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Optimise power designs with IGBT thermal calculations

Posted: 02 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:junction temperature? IGBT? waveforms? power dissipation? single die?

The charts for the IGBT and diode thetas are included in the datasheets for ON Semiconductor devices. The steady-state thetas are given on the charts shown in figures 7 and 8. These are 0.470 C/W for the IGBT and 1.06 C/W for the diode. There is one other thermal coefficient required for the calculations, and that is the Psi which is the thermal interaction constant between the two dice. Testing has shown that for the TO-247, TO-220 and similar packages, it is about 0.15 C/W, which is what will be used in the following example.

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Figure 7: IGBT Transient Thermal Impedance./i>

Figure 8: IGBT Transient Thermal Impedance.

The die temperature for the IGBT can be calculated from the following equation:

TJ-IGBT = (PIGBT RIGBT) + (PDIODE Psi ) + TCASE

Assuming the following conditions:

TC = 82C
RJC-IGBT = 0.470C/W
PD-IGBT = 65 W
PD-DIODE = 35 W
Psi-interaction = 0.15C/W
The temperature for the IGBT die is:
TJ-IGBT = (65 W 0.470C/W)+(35 W 0.15C/W ) + 82C
TJ-IGBT = 118C
Diode Die Temperture
RJC-diode = 1.06C/W
TJ-DIODE = (PDIODE RDIODE) + (PIGBT Psi) + TCASE

Likewise, the temperature for the diode die is:

TJ-DIODE = (35 W 1.06C/W ) + (65 W 0.15C/W ) + 82C
TJ-DIODE = 129C

Peak die temperatures
The temperatures calculated in the above analysis are for the average die temperature. This varies throughout the cycle and the peak die temperatures can be calculated by using the thermal transient curves in Figures 7 and 8. For this, it is necessary to read the transient information from the curves. If the line frequency is 60Hz, one half cycle would have a period of 8.3 ms. So, using the 50% duty cycle curve for an 8.3 ms period, the Psi values are:

IGBT 0.36 C/W
Diode 0.70C/W
TJpk-IGBT = TJ-IGBT + (PIGBT RIGBT)
The peak temperature for the IGBT die would be:
TJpk-IGBT =118C+ (65 W 0.36C/W )
TJpk-IGBT = 141C
And the peak diode die temperature would be:
TJpk-DIODE = TJ-DIODE + (PDIODE RDIODE)
TJpk-DIODE =129C+ (35 W 0.70C/W )
TJpk-DIODE =154C

Conclusion
Evaluating the temperature of the semiconductor dice in a multi-die package requires additional analytical techniques compared to those applicable for single die devices. It is necessary to have both DC and transient thermal information available from both dice to accurately calculate the die temperatures.

It is also necessary to measure the power dissipation in both devices and analyse those losses across the entire half sine waveform. This analysis will add confidence that the semiconductors in the system will be operating at a safe and reliable temperature for optimal system performance.

About the author
Alan Ball contributed this article.

To download the PDF version of this article, click here.


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