Thermal resistance is a key parameter for evaluating the heat dissipation performance of IGBT modules. It is commonly represented by symbols Rth or Rθ and is measured in°C/W. It reflects the resistance encountered during heat transfer, similar to the resistance that an electric current encounters in a conductor. A proper understanding of these thermal resistance parameters is of great significance for the heat dissipation design of IGBT modules, the improvement of reliability, and system optimization.
1.Definition and Explanation of Thermal Resistance Parameters
- RθJC (Junction to Case): Junction to Case thermal resistance, referring to the thermal resistance between the chip junction temperature and the module case surface.
- RθCS (Case to Sink): Case to Sink thermal resistance, referring to the thermal resistance between the module case and the heat sink contact surface.
- RθJA (Junction to Ambient): Junction to Ambient thermal resistance, referring to the total thermal resistance from the chip junction to the surrounding environment.
- RθJCD (Diode Junction to Case): Body Diode Junction to Case thermal resistance, usually the same as RθJC.
- RθCA (Sink to Ambient): Sink to Ambient thermal resistance, reflecting the heat dissipation capacity of the heat sink itself.
Among these parameters,
- JC represents "Junction-Case"
- CS represents "Case-Sink"
- JA represents "Junction-Ambient"
- CA represents "Sink-Ambient"
θ is sometimes replaced by "th", for example, RθJCD can also be written as RthJCD.
2.Physical Meaning and Heat Dissipation Path of Thermal Resistance
The heat generated by the IGBT chip needs to pass through the chip itself, the soldering layer, the module case (TAB or bare pads), the heat sink, etc., and finally be dissipated into the environment. The thermal resistance of metal materials (such as the module case and heat sink) is relatively small, and the main thermal resistance comes from the contact interfaces between materials, such as the contact surfaces between the chip and the substrate, the substrate and the heat sink, and the interface between the heat sink and the air.
In practice, improving the material of the heat sink has certain effects, but optimizing the interface thermal conduction (such as using high-quality thermal conductive silicone) often yields more significant results. Due to the non-flatness of metal surfaces at the microscopic level, the filling of the thermal medium is very crucial. Forced air cooling, water cooling, etc., are mainly used to reduce RθCA.
3.Thermal Resistance Relationship and Ohm's Law
The relationship between thermal resistances can be expressed as:
RθJA = RθJC + RθCS + RθCA
In terms of numerical values, usually RθJC < RθCS < RθCA, but the impact on the temperature rise of the device is exactly the opposite: RθJC has the greatest impact, followed by RθCS.
The relationship between thermal resistance and temperature rise and power conforms to the thermal Ohm's law:
Where, ΔT is the temperature rise (℃), and P_D is the power loss (W). This formula indicates that the temperature rise caused by a unit of power is directly determined by the thermal resistance.
4.Application of Thermal Resistance in Heat Dissipation Design
Manufacturers usually provide RθJC, RθCS and RθJA, while RθCA is mostly provided by the radiator supplier. The selection of the radiator needs to be calculated comprehensively based on the ambient temperature (TA), the required heat dissipation capacity (PS), and its thermal resistance value. The theoretical design should meet:
SHYSEMI reminds you: In actual design, safety margins must be retained. The safety factor is usually set between 0.5 and 0.8, and the heat dissipation effect must be verified through experiments.
For portable devices or large-scale products, optimizing the heat dissipation design can significantly reduce volume, weight and cost. For small-batch products, empirical design can be used, but it still needs to be verified in combination with actual conditions.
5.Summary
Thermal resistance is the core parameter of IGBT module thermal management. Understanding the meanings and relationships of each thermal resistance, reasonably selecting heat dissipation interface materials and heat sinks, and combining theoretical calculations and measurements can ensure that the IGBT module operates stably and reliably under high-power conditions.