Thermal Impedance - how to use this parameter

Matt

Please could you help clarify how the Thermal Impedance parameter (Zthjhs) is applied to thermal calculations (to SIC Modules with high and low side MOSFETs).

Referring to the CAB011M12FM3 data sheet Thermal Impedance graph (Fig 17), I understand that the x-axis 'Time, tp(s)' refers to the pulse duration (in seconds).

Q1) with a 0.5 duty, does the 'pulse duration' apply to the 'on time', or the 'cycle time' (on + off duration of each cycle)?

In our inverter application, the SpeedFit simulator predicts a 32C Junction to Case temperature rise for a CAB011M12FM3, which I believe is accurate. However, using hand calculations (based on data sheet Zthjhs), I calculate a significantly higher temperature rise (52C).

SpeedFit calculates total inverter loss to be 532W (three MOSFET modules, one per phase).

There are two devices per package, high-side and low-side. I note the each device comprises two separate junctions (areas on silicon), so four junctions in each module.

Q2) does the 'Zthjhs' parameter refer to each MOSFET device in the package (i.e. two devices, high and low), or to each junction (i.e. 4 junctions, two on low side MOSFET, two on high side MOSFET )?

Q3) what power dissipation should be applied to the Zthjhs parameter? I suspect that it should be the total loss per module (rather than per MOSFET device) i.e. total inverter loss divided by the three MOSFET modules 523W / 3 = 174W per module

Zthjhs (from Fig 17) 0.5 duty, 283Hz (3.53ms cycle time) = 0.3C/W (approx)

Junction to Case temperature rise = 0.3 x 174W = 52C, quite a lot higher than 32C.