Junction temperature and thermal resistance calculator
Enter the power dissipation, the ambient temperature and the thermal resistances from the datasheet to estimate the junction temperature Tj inside the chip. Two cases are compared, without a heat sink (θJA) and with one (θJC + θCS + θSA), each with its margin to the maximum junction temperature and the maximum power it allows.
Results
Junction temperature Tj
Margin to Tj(max)
Maximum power
No heat sink (θJA)
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With heat sink (–)
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Steady-state values from Tj = Ta + P × θ; the maximum power is (Tj(max) − Ta) ÷ θ. Transient heating from short power pulses is not included. θJA changes a lot with the board and airflow, so treat it as a guide.
See it as a graph
Other inputs stay at their current values while the chosen input varies from 1/4 to 4 times its current value. The red dot is the current value.
Thermal resistance works like an electrical circuit
Power dissipated in the chip turns into heat that flows from the junction through the package and the board or heat sink to the surrounding air. Thermal resistance θ is the temperature difference produced per watt of heat flow, in °C/W. Treating temperature difference as voltage and heat flow as current gives an Ohm's law, ΔT = P × θ, and resistances in series simply add. The junction temperature is therefore Tj = Ta + P × θJA. With the defaults of P = 1 W, Ta = 25 °C and θJA = 50 °C/W, Tj = 75 °C, leaving a 50 °C margin to a 125 °C Tj(max).
Why θJA depends on the board, and ψJT
θJA is not a property of the package alone. It changes greatly with board size, the number of copper layers, copper area and airflow. Most datasheet values are measured on a JEDEC standard test board in still air, so they are best used to compare packages. In a real product heat spreads through the board copper, and θJA can be higher or lower than the table value. In a real system, Tj is often estimated from the measured package top temperature Tt as Tj = Tt + P × ψJT. Because not all heat leaves through the top, ψJT is called a thermal characterization parameter rather than a thermal resistance.
The heat sink chain and interface material
With a heat sink, heat flows mainly from junction to case, case to sink and sink to ambient, so the total is θJC + θCS + θSA. θJC is internal to the package and listed in the datasheet. θCS is the contact between case and heat sink; tiny air gaps in the rough surfaces block heat, so a thermal interface material (TIM) such as thermal grease or a thermal pad fills them. θSA depends on the heat sink size, fin shape and airflow. The defaults total 4.5 °C/W, giving Tj = 29.5 °C at 1 W and a maximum power of about 22.2 W.
Derating and design margin
For silicon ICs and power devices, Tj(max) is most often 125 °C or 150 °C, and the allowed power is Pmax = (Tj(max) − Ta) ÷ θ. Because the margin shrinks as the ambient temperature rises, datasheets show a derating curve that lowers the allowed power linearly with ambient temperature. With θJA = 50 °C/W and Tj(max) = 125 °C, for example, the limit is 2.0 W at 25 °C but only 0.8 W at 85 °C. Higher junction temperatures also raise failure rates and shorten lifetime, so real designs normally keep extra margin rather than running right at Tj(max).
AIt is determined solely by the package structure, so the value does not change regardless of the board it is mounted on
BThe larger the copper area on the board, the more heat is trapped, so θJA increases
CIt is measured on a test board, so it varies greatly with the actual board's copper area, number of layers and airflow
Show answer and explanation
Answer: C. It is measured on a test board, so it varies greatly with the actual board's copper area, number of layers and airflow
θJA is measured with the IC mounted on a test board defined by standards such as JEDEC, inside an enclosure with still air. Much of the heat flows from the package through the pins and the exposed pad on the bottom into the board, so θJA depends strongly on the board. Even for the same package, it can differ by nearly a factor of two between a small single-layer board and a multilayer board with many solid planes. Enlarging the copper area or adding airflow makes θJA smaller (easier heat dissipation). Treat the datasheet value as a guide for comparison, and verify by measuring temperature on the actual equipment.
Junction temperature is given by Tj = Ta + P × θJA. Solving for P gives P = (Tj(max) − Ta)/θJA = (125 − 85)/50 = 0.8 W. The higher the ambient temperature, the smaller the allowable temperature rise, and so the smaller the allowable power. 2.5 W is the mistake of forgetting to subtract the ambient temperature (125/50), and 1.7 W is the mistake of computing 85/50. In practice, considering that θJA varies with the board and allowing derating margin, design to keep power below the calculated value.
AThe datasheet θJA (junction-to-ambient) plus the heatsink-to-ambient thermal resistance
BThe sum of θJC, the case-to-heatsink contact thermal resistance and the heatsink-to-ambient thermal resistance
COnly the datasheet θJA (junction-to-ambient); attaching a heatsink does not change the value
Show answer and explanation
Answer: B. The sum of θJC, the case-to-heatsink contact thermal resistance and the heatsink-to-ambient thermal resistance
θJC is the thermal resistance from the junction to the package's heat-dissipating surface (the case), measured with the case held at a constant temperature. With a heatsink attached, most heat flows from the case into the heatsink, so θJC, the contact thermal resistance θCS of thermal grease or similar, and the heatsink thermal resistance θSA are added in series. For example, 1.0 + 0.5 + 3.0 = 4.5 °C/W, so a 10 W loss gives a temperature rise of 45 °C. θJA is measured without a heatsink, with the device mounted on a test board, so it cannot be used in this case.
Silicon power devices typically have a maximum junction temperature of 150 °C or 175 °C. Beyond it, leakage current rises sharply, inviting thermal runaway, and lifetime shortens dramatically, so designs keep margin (operating around 125 °C, for example). SiC itself withstands over 300 °C, but package materials—solder, bond wires and resins—limit practical operation.
FAQ
When do I use θJA and when θJC?
θJA is the junction-to-ambient resistance when the part is cooled by the board and surrounding air without a heat sink. θJC is the junction-to-case resistance, which you add to θCS and θSA when a heat sink is attached. Adding θSA to θJA for a heat-sink design is a mistake.
How much power is allowed with Tj(max) = 125 °C, θJA = 50 °C/W and Ta = 85 °C?
Pmax = (125 − 85) ÷ 50 = 0.8 W. At an ambient temperature of 25 °C the limit would be (125 − 25) ÷ 50 = 2.0 W, so hotter environments allow less power.
Why does the datasheet θJA not match my board?
θJA is measured under fixed conditions such as a JEDEC standard test board, and it changes greatly with board size, copper layers and airflow. In real systems the junction temperature is often estimated from the measured package top temperature using ψJT.