That Mosfet has a 500V rating that you don't need and its on-resistance is very high. It has a loss of up to 3V at 6A.
Why nort use a lower-voltage Mosfet that has a much lower on-resistance and much lower voltage loss?

Why limit its gate voltage? Use 10V if it saturates well with 10V. Select a "logic-level" Mosfet if you have only 5V axailable to feed its gate.

Only if all of the following are true:
1. the PIC's HI voltage is enough to trigger, the MOSFET gate (pass the gate threshold voltage, and it is highly preferable to go even higher than that and have it push the MOSFET into saturation)
2. the MOSFET's source is referenced to the same ground as the PIC
3. you are switching the transistor slowly

1. If the PIC's 5V output isn't enough to pass the gate threshold voltage...well it won't turn on. Either choose a different transistor with a lower gate threshold voltage, or use a gate driver that allows a 5V signal to drive the gate at 10V (or whatever voltage is needed to turn the gate on).

2. MOSFET's connected on the high side (closer to +V than the load) have this problem. YOu need a floating gate driver or a boostrap capacitor gate driver or some other complicated method.

3. So if you are switching it fast, you need a gate driver since the PIC can't provide enough current to charge up the gate capacitance fast enough. And sometimes the source of the MOSFET is floating, so the 5V from a PIC so that if you apply it to the gate, will not produce a gate-source voltage difference that is higher than the gate threshold voltage (preferably one that will push the MOSFET into saturation).

1 and 3 may be problems for youru circuit depending on what you want. 2 is a yes for your circuit already.

Originally Posted by dknguyen

1. the PIC's HI voltage is enough to trigger, the MOSFET gate (pass the gate threshold voltage ...)

Forget about the threshold voltage because then the Mosfet is barely conducting with a drain current of only 0.25mA. This Mosfet fully turns on with a gate voltage of 10V.

He shows a Mosfet Driver that might have an output of 10V.

Audio you have a very valid point here.

Many people fall into the trap of assuming the MOSFET threshold voltage to be the operating VGS voltage. In fact it is the voltage that the MOSFET starts conducting.

One should really look at the data graphs in the datasheet that gives the corresponding VGS at the wanted load current and make sure the VGS to the MOSFET gate is higher than this value.

So if the motor max current is 6A, obtain from the graph the correct VGS voltage that gives 6A load current.

Hi L.Chung,
The graph shows typical Mosfets. The one you use might be minimum.
You might even find an entire production lot of minimum spec's.
I always design circuits so that all parts work, not just some parts.

Originally Posted by audioguru

"1. the PIC's HI voltage is enough to trigger, the MOSFET gate (pass the gate threshold voltage ...) "

Forget about the threshold voltage because then the Mosfet is barely conducting with a drain current of only 0.25mA. This Mosfet fully turns on with a gate voltage of 10V.

He shows a Mosfet Driver that might have an output of 10V.[/

"..." = "and it is highly preferable to go even higher than that and have it push the MOSFET into saturation"

???

Originally Posted by dknguyen

???

When talking about turning on a high current switching Mosfet, the threshold voltage should not be mentioned.
The threshold voltage is when it is turning off.

One important parameter he left out is (according to his posts on another forum) this motor requires 180V (DC, I hope ). This somewhat limits the choice of MOSFETs, but there are still plenty to choose from. I think it does severely limit, or possibly exclude, logic level MOSFETs as a choice. We (or I, at least) missed the PWM aspect of the design.

Originally Posted by Roff

One important parameter he left out is (according to his posts on another forum) this motor requires 180V (DC, I hope ). This somewhat limits the choice of MOSFETs, but there are still plenty to choose from. I think it does severely limit, or possibly exclude, logic level MOSFETs as a choice. We (or I, at least) missed the PWM aspect of the design.

sure, 180V dc
i choice that MOSFET because its available, about the logic level MOSFET
if you can send to me the type no. of it.
and thanks for your note
and the design is for PWM control for the DC-MOTOR

Originally Posted by h.d

sure, 180V dc
i choice that MOSFET because its available, about the logic level MOSFET
if you can send to me the type no. of it.
and thanks for your note
and the design is for PWM control for the DC-MOTOR

Fairchild has a few 200 volt MOSFETs with Rds(on) guaranteed at 5 volts Vgs. 200V may not have a comfortable margin in a 180V application. That's up to you.
The lowest Rds(on) I found was FQP34N20L. Rds(on) is 0.08 ohms max.
The Fairchild MOSFETs can be found by going here.
Here is that same table after filtering for N-channel 200V parts. I couldn't find any logic level MOSFETs with breakdown voltage higher that 200V