MOSFET help

Zap-o

MOSFET help

When using a MOSFET and its turned completely "on" how do I find the "Resistance of the Drain to Gate"? In other words: I want to use a MOSFET that is rated at 7amps continuous but has a power rating of 1Watt. This is drawing some concern for me because I intend on running 12-13Volt supply. I personally think that the MOSFET will be fine because the resistance will be next to zero so, the power dissipated should be relativity small. Am I correct? Please teach me...

Phil Pilgrim (PhiPi)

Power dissipation will equal (V_D - V_S) * I, or R_DS * I². You can either measure the quantities in the first formula to get your answer or find the resistance in the second formula from a graph in the datasheet that relates it to gate voltage.

-Phil

Zap-o

Thanks for the quick response. So according to the data sheet below and your formula I can assume that the total power dissipated at 5 amps will be 875 mWatts. This brings me to another question whats is the Vgs do? I see in the data sheet that its tied to the Rds(on)

www.fairchildsemi.com/ds/ND/NDT451AN.pdf

Phil Pilgrim (PhiPi)

Rds(on) is a function of the gate-to-source voltage. You have to look at the graph to see what this value is with your particular gate drive voltage. You can't just use the values given in the "Characteristics", unless one just happens to coincide with the gate voltage your system supplies. Also, if you're PWMing your load, the gate rise-time (a function of gate capacitance and the source impedance of your dirver) is important, too. If the gate drive spends a signficant fraction of its time transitioning between high and low, the power dissipation will rise.

-Phil