Experimental evaluation and modeling of the therminal behavior of a TO-247 Mosfet

Abstract

In this thesis, analyticalmodeling of aMOSFET as well as investigation of its gate drive was performed. Firstly, the performance of the gate drive circuit during switching mode was tested. In order to do so, four simple case set-ups consisting of RC circuit were switched by the gate drive. For the first case a large resistor and small capacitor was selected (C = 2.2nF, R = 4.7 ). For the second case a small resistor and small capacitor were selected (C = 2.2nF, R = 1 ). For the third case a large resistor and large capacitor were selected (C = 10nF, R = 4.7 ). For the fourth case a small resistor and large capacitor were selected (C = 10nF, R = 1 ). The results in these four cases were in agreement with the results from the simulation test set-ups. The next step was modelling a MOSFET thermally using PLECS/Simulink. The analogy between the electrical and thermal components was used to obtain thermal equivalent circuit diagrams. The sources of power dissipation which was generated by the device was expressed as P = I ∗ VTJ which correspond to current sources, and RC elements were used to represent the thermal impedances Zth of the MOSFET which could be obtained from the manufacture datasheet. The energy losses of all internal components in the module would be dissipated through the thermal impedance Zth. The power dissipated in theMOSFET in DC operationmode, which was generated by conduction losses (Pc), was calculated by multiplying the square of the root mean value of the drain current by the on-state resistor. In this experiment, the first power supply was used to control the drain current through the MOSFET while the second power supply was used to keep theMOSFET on by supplying a DC source at the gate pin. The power dissipation and the ambient temperature was measured for each value of the drain current. The temperature was measured by using a resistor sensor. The tip of one thermocouple was attached to the MOSFET while the other thermocouple was positioned approximately 10 cm away from the device to measure the ambient temperature in the vicinity of the MOSFET device. The temperature measurement circuit was designed and then the PCB was built and analyzed in the simulation software Target. After that the PCB was ordered. Three case set-ups were studied. For the first case the MOSFET without heat sink was tested. The case temperature was measured and compared with the simulation results. For the second and third tests the MOSFET was screwed onto a small and big heat sink respectively. In the third and fourth test a thermal isolation pad with thermal resistance of 3 ◦C W was put between theMOSFET and heat sink. The case temperature, heat sink temperature, and ambient temperature were measured. The measurement results were in agreement with the results from the simulation test setups.