I wanted to verify the following or get some helpful pointers.
So, I have a gate driver which can supply a maximum current of 0.5 A , and I am driving a MOSFET Vgs of 15 V, based on these two specifications I should be selecting a gate resistance of at least 30 ohm (preferably even higher). This way the gate driver doesn't get damaged when the MOSFET is switched ON and its Cgs is charged up.
The peak power that the resistance should be able to handle is
\$ 15 W \cdot 0.5 = 7.5 W \$.
So while selecting the SMT resistance, I was looking for something like a R~50 ohm, P=10 W and guess what, its hard to come by something like that (in a 1210 package size or similar), and not to mention they are pretty expensive (highest I found was 3.5 W for $ 3!)
So, instead of considering peak power, I assume we should consider avg. power?
\$P_{\text{avg}} = \frac{V_{\text{rms}}^2}{R} \$
While considering Pavg in LTSpice, it came to around 0.5 W (when Vgs was charged to around 80%)
So, is it fine to use a resistor of 1 W rating in this scenario?
Any other pointers would be much appreciated.
Best Answer
Normally you can size the resistor for the average power rather than the instantaneous power. And given certain assumptions, there is an easy way to calculate the average power dissipated in the resistor:
\$P = CV^2F\$
Where P is power, C is gate capacitance, V is the gate voltage, and F is the switching frequency. Note that the resistor value is not part of the formula. That is because given certain assumptions, the resistor value doesn't change the average power dissipation in the resistor.
Of course, the resistor has a strong effect on the overall power dissipation because it affects the turn-on and turn-off time of the transistor. As the gate resistor gets larger, the transistor power dissipation increases (because it switches more slowly). But if the resistor is too small, then there can be other undesireable effects such as ringing or miller capacitance coupling into the driver IC through the output, etc. But that is not what you asked about.