# Rthjc why is smaller than Rthca

##### Member level 1
Hello,
does anyone knows why the Rthjc is smaller than Rthca on a Mosfet? The Rthja I read that is Rthja=Rthjc + Rthca. The Rthjc shouldn't be higher than the Rthca becase the Rthjc is the temprature closer to the junction (the core of the mosfet)?

IRF540N:
Rthja: Junction-to-Ambient - 62 Celcius/W
Rthjc: Junction-to-Case - 1.15 Celcius/W
Rthca: Case-to-Ambient

Rthja = Rthjc + Rthca -> Rthca = Rthja - Rthjc = 62 Celcius/W - 1.15 Celcius/W = 60.85Celcius/W

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#### KlausST

##### Super Moderator
Staff member
Hi,

Rthja is when the device is used "without heatsink". This is the total thermal resistance to calculate the temperature rise.

Rthjc is the thermal resistance from junction to device case .... when the device is used with heatsink.
This is just one part of the total thermal resistance. There additionally is the transition from device_case to heatsink...and the transition from heatsink to ambient air.

Klaus

#### Easy peasy

the thermal path from junction to case is very good, hence low figure, the thermal path to air is not so good, hence much higher figure - if you bolt the device to a large heatsink you have changed the thermal path to air to being very good - and will get a lower figure, based on heatsinkk size and design and orientation, and air flow ...

##### Member level 1
Thanks a lot for the replies... Also I saw the below videos where are very helpful.

*** Heatsinks ***
EEVblog #105 - Electronics Thermal Heatsink Design Tutorial

EEVblog #744 - SMD Thermal Heatsink Design - µSupply Part 15

EEVblog #1196 - Custom Heat Sink Design

EEVblog #1347 - SMD Thermal Jumpers could be GAME CHANGING!

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