ku637
Advanced Member level 4
Hello ,
I was trying to calculate the junction temperature of a device MMDT5451 .
The datasheet states it can handle power up to 320mW , if proper PCB layout is taken care and provides an equivalent 𝑅𝑡ℎ𝐽A=390°C/W.
The power dissipation in application can come close to 195mW which is less than rated value of 320mW
But if i use the junction temperature equation Tj=Ta+ Pd x𝑅𝑡ℎ𝐽A ( with ambient around 100°C) Tj=100+0.195x390=176.05°C which will violate the Tjmax spec 150°C.
I understand there is an 𝑅𝑡ℎ𝐽C=140°C/W and the usage of 𝑅𝑡ℎ𝐽A=390°C/W alone in the equation may not be accurate.
I'm not using any heatsink , no exposed pad just the plastic case and soldered on to the PCB.
Is there any better way by modifying the Tj equation to result in more a realistic Tj estimate.
I was trying to calculate the junction temperature of a device MMDT5451 .
The datasheet states it can handle power up to 320mW , if proper PCB layout is taken care and provides an equivalent 𝑅𝑡ℎ𝐽A=390°C/W.
The power dissipation in application can come close to 195mW which is less than rated value of 320mW
But if i use the junction temperature equation Tj=Ta+ Pd x𝑅𝑡ℎ𝐽A ( with ambient around 100°C) Tj=100+0.195x390=176.05°C which will violate the Tjmax spec 150°C.
I understand there is an 𝑅𝑡ℎ𝐽C=140°C/W and the usage of 𝑅𝑡ℎ𝐽A=390°C/W alone in the equation may not be accurate.
I'm not using any heatsink , no exposed pad just the plastic case and soldered on to the PCB.
Is there any better way by modifying the Tj equation to result in more a realistic Tj estimate.