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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?

Calculating the junction temperature for most semiconductor devices is a process familiar to most of us. Typically, the case or lead temperature is known. The power dissipation of the die is measured and multiplied by the theta for the die to package to calculate the temperature rise from the case to the junction. This method applies to all single die packages including BJTs, MOSFETS, diodes and thyristors. For multiple die IGBTs, however it proves to be insufficient.

Some IGBTs are single die devices, either with a monolithic diode or no diode; however, the majority of them come with a co-packaged diode. Most manufacturers provide a single theta for the junction-to-case thermal resistance calculation. This is a simplified approach to calculating the die temperature and will lead to incorrect analysis of the two junction temperatures involved.

For multiple die devices, the thetas are typically different and the power dissipation of the two dice is also different, requiring a separate calculation for each. In addition, each die provides thermal energy to the other, so the interaction must also be considered.

Figure 1: IGBT & Diode Mounted on Lead Frame of a TO-247 Package.

Power calculations
The voltage and current waveforms must be multiplied and then integrated to measure the power. Although the simple multiplication of the voltage and current will give the instantaneous power, it is not simple to derive the average power from this, so the integral is used to convert this to energy. The sum of the energies of the various losses can then be used to calculate the average power across the waveform.

It is important to define the boundaries of the turn-on, conduction and turn-off losses before starting as it would create errors in the measurement if some areas of the waveform were left out or alternately if some areas were duplicated. For this analysis we will use the 10% points; however, although this is a common method of doing this, other levels such as 5% or 20% can be used, as long as they are applied to all of the components of the losses.

Normally the waveforms are taken at the peak of the sine wave that is being formed. This is the peak power dissipation. The average power is 50% of this value (factored by 2 for the voltage and 2 for the current).

In general, at the peak of the voltage waveform, the IGBTs will be conducting and not the diodes. To measure the diode losses, a reactive load, such as a motor is required and the waveform needs to be captured when the current is in the reactive state i.e. being fed back into the source.

Figure 2: IGBT Turn on Waveforms.

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