# How to do Thermal Impedance modelling for Foster circuit?

**URL:** <https://forum.plexim.com/t/how-to-do-thermal-impedance-modelling-for-foster-circuit/472>\
**Category:** PLECS\
**Tags:** thermal-model, power-semiconductors, wolfspeed\
**Created:** [August 30, 2018, 5:04pm UTC](https://forum.plexim.com/t/how-to-do-thermal-impedance-modelling-for-foster-circuit/472 "2018-08-30T17:04:23Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![snehathakur](https://avatars.discourse-cdn.com/v4/letter/s/cab0a1/32.png) [@snehathakur](https://forum.plexim.com/u/snehathakur)\
**Post date:** [August 30, 2018, 5:04pm UTC](https://forum.plexim.com/t/how-to-do-thermal-impedance-modelling-for-foster-circuit/472/1 "2018-08-30T17:04:23Z")

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Hi All

I have followed the demo buck converter model for understanding thermal modelling. Now I have few questions:

1. I have extracted the Ri, Ci… parameters of foster circuit from datasheet for calculating Zth\_Jc (junction to case thermal impedance). I want to verify these parameters by giving the power input to this foster circuit and observe the junction temperature. e.g at Ambient temp = 25 C, power dissipated is 92 W, R1 to R4 = 0.1746 0.6273 0.6306 0.0033, C1 to C4 = 0.0094 0.9217 1.1628 0.3433

How do I give an input of 92 W to this circuit? Is there a block of heat flow to physical signal converter available in plecs? How do I model this circuit?

Thanks

Sneha

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**Author:** ![Kris\_Eberle](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/kris_eberle/32/6_2.png) [@Kris\_Eberle](https://forum.plexim.com/u/Kris_Eberle)\
**Post date:** [September 6, 2018, 2:42pm UTC](https://forum.plexim.com/t/how-to-do-thermal-impedance-modelling-for-foster-circuit/472/2 "2018-09-06T14:42:38Z")

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Hi Sneha,

Actually, yes. In the thermal domain component library there is both a Constant Heat Flow block and a Controlled Heat Flow block, whose input signal is generated in the Control domain. You can then place a Thermometer at the desired node and view the resulting temperature.

Best,

Kris

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**Author:** ![snehathakur](https://avatars.discourse-cdn.com/v4/letter/s/cab0a1/32.png) [@snehathakur](https://forum.plexim.com/u/snehathakur)\
**Post date:** [September 6, 2018, 4:48pm UTC](https://forum.plexim.com/t/how-to-do-thermal-impedance-modelling-for-foster-circuit/472/3 "2018-09-06T16:48:09Z")

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Thanks Kris.