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:
- 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
- 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
- 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
- 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.