In this article, we will share the calculation methods for several key temperature parameters in MOSFETs: TJ (junction temperature), TA (ambient temperature), and TC (case temperature).
1. Importance of MOSFET Temperature Parameters
In power electronics applications, temperature is a critical factor affecting the performance and lifespan of MOSFETs. Excessively high temperatures can lead to degraded device performance, or even damage. Therefore, understanding and calculating these temperature parameters is crucial to ensuring the stable operation of MOSFET devices.
2. Definitions of Temperature Parameters: TJ, TA, TC
a. TJ (Junction Temperature): Refers to the temperature of the PN junction inside the MOSFET chip. It is the maximum temperature limit that the MOSFET can withstand during operation; exceeding this temperature may cause degraded performance, damage, or even failure of the device.
b. TA (Ambient Temperature): Refers to the temperature of the surrounding environment where the MOSFET is located.
c. TC (Case Temperature): Refers to the temperature of the surface of the MOSFET's case.
Calculating junction temperature requires the use of thermal resistance parameters, which are introduced below.
3. Definition and Calculation of Thermal Resistance
Thermal resistance (Rθ) is a parameter that measures the difficulty of heat transfer.
a. Junction-to-Case Thermal Resistance (RθJC): Represents the thermal resistance from the MOSFET's junction to its case.
b. Case-to-Ambient Thermal Resistance (RθCA): Represents the thermal resistance from the MOSFET's case to the surrounding environment.
c. Junction-to-Ambient Thermal Resistance (RθJA): RθJA = RθJC + RθCA.
MOSFETs typically provide thermal resistance parameters such as junction-to-case (RθJC) and junction-to-ambient (RθJA). These thermal resistance values can be obtained from the data sheet.
4. Relationship Between TJ, TA, and TC
TJ (Junction Temperature) = TC (Case Temperature) + Power Loss × Junction-to-Case Thermal Resistance (RθJC); (Formula 1)
TC (Case Temperature) = TA (Ambient Temperature) + Power Loss × Case-to-Ambient Thermal Resistance (RθCA); (Formula 2)
Substituting Formula 2 into Formula 1, we get:
TJ (Junction Temperature) = TA (Ambient Temperature) + Power Loss × (Junction-to-Case Thermal Resistance (RθJC) + Case-to-Ambient Thermal Resistance (RθCA))
Among them, power loss (Pd) mainly consists of conduction loss and switching loss.
Conduction loss = I2 × Rds(on) (where I is the conduction current, and Rds(on) is the on-resistance).
The calculation of switching loss is more complex, as it usually needs to consider factors such as switching frequency and drive voltage. It may also require reference to the formulas or curves provided in the MOSFET's data sheet.
5. Examples of Temperature Calculation
The following are practical examples of calculating MOSFET temperature parameters:
Example 1:
A MOSFET has an on-resistance RDS(on) of 0.1Ω, a conduction current Id of 10A, a junction-to-ambient thermal resistance RθJA of 50°C/W, and an ambient temperature TA of 25°C.
First, calculate the power loss: P = Id2 × RDS(on) = 102 × 0.1 = 10W.
Then, calculate the junction temperature: TJ = TA + P × RθJA = 25 + 10 × 50 = 525°C.
Example 2:
Another MOSFET has an on-resistance RDS(on) of 0.05Ω, a conduction current Id of 5A, a junction-to-case thermal resistance RθJC of 2°C/W, a case-to-ambient thermal resistance RθCA of 30°C/W, and an ambient temperature TA of 20°C.
First, calculate the conduction loss: P = Id2 × RDS(on) = 52 × 0.05 = 1.25W.
Since thermal resistances are in series, the total thermal resistance RθJA = RθJC + RθCA = 2 + 30 = 32°C/W.
Junction temperature TJ = TA + P × RθJA = 20 + 1.25 × 32 = 60°C.
Example 3:
A MOSFET in a high-frequency switching application has a switching loss of 5W and a conduction loss of 3W, a junction-to-ambient thermal resistance RθJA of 60°C/W, and an ambient temperature TA of 30°C.
Total power loss P = 5 + 3 = 8W.
Junction temperature TJ = TA + P × RθJA = 30 + 8 × 60 = 510°C.
6. Conclusion
From the above calculations, we can see that the junction temperature of a MOSFET can reach very high levels. Generally speaking, the maximum junction temperature that a MOSFET can withstand is limited. During design and use, it is necessary to ensure that the junction temperature does not exceed this limit. Therefore, designing an appropriate heat dissipation scheme and monitoring the temperature are crucial. As engineers from Shanghai Leimou Electronics, we are always committed to providing high-performance MOSFET devices and offering customers accurate parameter calculation guidance to ensure the long-term stable operation of the devices.
Please note that the calculations in this article are for illustrative purposes only. In practical applications, calculations should be performed based on specific device parameters and operating conditions. The device data sheets and technical support provided by Jinan Lujing Semiconductor will help you more accurately calculate and evaluate temperature parameters.
