of how you built your circuit. If you are attempting to use PWM at higher frequencies, how you have laid the circuit out will make a difference. Long wires will hurt circuit performance.

 

What is the PWM frequency that you are using, or the time between the ON pulses?

I am using 40KHz for the PWM frequency. This setting was set at the MCU setting and tested to be correct...

I don't understand what you are trying to say here:

Are you trying to say that the Vgs stays at 0v when you try to increase the PWM duty cycle beyond 85%?

Did you verify that the output from the PIC pin was showing the proper signal?

Yes, Vgs suddenly drops to zero when i try to drive the PWM On Duty Cycle beyond 85%...

It was working fine when i drive the circuit starting from 8% PWM On Duty Cycle getting 1 Volts output... And i worked my way up, trying to tune my PWM so that i can know what PWM gives what output voltage... It was at interval of proportional steps, for 1V, 2V, 3V... up to 6 Volts output. (i.e. I would only need to increase the PWM by around 6% steps to get the the next voltage level.)
For example:
To get 1V o/p: i pumped the PWM On Duty Cycle from MCU at 8% on.
To get 2V o/p: i pumped the PWM On Duty Cycle from MCU at 14% on.
To get 3V o/p: i pumped the PWM On Duty Cycle from MCU at 20% on.
To get 4V o/p: i pumped the PWM On Duty Cycle from MCU at 26% on.
To get 5V o/p: i pumped the PWM On Duty Cycle from MCU at 32% on.
To get 6V o/p: i pumped the PWM On Duty Cycle from MCU at 38% on.

However, when i try to get 7V o/p: i pumped the PWM On Duty Cycle from MCU at 80% on.

When i try to get 7V output, i had to squeeze the PWM a lot more higher. I could no longer use the usual 6% step. Therefore, when i try to get 8 Volts o/p, and tried to generate a PWM with >85% PWM On Duty Cycle, my measurement shows that the Driver circuit PWM waveform collapse to 0% ON PWM waveform... output becomes 0 Volts.

I need to drive it beyond 85% because at 80% On PWM Duty Cycle, i am only able to get 7 Volts of output... But i am trying to get a output voltage range of up to 12 Volts...

[eta]
Post a photo of how you built your circuit. If you are attempting to use PWM at higher frequencies, how you have laid the circuit out will make a difference. Long wires will hurt circuit performance.

I don't think the wires used is that long, Their length does not exceed 15cm each...
I would expect maybe the circuit would collapse nearer to the higher end, (maybe 11 Volts or 12 Volts), but it failed at 8 Volts level already...

Should i reduce the PWM frequency? Any component ratings that i can adjust to reduce this effect?

It should not be the cause of the Driver Circuit right? Cause it is showing fine when i simulated with it all the way to 12 Volts...

Thanks in advance for any advice.

 

15cm is about 5.9 inches. If the wire is .32mm, 15cm will have about 200nH inductance.

Here is an online calculator for the inductance of straight wire: http://www.consultrsr.com/resources/eis/induct5.htm

If you have the circuit on a breadboard, it probably won't work very well. It needs to be built on a PCB, with all of the parts close together. It will also need low-ESR capacitors from +V to ground.

In the meantime, try reducing your PWM frequency. Try around 10kHz. You will have much more noise on the output, and the inductors may saturate. I do not know what you are using for inductors; they may be not rated sufficiently.