13 July 2017

Revolutionary Approach for Thermal Conductance Improvement

This application note presents the new ISOPLUS247 power package featuring 2500 V internal isolation, which gives a revolutionary approach that improves thermal conductance and reliability. The document also describes the advantages of the isolated and hole-less packaging concept and a brief table and graphical comparisons of some characteristics of the IXFR55N50 to IXFX55N50. The new ISOPLUS247 has an internally isolated mounting tab with a 2500 V (RMS) isolation voltage rating. The package meets the standard JEDEC TO-247 outline and is intended for pressure mounting applications since there is no mounting screw hole. As can be seen in Figure 1, the usual copper lead frame has been replaced by a direct-copper-bonded (DCB) alumina substrate possessing both high thermal conductance and high electrical isolation (2500 V).

 

There are multiple advantages to this isolated and hole-less packaging concept.

1. The dielectric breakdown voltage of the DCB is 6 00 V although the package is only rated to 2500 V (RMS) due to the creep and strike distances of the TO-247 outline;
2. The electrical insulation barrier is plastic encapsulated and therefore protected from the environment
3. Very low, isolated thermal resistance junction-to-heatsink due to the use of DCB and only non-soldered interface between the silicon chip and the heatsink;
4. Ease of assembly due to pressure mounting (no screws) and no additional isolation interface material required for mounting to a heatsink
5. Increased temperature and power cycling capability because the thermal expansion coefficient of the silicon die matches that of the DCB
6. Small chip-to-heatsink stray capacitance for reduced EMI/RFI emissions #
7. The DCB substrate can be etched like a PC board allowing for more circuit configuration than normally found in a non-isolated TO-247, e.g. epitaxial ultra-fast recovery diodes and Schottky connected common anode or in series, multiple diodes in series, MOSFET connected in series with a Schottky diode with or without an antiparallel diode.

 

The primary advantage of ISOPLUS247 packaging is the very low thermal resistance achievable in a rugged, high voltage, isolated mounting system. Figure 2 shows the allowable current handling capabilities of the various mounting conditions.

 

 

Conversely, the chip can run up to 29°C cooler for the same operating conditions that translate into more reliable operation. Because there is such a potentially large decrease in junction-to-case thermal resistance, it may be possible to use a smaller chip for the same current, which would more than cover for the extra price.

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Source : IXYS

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