# Question about MOSFET switching loss with external Coss and custom commutation current in PLECS 4.7.4 **URL:** \ **Category:** PLECS\ **Tags:** thermal-model\ **Created:** [August 10, 2026, 1:51pm UTC](https://forum.plexim.com/t/question-about-mosfet-switching-loss-with-external-coss-and-custom-commutation-current-in-plecs-4-7-4/3264 "2026-08-10T13:51:32Z")\ **Posts on this page:** 1\ **Showing post:** 2
**Author:** ![Bryan\_Lieblick](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/bryan_lieblick/32/10_2.png) [@Bryan\_Lieblick](https://forum.plexim.com/u/Bryan_Lieblick)\ **Post date:** [August 10, 2026, 5:52pm UTC](https://forum.plexim.com/t/question-about-mosfet-switching-loss-with-external-coss-and-custom-commutation-current-in-plecs-4-7-4/3264/2 "2026-08-10T17:52:18Z")
The short answer is that you should not use an explicitly modeled Coss in conjunction with a PLECS thermal model as it invalidates the assumptions of the thermal model - namely an instantaneous transition from the off state to the on state. You end up chasing various artifacts 1) the instantaneous current discharge of the Coss through the switch on-state resistance and 2) the parallel capacitance means the voltage across the switch voltage cannot change instantaneously at the switching moment. These distort the inputs into the look-up tables. You can use the workarounds you mention in your post like using the tank current, perhaps eliminating the artificial power losses from discharging Coss, but this isn’t a consistent engineering workflow one can make broad recommendations on. The typical recommendations are twofold: - Clearly separate the thermal model used for loss analysis (requires some assupmtions, for example ZVS is always achieved or ZVS losses) from the model used to analyze ZVS/dead-time commutation. This is explained further in [this post](https://forum.plexim.com/t/soft-switching-implementation-in-a-dab-converter/2666/7). - Use PLECS Spice which, with the appropriate model of the semiconductor device, can explicitly model the Coss, ZVS behavior, and power loss. --- _[View the full topic](https://forum.plexim.com/t/question-about-mosfet-switching-loss-with-external-coss-and-custom-commutation-current-in-plecs-4-7-4/3264)._