Little help needed to understand Dimmer for DC lamp?

[Charles Linquis]

Use an N-channel FET if you are going to switch the GND side of the circuit. Use a P-channel FET if you are going to switch the positive (high) side. If you switch the high side, you will need a level shifter like Aratti has shown. If you switch the low side, you don't need the level shifter and can drive the FET directly.

Switching the high side is necessary if one side of your lamp must be grounded. Since I doubt that is the case, then I would recommend using an N-channel logic-level FET with the source connected to GND, the drain connected to the lamp (and the other lamp terminal connected to 12V), and the GATE connected to the PIC. Use a 10K pull DOWN resistor between the gate and GND, so that the lamp is off until your program gets started.

At the low frequencies you are running, most of your losses will be resistive. Look at the RDSON with the gate drive you will provide (probably around 4V). If it is 50 milli-ohms, then at 10A the FET will have a (.050*10) = .5V drop. .5V X 10A = 5 Watts. The lower RDSON, the lower the amount of heat produced. My guess is that you will produce maximum heat at around 80-90% of full brightness.

[Megahertz]

I found one mosfet P30NF10
datasheet: http://www.st.com/stonline/products/...re/ds/7817.pdf

The setup worked upto some extent but with few concerns.
firstly when system was fired up, the lamp started glowing but very DIM. I increased the resistance with 1.5K more with 4.7K and this problem got sorted. Now the lamp stays OFF when system is powered up

The problem is that the lamp is not glowing to its full potential. Any ideas please as what changes I can try in the hardware to make this work, i.e. make the lamp off at value 0 & glow at full potential at 255.
I am using 20KHz as frequency with HPWM command.
Thanks