Method for measuring degradation of thermal resistance between power semiconductor and heat sink, and control device for power semiconductor

A method for measuring a degradation of a thermal resistance RTh between a power semiconductor in a power module assembly and a heat sink of said power module assembly where said power semiconductor has an internal gate resistance, comprising:

• measuring a first initial parameter, K0, related to a power semiconductor junction temperature at a stable operating temperature,
• heating the internal gate resistance through applying an AC voltage VAC of an AC source with a frequency fl on a gate/source junction of said power semiconductor, and measuring a second initial parameter, K1, related to a power semiconductor junction temperature after said heating, and
• saving the first and second initial parameters K0 and K1 in a memory, comprising after at least one discrete time interval dϵ{d1, . . . , dn} during a lifetime of the power module assembly a subsequent process of:
• measuring a first subsequent parameter, K0dϵ{K0d1, . . . , K0dn}, related to the power semiconductor junction temperature at a stable operating temperature T0dϵ{T0d1, . . . . T0dn},
• heating the internal gate resistance through applying the AC voltage VAC with a frequency f1 on the gate/source junction of said power semiconductor, and measuring a second subsequent parameter, K1dϵ{K1d1, . . . , K1dn} related to the power semiconductor junction temperature T1dϵ{T1d1, . . . , T1dn} after said heating,
• calculating a thermal resistance degradation, ΔRTh with the subsequent parameters related to power semiconductor junction temperature K0dϵ{K0d1, . . . , K0dn}, and K1dϵ{K1d1, . . . , K1dn}, and the initial parameters K0 and K1 as read from the memory, where
• Δ
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• =
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• where
• R
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• -
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• where
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• ,
• where d=d1, . . . , dn and where PR is the power dissipated by the internal gate resistance under said AC voltage VAC,
• comparing ΔRTh with a limit value ΔRThmax, and
• raising a fault flag in case ΔRTh is above said limit value or repeating said subsequent process in case ΔRTh remains under said limit value.