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. – PlasmaHH Feb 12 '16 at 20:34
• Nice related article:mcmanis.com/chuck/robotics/projects/esc2/FET-power.html – jippie Feb 12 '16 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 Feb 12 '16 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? – Oleksandr R. Feb 13 '16 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). – Oleksandr R. Feb 13 '16 at 5:48