# MOSFET power dissipation calculations - Diodes Inc. datasheets

Looking at Diodes Inc. datasheets, I am having trouble following their power dissipation limit calculations for their MOSFETS.

E.g. for DMG4496SSS http://www.diodes.com/_files/datasheets/ds32048.pdf

They specify on page 1

• I_D(max) = 8A @ V_GS=4.5V (with a R_DS(on) = 0.029 ohm)

But then the datasheet also gives on page 2:

• Power dissipation P_D = 1.42 W
• Junction temperature T_J = 150°C
• Thermal resistance R_\theta = 88.49 K/W

And on page 3:

• R_DS(on) @ V_GS=4.5V, I_DS=8A approximately 0.024 ohm

To me this looks like one big mess:

1. P = 0.029 ohm * (8A)^2 = 1.86 W which is significantly larger than the permissible power dissipation of P_D = 1.42 W from page 2
2. even with the R_DS(on)=0.024 ohm value from page 3, P = 1.54 with is still larger than the permissible power dissipation
3. the permissible power dissipation figures are at least self-consistent: P_D =(T_J-T_A) / R_\theta = (150°C-25K) / 88.49 K/W = 1.41 W
4. However, the R_DS(on) vs V_GS and I_D vs V_DS graphs appear to be inconsistent: Looking at the case of V_GS = 3.5 V: In fig 1, the tangent at the point (V_DS=0.5V, I_D=10A) is about 6A/0.5V which seems to imply a R_DS(on) = 0.5V/6A = 0.083 ohm. Looking at fig. 3 however, the R_DS(on) is more like 0.048 ohm at 10A.

How to use Diodes Inc datasheets?

So given the datasheet, how would one calculate I_DS(max) provided some V_GS and some V_DS? E.g. V_GS = 6V and V_DS = 12V.

• Have a +1 from me, solely for reading a datasheet in that detail. Commented Feb 12, 2016 at 20:34
• Nice related article:mcmanis.com/chuck/robotics/projects/esc2/FET-power.html Commented Feb 12, 2016 at 20:41
• @jippie Thanks for the reference, unfortunately that explains why the power rating of the MOSFET is LOWER than suggested by the P_D and R_DS(on) figures. In the datasheet I referenced, the power rating is HIGHER than suggested by P_D and R_DS(on)... - The first is completely logical, the latter should not be physically possible!
– ARF
Commented Feb 12, 2016 at 21:07
• 1. I_Dmax is usually specified at V_GS = 10V or perhaps 5V for a logic level MOSFET. 2. I_Dmax is not limited by power dissipation in the way that you think--imagine 100ns pulses with a duty cycle of 1%. In such a case it would be possible to pass 30V/0.024Ohm = much more than 8A without ever exceeding the power dissipation limit, yet still destroy the device. The first-page specifications are often typical rather than guaranteed values, so I wouldn't take them too seriously if they're slightly contradicted elsewhere. Does that help a bit? Commented Feb 13, 2016 at 5:39
• I should also say that thermal resistance is not a static quantity, but time-dependent because the MOSFET has a certain heat capacity and thermal diffusion rate. Infrequent, powerful current pulses will heat it basically only to the extent of their RMS value, not instantaneously to incandescence. See also (35352). Commented Feb 13, 2016 at 5:48