Thermal simulation query

T

Techman_7

Full Member level 3
Joined
Dec 21, 2013
Messages
159
Helped
0
Reputation
0
Reaction score
1
Trophy points
1,298
Activity points
2,552
Hello,
In cauer thermal simulation as below -

Source1 is the PWL pulses acting as power dissipation in MOSFET. The simulation works but if the Rthca (R5) is changed, then still the junction temperature remains the same. (Logically its correct, it should not change as per circuit configuration).

But practically Tj should change if Rthca (R5) is changed.

What modifications are needed in above simulation?
 

Sort by date Sort by votes
Hi,

In my understanding source1 needs to act like a constant_current not like a constant_voltage.

Klaus
 
Reactions: FvM

Upvote 0 Downvote

Upvote 0 Downvote
KlausST said:
Hi,

In my understanding source1 needs to act like a constant_current not like a constant_voltage.

Klaus
Click to expand...
I have created behavioural voltage source whose values are equal to power dissipation in the MOSFET. The source1 waveform looks fine w.r.t. power dissipation in the FET.
 

Upvote 0 Downvote
Hi,

this means: You don´t want to correct it?
If so: why do you ask for "what needs to be modified"?

Klaus
 

Upvote 0 Downvote
Techman_7 said:
I have created behavioural voltage source whose values are equal to power dissipation in the MOSFET. The source1 waveform looks fine w.r.t. power dissipation in the FET.
Click to expand...
The waveform may look correct, but the model uses no physically plausible mapping of power/temperature, thus can't give correct temperature values.
Please review the linked Infineon application note. If you map thermal resistance to electrical resistance, temperature corresponds to voltage and power to current, not voltage. Therefore power dissipation must be modelled by a current source.
 

Upvote 0 Downvote
Hello,
I tried with current source but Tj variation is inversely proportional to Rthca value. I think Tj and Rthca should be directly related to each other....
--- Updated ---

KlausST said:
Hi,

this means: You don´t want to correct it?
If so: why do you ask for "what needs to be modified"?

Klaus
Click to expand...
Because, the junction temperature not changing even if Rthca is changed. This should not happen, thats why asking what modifications needed.
Rthca and Tj should be directly proportional...
 
Last edited:

Upvote 0 Downvote
Hi,

Techman_7 said:
Because, the junction temperature not changing even if Rthca is changed. This should not happen, thats why asking what modifications needed.
Rthca and Tj should be directly proportional...
Click to expand...
if you have a voltage source .. an a resitor connected to it... Why should the voltage change when you change the resistor?

I can only repeat: use a current source.

Techman_7 said:
I tried with current source but Tj variation is inversely proportional to Rthca value. I think Tj and Rthca should be directly related to each other....
Click to expand...
makes no sense at all.

--> Current through a resistor:
* voltage is directly proportional to resistor
* voltage is directly proportional to current.

Nothing "inversly"..

Klaus
 

Upvote 0 Downvote
Techman_7 said:
I tried with current source but Tj variation is inversely proportional to Rthca value.
Click to expand...
Not sure what Tj is in your model. I'm my simulation it's potential of upper left node. V(tj) = I(p,avg) * sum(Rth) + V(tamb), thus temperature proportional to Rth.
 

Upvote 0 Downvote
You must log in or register to reply here.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…