Hi,
Could anybody advise me on MOSFET driving circuit ?
The push/pull driver is very popular using a PNP and NPN transistor with bases tied together as the input and emitters tied together as the output. A low value of resistor around 22-100Ohms is conected from this output to the gate to prevent parasitic oscillations when switching. The 2 collectors are tied to the supply rails according to their proper polarities. Some times a backward diode is connected over the gate resistor to help speed-up the turn off cycle.
Based on datasheet, it is true that the higher the VGS for a N-Channel MOSFET will result in a lower RDS(on) ?
There's rarely ever a need to switch above 9V or so on the gate as any voltage above this will only make a very marginal improvement upon the on-resistance, and also remember that you need to discharge the input and that takes time and current that rapidly increases with switching frequency. The total capacitance you have to charge-discharge on the gate will also increase with the Miller capacitance reflected back into the gate which again depends on the voltage swing on the output of your device. This switching off period is very important with inductive loads.
And if that's the case, isn't it will be much easier to operate a P-Channel MOSFET than N-Channel since it only require a low on the gate ?
This device operates on a different polarity, low voltage to turn on and high to turn off. The drain connects -ve with respect to the source. P-Chan devices normally have higher on-resistances for the same die size.
Where do we usually find the minimum voltage to turn on a N-Channel or P-Channel MOSFET ?
There's normally a curve of VGS/V against gate charge QG/nC on the datasheet. The part where the curve starts to be horizontal the device starts turning on properly. On average around 3-4V
For a P-Channel circuit, why do designer usually attached a huge resistor (in KOhm to MOhm range) between the gate and source ?
This will prevent the device from accidently turning on by keeping the gate discharged with no drive connected.