Simulating Power Losses in an IGBT Module
As a result of the increasing use of power electronics applications, accounting accurately for temperature is becoming increasingly important. Thermal analysis of switch-mode power supply allows studying the effect of power dissipation and operating temperature on the response of the device. This characterization is important as it affects the expected lifetime of power switching devices.
When an IGBT conducts current, there is a non-zero voltage drop across it. This results in losses that are converted almost entirely into heat. Consider the simplified structure of a typical use case of an IGBT silicon chip and a diode silicon chip, mounted to a case that is mounted to a heat sink.
For both the IGBT and diode, the heat power originates in the junction, where its value is the highest. The instantaneous value of power is equal to the resistance \(I \cdot V\) of the IGBT or diode. The heat flows through the thermal impedance of the structure and dissipates in the ambient environment. The lower the thermal impedance, the lower the rise of the silicon temperature above ambient is.
In Caspoc a the thermal model is a trade-off between speed and accuracy. A commonly-applied process is the modeling of heat flow only along one dimension. Important is the modeling of the thermal path using lumped thermal impedances at the interface of any two surfaces, such as the junction-to-case interface.
With these modeling method, the thermal process can be represented and solved using thermal circuit elements. The power, as it flows through the structure in the form of heat, is represented as flow. The \(I \cdot V\) products can therefore be represented using flow sources. The temperature, as it exists in various physical points along the thermal network, is represented as a temperature. The ambient environment is represented as a temperature source. And the thermal impedances are represented using thermal elements.
There are several IGBT modules in Caspoc that can be used to estimate the losses and thermal behavior. This tutorial will show you how to simulate the losses in the converter/inverter. If we include the control and load of hte power electronics inverters and with the help of parameters in the datasheet, we can simulate the total power loss of the module and hence the junction temperatures.
Introduction
The power loss in an IGBT consists of conduction loss Pcond and switching loss Psw. The switching loss in the IGBT is given by Esw=Eon+Eoff whereas, in the Diode it is given by the reverse recovery loss Erec. All these switching energies can be added together multiplied by the switching frequency to give the total module switching losses. This tutorial describes the theory behind the model and shows how to simulate the power losses for the IGBT and Diode and the junction temperatures respectively.
Types of Power Loss
In an IGBT module there are several IGBT and diode chips depending on the module and requirements of the application. All chips dissipate power when they are conducting or switching from one state to another.
Here we will only look at the IGBT.
\[\theta = pp \cdot \theta_m\]Operating junction temperature
The parameter Tj is extremely important for the design. Although the device will not fail immediately once the limit is exceeded, the maximum junction temperature should never exceed its maximum rating. This will lead to device degradation and reduced lifetime.
In an application with a given thermal setup, a device with higher specified maximum junction temperatures could achieve longer life times in comparison to conventional IGBTs with lower specified temperature rating. In other words, customers are able to drive higher current out of the same power system, corresponding to higher power density.
Thermal resistance Rth(j-c)
The thermal resistance Rth(j-c) characterizes the thermal behavior of power semiconductors at steady state. Correspondingly, the thermal impedance Zth(j-c) describes the devices thermal behavior during transient pulses.
| IGBT Thermal resistance, junction-case | Rth(j-c) | 0.5 | Celsius/Watt |
| Diode Thermal resistance, junction-case | Rth(j-c) | 1 | Celsius/Watt |
The IGBT/diode case should be considered as the leadframe of device. In case of a Module, the central pin should be considered as the case. The maximum value stated in the datasheet takes the tolerance during mass production into consideration. It is the value to be used for the simulation
The thermal resistance junction to case Rth(j-c) is a key parameter to determine the thermal behavior of semiconductor devices. However in any design, it is not enough to compare this value directly from one product to another. In the thermal dissipation path of a power system, the thermal resistance junction to ambient Rth(j-a) plays the most important role, as it dictates the thermal limits in operating conditions. It consists of a resistance case to ambient Rth(j-h) + Rth(j-a) and the resistance from junction to case Rth(j-c). In most cases, the Rth of the thermal interface material, isolation - if applicable - and heatsink is dominating the Rth(j-a).
For a typical inverter IGBT module, the Rth(j-c) is 0.5 K/W. The thermal resistance value of typical thermal interface material (TIM) and isolation like isolation foil could be as low as 1 K/W and the thermal resistance heatsink to ambient could range anywhere from 1 K/W with forced ventilation to tens of K/W without ventilation. Therefore, the Rth(j-c) impact is only in the order of some single digit percent to some tens of percent compared to the total Rth(j-a).
However, the temperature rise on the junction is much faster than the temperature rise on the heatsink. Therefore the temperature on the junction will rise very fast compared to the heatsink temperature and can destroy or limit the lifetime of the semiconductor.
Therefore a simulation can predict the temperature swing at the semiconductor junction during operation and detailed parameters from the module are required during the simulation.
