902 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 4, JULY 2004 A Thermal Model for Insulated Gate Bipolar Transistor Module Zhaohui Luo, Hyungkeun Ahn, and Mahmoud A. El Nokali, Senior Member, IEEE Abstract—A thermal resistor–capacitor ( ) model is intro- the thermal network can be extracted from the thermal dy- duced for the power insulated gate bipolar transistor (IGBT) namic curve [7]. modules used in a three-phase inverter. The parameters of the The manufacturer of IGBT module provides the transient model are extracted from the experimental data for the transient thermal impedance curve of junction to case to the users. In thermal impedance from-junction-to-case j and case-to-am- bient . The accuracy of the model is verified by comparing a real application environment, the IGBT module is mounted its predictions with those resulting from the three–dimensional on a heatsink in order to keep the device temperature in the finite element method simulation. The parameter extraction safe operation area. Natural air, forced air or water-cooling [8] algorithm is easy to adapt to other types of power modules in an are typical methods of cooling used. The thermal behavior of a industrial application environment. system is therefore determined by the thermal impedance of the Index Terms—Finite element method (FEM), insulated gate IGBT from junction to case, the thermal impedance of the inter- bipolar transistor (IGBT), thermal resistor–capacitor ( ) face (thermal grease, etc.) between the IGBT and the heatsink network, transient thermal impedance. in addition to the heatsink thermal behavior. If we assume the system to be linear and use a one-dimensional formulation, I. INTRODUCTION we can extend the thermal network from junction-to-case OMMERCIALLY available since 1988, the insulated gate to a network from junction-to-ambient. However, since the C bipolar transistors (IGBTs) are widely used in today’s junction temperature is not easy to measure in an actual power conversion systems for high switching frequency and system, the user is unable to produce the thermal impedance medium power ranges [1]. The IGBT combines the advantages curve from junction to ambient directly from experiment. An of high current density associated with bipolar operation alternative approach would collect thermal measurement data with fast switching and low drive power of metal oxide for the module case which is accessible and easy to obtain semiconductor (MOS) gated devices. Additional advantages and combine it with the thermal impedance data to extract include low steady-state losses, very low switching losses, the network parameters for the whole system. With this high short-circuit capability, and the easiness of connecting thermal network, we can predict the junction temperature the devices in parallel. As the power density and switching of the IGBT in a real time application. This method is verified frequency increase, thermal analysis of power electronics by the 3-D FEM simulation results. system becomes imperative. The analysis provides valuable information on the semiconductor rating, long-term reliability, II. TRANSIENT THERMAL IMPEDANCE FROM and efficient heatsink design. JUNCTION-TO-CASE Thermal resistor–capacitor ( ) networks are widely used The term IGBT module used in this work refers to what the for thermal analysis because they are easy to integrate into ex- manufacturer labels as single module. The module used in this isting circuit simulators, like SPICE or SABER making them paper has a rating of 1200 A/1700 V. The module contains capable of simulating both the electrical and thermal character- IGBTs and freewheeling diodes and is used as a power switch istics of systems. The thermal model is flexible and can be in various applications. This is different from an inverter that used to describe one-dimensional (1-D) [2], two-dimensional would normally be built using six of these modules. (2-D) [3], or three-dimensional (3-D) [4] problems. The model Usually the manufacturer of power semiconductor devices can be built through the discretization of the thermal conduction will provide the user with the transient thermal impedance equation by using either finite difference [2] or finite element curve. Fig. 1 depicts the transient thermal impedance data for method (FEM)[5]. The two approaches are analyzed and their both diodes and IGBTs inside the module. In this work, we accuracies are compared in [6]. Alternatively, the elements of have not considered the thermal coupling between the diodes and the IGBTs. In other words, only the IGBTs were powered to obtain the thermal impedance data that were then used to Manuscript received October 18, 2003; revised February 7, 2004. Recom- extract the network model for the IGBT chips. The same mended by Associate Editor M. C. Shaw. Z. Luo and M. A. El Nokali are with the Department of Electrical En- concept applies to the extraction of the thermal network gineering, University of Pittsburgh, Pittsburgh, PA 15261 USA (e-mail: model for the diode chips when only the diodes are powered. [email protected]). In order to understand the definition, derivation, assumption H. Ahn is with the Department of Electrical Engineering, Konkuk University, Seoul, Korea. and application of the curve, the measurement process is Digital Object Identifier 10.1109/TPEL.2004.830089 introduced. 