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Technology Now

Introducing Mitsubishi Electric Power Device Technologies and Product Trends

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2-5: Technological development of diodes evolving together with IGBT chips

Diode technology development has also progressed together with the evolution of IGBT chips from the 5th to 6th generation. Our target is to achieve a positive VF temperature coefficient and further reduce the ON voltage by making the backside surface of the cathode portion thinner.

Thin-wafer Diode

• Thickness reduced to 100-and-several-tens of micrometers, less than half compared to the 250-300µm of existing cathodes.

VF - IF Characteristics

By reducing the thickness of the wafer and optimizing the impurity concentration, the cross point (Forward voltage (VF) - Forward current (IF) curve at Tj=125°C and VF - IF curve at Tj=25°C intersect) is set slightly lower than the rated electrical current. Therefore, when the diode is used in an overload situation, the positive temperature coefficient prevents constriction of the electrical current, realizing a lower VF and a positive temperature coefficient.

Another major issue with diodes is that they become a source of noise because voltage oscillation is generated during reverse recovery. The characteristics of the diode are such that during reverse recovery current oscillations occur under the conditions of low-temperature, high-voltage and low electrical current. However, the diode that will be used paired with the 6th-generation IGBT chip is designed so that voltage oscillation is controlled through impurity concentration optimization.

Recovery Characteristics

• Verification using 1200V/150A diode chips. Recovery waveform with switching performed at a low electrical current of 23A/cm², approximately 1/15 of the rated current (300A).

Improvement in Diode Characteristic Trade-off

• The relationship between ON voltage and recovery loss. The lower ON voltage increases the reverse recovery electrical field. However, the 6th-generation chip shows an improvement of approximately 35% compared to the previous chip.

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