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# UC2843A: Temperature stress of UC2843A

Part Number: UC2843A
Other Parts Discussed in Thread: UC2842A

Hi team,

My customer test the surface temperature of UC2842A and UCC2843A is greater than 85℃. And they calculate the junction temperature by the function--

Tj =Tc+PD*RθJC, where PD is the power consumption of the device. But they don't it can verify if the devices are working in a normal temperature range. Can you provide some comments on this?

 Rated working tempreature 37V 48V 57V UC2842AD8TR 85Ta 80.93℃ 90.59℃ 94.43℃ UC2843AD8TR 85Ta 71.37℃ 84.35℃ 89.05℃

Below is the power consumption for these two devices.

 VIN=37VDC VIN=48VDC VIN=57VDC PD(U2)/(mW) 570.7 576.4 548.89 PD U4/(mW) 197.16 190.96 190.96

And Tj result is shown below.

 Rated Tj tempreature 37V 48V 57V Tc Tj Tc Tj Tc Tj UC2842AD8TR 150℃ 80.93℃ 104.8℃ 90.59℃ 114.8℃ 94.43℃ 117.4℃ UC2843AD8TR 150℃ 71.37℃ 79.6℃ 84.35℃ 92.3℃ 89.05℃ 97.07℃
• Hi Lenna Yan ,

The method used here is not accurate. It assumes that all of the dissipated power is extracted through the top of the case which is not true.

Instead of RθJC you should use the parameter ΨJT. While this is not a 'true' thermal resistance it includes the fact that more power is extracted via the leads than through the plastic case. Please see the extract below taken from the link below the Thermal Information table in the datasheet.

Thanks

Joe Leisten

• Hi Joe,

Thank you. Do you mean they should use the below function to calculate the junction temperature?  Tj =Tc+PD*ΨJT. If so, the Tj calculation result is lower than the original result. If it's wrong, how can we verify the device is working in a normal temperature range?

I asked a similar questions several weeks ago. And I got a answer from Peter. If the controller is not in a forced air environment then the surface temperature of the device will be within a few degrees of the junction temperature.

Best regards,

Lenna

• Hi Lenna,
For a typical cooling arrangement the expression you have written in your last post should give the correct result. The parameter ΨJT is given in the datasheet so customers have and easy way to check the junction temperature of their designs.
It is possible to verify the junction temperature by experiment, but the process is long and not normally required. If you customer feels this is necessary please contact me by Email and I will describe a process by which this can be done.
The method suggested by Peter is approximate, but normally also gives a result that is not too far from the accurate figure.
Thanks
Joe (j-leisten@ti.com)