# Mosfet coss modeling **URL:** \ **Category:** General\ **Tags:** plecs, mosfet\ **Created:** [December 17, 2025, 2:19pm UTC](https://forum.plexim.com/t/mosfet-coss-modeling/2761 "2025-12-17T14:19:07Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![mazers26](https://avatars.discourse-cdn.com/v4/letter/m/5f9b8f/32.png) [@mazers26](https://forum.plexim.com/u/mazers26)\ **Post date:** [December 17, 2025, 2:19pm UTC](https://forum.plexim.com/t/mosfet-coss-modeling/2761/1 "2025-12-17T14:19:07Z")
Hello guys. I am working on PSFB converter and I want to be sure and see zero voltage transient(ZVT). I need to add a capatiance between drain and source in mosfet to ensure resonance with leakage inductance of primer winding. When I add a thermal model of a mosfet , is it included Coss parameter ? as second option, When I connect a capacitance paralell to mosfet , the simulation doesnt work. I connected a series resistance to capacitance to prevent this but is this a good solution to model a Coss capacitance If yes, what should the resistance value be for optimal performance? What are your thoughts ? ---
**Author:** ![Munadir\_Ahmed](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/munadir_ahmed/32/1540_2.png) [@Munadir\_Ahmed](https://forum.plexim.com/u/Munadir_Ahmed)\ **Post date:** [December 17, 2025, 2:53pm UTC](https://forum.plexim.com/t/mosfet-coss-modeling/2761/2 "2025-12-17T14:53:42Z")
Hi mazers26, Does this post help answer your question: [Soft switching implementation in a DAB converter - #5 by Munadir\_Ahmed](https://forum.plexim.com/t/soft-switching-implementation-in-a-dab-converter/2666/5) . Let me know if you still have any open questions. Also, sharing a model would help clarify what is happening when you say “the simulation doesn’t work.” ---
**Author:** ![mazers26](https://avatars.discourse-cdn.com/v4/letter/m/5f9b8f/32.png) [@mazers26](https://forum.plexim.com/u/mazers26)\ **Post date:** [December 18, 2025, 6:21am UTC](https://forum.plexim.com/t/mosfet-coss-modeling/2761/3 "2025-12-18T06:21:46Z")
Hello, Firstly, If I work without Coss capacitances , mosfets ensure the ZVS in any primer current. However, ZVS cant be ensured in any primer current therocially . There is a critical value of primer current to ensure ZVS . Thus, the simulation is not accurate at all. I mean my first clear question is how can I observe the ZVS in this circumstances. Second option , connecting a paralell coss capaticance to mosfets . In this case , When a gate signal is high and if the primer current is under the critical value to get in ZVS , mosfets current go to peak instantly then stabilize. ![image](https://europe1.discourse-cdn.com/flex006/uploads/plecs/original/2X/5/5d7f549ee0b3658d1e2ea5c22309f6c463af979c.png) like that. I dont understand that why these peak currents happen ? ![image](https://europe1.discourse-cdn.com/flex006/uploads/plecs/original/2X/1/119a3f30610bb2b0b2acd657173a4c3673a2af57.png) This is my full bridge structure and those capacitance are supposted to be Coss and I have to connect a series resistance each of them . Otherwise , simulation goes to error like the image below ![image](https://europe1.discourse-cdn.com/flex006/uploads/plecs/original/2X/0/0bf205d702d88dc1b4a839f537c474867d7d9ef4.png) I wonder that Why mosfet current goes to peak to high values when I connect capactiance under critical value of primer current for ZVS ? and how can I observe thermal charasteristic in this situtaion ---
**Author:** ![Munadir\_Ahmed](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/munadir_ahmed/32/1540_2.png) [@Munadir\_Ahmed](https://forum.plexim.com/u/Munadir_Ahmed)\ **Post date:** [December 18, 2025, 8:31pm UTC](https://forum.plexim.com/t/mosfet-coss-modeling/2761/4 "2025-12-18T20:31:22Z")
Please see my responses below: _Firstly, If I work without Coss capacitances […] this circumstances._ You are absolutely right, without modeling the resonating capacitor you will not be able to observe the ZVS functionality. _Second option , connecting a paralell coss […] then stabilize._ Hard to tell what is going on without a PLECS model. What I will say is if you don’t achieve ZVS and turn on the FET you are effectively shorting a capacitor (Coss) across a very small resistor (Rdson). For example, if your Rdson is 1mOhm and you have 1V then the instantaneous current would be 1000A with a decay defined by the Rdson\*Coss time constant. _This is my full bridge structure […] to error like the image below_ Please see the following description of the error you are experiencing: [What does the error message "state/source dependence" mean? | Plexim](https://www.plexim.com/support/solutions/166) _I wonder that Why mosfet […] thermal charasteristic in this situtaion_ A potential solution might be the one I proposed in this comment on that thread I had sent you earlier: [Soft switching implementation in a DAB converter - #7 by Munadir\_Ahmed](https://forum.plexim.com/t/soft-switching-implementation-in-a-dab-converter/2666/7) . Effectively having a single configuration to model both ZVS and thermals may not be a feasible option.