The maximum junction temperature allowed for most Infineon automotive linear voltage regulators is 150°C. The thermal shutdown protection can prevent the device from direct damage caused by an excessively high junction temperature. Moreover, exceeding the specified maximum junction temperature reduces the lifetime of the device. A proper design must ensure that the linear regulator is always working beneath the allowed maximum junction temperature as specified in the datasheet of the device.
Thermal resistance is the temperature difference across a structure in the presence of a unit of power dissipation. It reflects to the capacity of the package to conduct heat outside the device. It is the key parameter to be considered in the thermal design. The most useful thermal resistance for thermal calculation is the junction-to-ambient thermal resistance R thJA. In most datasheets, junction-to-ambient thermal resistance R thJA is specified in accordance with JEDEC JESD51 standards defining PCB types and heat sink area.
Cross section JEDEC 1s0p board
Cross section JEDEC 2s2p board
Knowing the input voltage, the output voltage and the load profile of the application, the total power dissipation can be calculated:
P D = (V IN – V OUT) × I OUT + V IN × I q
The maximum thermal resistance R thJA can then be calculated:
R thJA,max = (T j,max– T a)/P D
Based on the above calculation the proper PCB type and the necessary heat sink area can be selected with reference to the thermal resistance table in the regulator’s datasheet. Below is an example of the thermal consideration for an application with TLE42754G.
Example: TLE42754G thermal resistance
|Junction to case 1)||R thJC||-||3.7||-||k/W||-|
|Junction to ambient||R thJA
|-||70.0||-||k/W||Footprint only 3)|
|-||42.0||-||k/W||300mm 2 heatsink area on PCB 3)|
|-||33.0||-||k/W||600mm 2 heatsink area on PCB 3)|
1)Not subject to production test, specified by design.
2)Specified R thJA value is according to Jedec JESD51-2, -5, -7 at natural convection on FR4 2s2p board; The product (chip + package) was simulated on a 76.2 x 114.3 x 1.5 mm³ board with 2 inner copper layers (2 x 70µm Cu, 2 x 35µm Cu). Where applicable a thermal via array under the exposed pad contacted the firstinner copper layer.
3)Specified R thJA value is according to JEDEC JESD 51-3 at natural convection on FR4 1s0p board; The product (chip + package) was simulated on a 76.2 x 114.3 x 1.5mm³ board with 1 copper layer (1 x 70µm Cu).
Example:Thermal calculation for TLE42754G
Determination of R thJA:
As a result, the PCB design must ensure a thermal resistance R thJA lower than 28.76K/W. Referring to the thermal resistance table of the TLE42754G, only a FR4 2s2p board could be used.
Thermal resistance constant R thJA reflects the steady-state condition of the power dissipation.
In other words, the amount of heat generated in the junction of the device equals the heat conducted away. In some applications, the worst case conditions for power dissipation occur during the transient state. The duration in transient could be far shorter than steady-state.
Thermal impedance curves characterize delta temperature rise (between junction and ambient) versus power dissipation as a function of time. In this case, the junction temperature will be a function of time: T j(t) = Z thJA(t) × P D(t) + T a
Thermal impedance curve of TLE42754 in PG-TO263 package
Calculation example in transient based on TLE42754G. The following load current profile is applied.
Determination of junction temperature T j:
The calculation result shows that the junction temperature of TLE42754G never exceeds the maximum threshold of 150°C. This is a valid thermal design.