Thermal Design of IGBT Modules
Due to the inherent power consumption of IGBT modules, they generate heat during operation. Under certain thermal dissipation conditions of the module casing, power devices will experience a temperature rise (i.e., the difference between case temperature and ambient temperature).
The surface area of the IGBT module casing directly affects the temperature rise. A rough estimation of the temperature rise can be made using the following formula:
Temperature Rise = Thermal Resistance × Power Device Power Consumption
For a blackened copper casing like P25xxx (used in the SMP-1250 series products), the thermal resistance coefficient is approximately 3.76°C/W.
This temperature rise and coefficient are based on tests where the power device is mounted vertically with a 1cm clearance below it, allowing natural air convection.
In high-temperature environments, derating of the IGBT module is necessary to reduce its power consumption, thereby lowering temperature rise and ensuring the case temperature does not exceed its maximum allowable limit.
For higher-power devices, appropriate heat sinks must be added to reduce the temperature rise of the power device.
Different heat sinks have different thermal resistance to ambient air under natural convection. The main factor affecting heat sink thermal resistance is its surface area. Also, considering air convection, finned heat sinks should be oriented to not obstruct natural airflow.
Heat Generation and Management in Power Devices
All power devices generate heat due to internal power loss during operation. In every application, it’s necessary to limit this self-heating so that the case temperature of the power device does not exceed its specified maximum.
Most power device manufacturers use power density as a benchmark to measure the effectiveness of their products. Power density is usually expressed in watts per cubic inch (W/in³).
Understanding the conditions under which power density is defined is critical.
If users cannot operate the power device within the specified maximum ambient temperature range, the maximum output power stated in the datasheet may not be achieved. The available average output power of the power device represents the available power density, which depends on the following factors:
Required Output Power: This is the maximum average power needed by the application.
Thermal Impedance: Defined as the temperature rise caused by power dissipation, usually measured in °C/W.
Maximum Case Operating Temperature: Every power device specifies a maximum case operating temperature, which is the highest temperature its internal components can endure. For reliability, the device should operate below this temperature.
Ambient Operating Temperature: This refers to the worst-case ambient temperature during device operation. If the heat generated is too high and cannot be dissipated in time, the power device may fail due to exceeding its guaranteed operating temperature. Therefore, selecting an appropriate heat sink is critical for reliable component operation.
Key Parameters for Thermal Design of Power Devices
Operating Junction Temperature (Tj): The maximum allowable junction temperature during operation. This parameter is provided by the manufacturer or mandated by product standards.
Power Loss (Pz): The average steady-state power dissipation generated by the device during operation, defined as the product of the RMS output current and the RMS voltage drop.
Dissipated Power (Q): Refers to the heat dissipation capability of a specific cooling structure.
Thermal Resistance (R): Represents the temperature rise per unit of power dissipation during heat transfer between media.