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The invention regards a method for estimating the temperature of a semiconductor chip accommodated in a power semiconductor device in operation, such as an IGBT power module, the method comprising the steps of; while the power semiconductor device is in operation determining a voltage drop over the

The invention regards a method for estimating the temperature of a semiconductor chip accommodated in a power semiconductor device in operation, such as an IGBT power module, the method comprising the steps of; while the power semiconductor device is in operation determining a voltage drop over the power semiconductor device for a value of applied load current and; estimating the temperature of the semiconductor chip by evaluating the relationship between the determined voltage drop and the value of applied load current on the basis of a semiconductor chip temperature model. The invention also regards the corresponding apparatus for estimating the temperature of a semiconductor chip, as well as a wind turbine comprising such an apparatus.

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1. Method for estimating the temperature of a semiconductor chip accommodated in a power semiconductor device in operation, the method comprising the steps of while the power semiconductor device is in operation, by means of a measuring circuit, determining a voltage drop over the semiconductor chip

1. Method for estimating the temperature of a semiconductor chip accommodated in a power semiconductor device in operation, the method comprising the steps of while the power semiconductor device is in operation, by means of a measuring circuit, determining a voltage drop over the semiconductor chip for a value of applied load current andestimating the temperature of the semiconductor chip by evaluating the relationship between the determined voltage drop and the value of applied load current on the basis of a temperature model,wherein the temperature model comprises parameters for the relationship between the determined voltage drop and the determined applied load current, established byestablishing a temperature of the power semiconductor device when the power semiconductor device is not in operation, anddetermining a voltage drop over the semiconductor chip for at least one predetermined current applied,wherein at least one parameter of the temperature model is established a plurality of times during the lifetime of the power semiconductor device. 2. Method according to claim 1, wherein the chip temperature model describes an ideal diode, a thermistor or a combination of an ideal diode and a thermistor. 3. Method according to claim 1, wherein the chip temperature model is at least partly based on data established by tests. 4. Method according to claim 1, wherein the estimated temperature is used as basis for determining a derating factor for the load current handled by the power semiconductor device. 5. Method according to claim 1, wherein establishing the temperature is performed by determining the temperature of the semiconductor chip accommodated in the power semiconductor device and/or performed by heating the power semiconductor device to a predetermined temperature, and/or performed when the power semiconductor device is not in operation. 6. Method according to claim 1, wherein the power semiconductor device comprises one or more thermal stack, and wherein the power semiconductor device has not been in operation for at least five times a thermal time constant of the thermal stack, when the parameters of said semiconductor chip temperature model is established. 7. Method according to claim 1, wherein at least one predetermined current is applied for less time than 10% of the thermal time constant for the chip. 8. Method according to claim 1, wherein said power semiconductor device is an IGBT power module. 9. Method according to claim 3 wherein said tests are accelerated tests. 10. Apparatus for estimating the temperature of a semiconductor chip accommodated in a power semiconductor device in operation, comprising: means for determining a voltage drop over the semiconductor chip for a value of applied current load while the power semiconductor device is in operation, andmeans for estimating the temperature of the semiconductor chip by evaluating the relationship between the determined voltage drop and the value of applied load current on the basis of a temperature model, wherein the temperature model comprises parameters for the relationship between the determined voltage drop and the determined applied load current, and the apparatus comprises a system for establishing the parameters of the temperature model, comprising: means for establishing the temperature of the power semiconductor device while the power semiconductor device is not in operation, andmeans for determining the voltage drop over the semiconductor chip for at least one predetermined current applied, where the current is applied when the power semiconductor device is not in operation, wherein the system establishes at least one parameter of said temperature model a plurality of times during the life time of the power semiconductor device based on the relationship between the determined voltage drop, the determined applied current and the established temperature. 11. Apparatus according to claim 10, wherein the means for determining the voltage drop comprises a measuring circuit, which measuring circuit comprises a low power semiconductor device and at least one gate drive for the low power semiconductor device, wherein at least part of the circuit for determination of the voltage drop over the semiconductor chip is integrated in the gate drive. 12. Apparatus according to claim 10, wherein the means for establishing of the temperature heats the power semiconductor device to a predetermined temperature. 13. Wind turbine, comprising a power semiconductor device and an apparatus according to claim 10. 14. Wind turbine according to claim 13, comprising a liquid system in thermodynamic contact with the power semiconductor device, wherein the liquid system is configured for controlling the temperature of liquid in the liquid system. 15. Wind turbine according to claim 14, comprising a wind turbine control system operationally connected to a power converter, comprising the power semiconductor device and where the liquid system is operationally connected to the power converter. 16. Wind turbine according to claim 14, wherein the wind turbine control system is operationally connected to the liquid system. 17. Apparatus according to claim 10, wherein said power semiconductor device is an IGBT power module. 18. Apparatus according to claim 12, wherein said heating of the power semiconductor device to a predetermined temperature is configured to be provided by use of a liquid in thermodynamic contact with the power semiconductor device. 19. Wind turbine according to claim 14 comprising a heater suitable to heat the liquid to the desired temperature.