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Power for Mosfet's gate resistor

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Dimitrisvlamis

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Hello,

I am designing a pcb and i would like to ask if i can use 0805 resistor for my design. The mosftet i use is IRF3205 and the circuit says i need a 10 Ohm resistor, i know 1/4W resistor is ok but 0805 is 1/8W, can i use it ?
 

Hi,

At the moment only you can know if you can get away with a 0.125W resistor... If you know the gate drive current then you can just multiply I^2 * R to find out. 100mA into a 10R is only 100mW. Also, the resistor datasheet or catalogue will/should show pulsed power in writing and in a graph, another way of not exceeding SOA.
 

It is not expected much above 150nC for the gate of the IRF3205. The energy stored at Vgs should be dissipated at Rg in the worst case, therefore if I were to estimate the power of the resistor I would do the following calculations

Code: 
1/2 C.V[SUP]2[/SUP] = V[SUP]2[/SUP]/R.t

Let us agree that by a preliminary estimate it is clear that the power dissipated in the resisotr would be negligible :

Code: 
P = (150/10[SUP]9[/SUP]).f
 

Unless it's a special pulse rated resistor, you most likely won't find significant data characterizing the resistor below 1 us.

Experience suggests that standard 0805 thin or thick film resistors won't have problems in this operation range, although the peak power during gate charge and discharge is probably above specified limits. It's presumed that the average power limits according to gate charge aren't exceeded.
 

More details needed. Vds in the off state or preferably the total gate charge at the specified operating voltages, as well as switching frequency.
In the worst case scenario, where all the driving power is dissipated in the resistance, the maximum switching frequency I get is ~69.5 kHz, assuming 12 V gate drive and 150nC.
 

for 146nC on the gate capacitance (data sheet) to 10Vgs, this gives Ceff = 12nF, for 100kHz operation, the power in the R is approx: C.V^2.freq = 174mW so use 1206 or 2 x 22 ohm in parallel (0805), for 50kHz you get 87mW... and so on ...
 

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