0885-8993/04$20.00 © 2004 IEEE LUO et al.: THERMAL MODEL FOR INSULATED GATE BIPOLAR TRANSISTOR MODULE 903 Fig. 2. Thermal g network from junction to case. TABLE II PARAMETERS OF THE RC NETWORK OF FIG.2 In order to derive the parameters of the thermal network, (3) needs to be expressed in the following form: Fig. 1. Experimental transient thermal impedance from junction to case of an IGBT module (1200 A/1700 V). (4) TABLE I EXPONENTIAL TERMS EXTRACTED FROM THE TRANSIENT THERMAL IMPEDANCE CURVE IN FIG.1FOR IGBT CHIP This can be achieved by using a continuous-fraction expan- sion (denominator divided by numerator continuously). Four or five exponential terms are enough to curve-fit with enough accuracy for the intended application. The number of exponential terms determines the number of rings in the thermal network. By using the method outlined above, the model de- scribing the thermal behavior of a power module mounted on By using a thermal control system [10], the temperature of an ideal heatsink is derived. Fig. 2 shows the resulting the power module case, , is set at a fixed value (such as the thermal network for an IGBT module from junction to case. ambient temperature). A single square power pulse with ampli- Table II lists the value of the RC parameters. It is instructive tude is applied to the module until the junction temperature to note that only ideal or nearly ideal heatsink can keep the reaches its steady state. The module junction temperature case temperature constant while applying power through the is measured at different instances by way of thermal imaging module. or by using temperature-sensitive thermometers. The transient thermal impedance is defined at time as III. TRANSIENT THERMAL IMPEDANCE FROM CASE-TO-AMBIENT (1) When an IGBT module is mounted on an ideal heatsink, the thermal network parameters can be extracted from the transient thermal impedance from junction to case as outlined From a network perspective, the transient thermal impedance above. When the IGBT module is mounted on a nonideal curve is equivalent to a step response with zero-initial conditions heatsink which is more realistic to expect, then knowing the and therefore it fully describes the system under consideration. transient thermal impedance from junction to ambient can The experimental transient thermal impedance is then fitted into lead to the derivation of the network parameters that describe a series that consists of a finite number of exponential terms the thermal behavior of the module. However, as mentioned before, the thermal impedance curve from junction to ambient (2) cannot be obtained directly from experiment since the junction temperature of the device is not easy to measure. The case temperature of the module however can be measured in real where , , and are specified by the manufacturer. For the application such as a three-phase inverter. Fig. 3 shows the curve in Fig. 1, the values of these parameters are listed in system schematics of a three-phase inverter using single IGBT Table I [9]. modules as power switches. Eight single IGBT modules are The transfer function (input impedance) of the thermal mounted on the heatsink which is made of aluminum. In this network is found by applying Laplace transform to (2) application, six modules are used to drive an ac motor and other two modules are for the braking process. The thermal (3) impedance data is measured by turning ON six IGBT modules and connecting them in series. Given that the current through 904 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 4, JULY 2004 Fig. 3. Schematics of IGBT modules used in a three-phase inverter. Fig. 4. Thermal g network with extension to include heatsink. Fig. 5. Experimental transient thermal impedance from case to ambient of IGBT chips with diodes unpowered. the circuit is , and assuming that the voltage drop across each module is , then the power losses in one module is given by (5) After being subjected to this constant power losses for a duration , the increase in the case temperature above the am- bient temperature is measured as . The thermal impedance at time is then equal to Fig. 6. Thermal g network including heatsink. (6) TABLE III PARAMETERS OF THE g NETWORK OF FIG.6 The case temperature is measured every 1 second until it reaches its steady state. Based on the experimental data col- lected, we can extend the thermal network that appears in Fig. 2 to include the heatsink, the interface between the device and the heatsink, and the cooling method used in a real system.