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Spurious Turn-On inside a Power Module of Paralleled SiC MOSFETs
Keywords: Paralleled SiC module, crosstalk and spurious turn-on, layout symmetry and parasitics
Silicon carbide (SiC) dice are often paralleled to realize power modules with high-current rating. Package parasitics significantly impact the switching performance of SiC module [Chen][Kim][Li] [Liang][Mantooth][Peftitsis]. Owing to the large network of interconnect (parasitic) impedances, terminal waveforms could appear benign while the dice experience detrimental fault currents generated by spurious cross-turn-on [Heer][Xu][Zhang]. This paper will quantify the nonuniform distribution of current stress and switching energy among the dice, as well as the penalty caused by cross-turn-on, versus layout symmetry, number of dice, gate resistance, input voltage, load current, and free-wheeling diode. A reliable model for the package is essential. This is demonstrated for a commercial power module with 16 dice, 124 layout inductances, and the companion parallel resistances extracted from finite-element simulation. For a module tested at 800 V / 300 A and carrying six paralleled dice per switch, the cross-turn-on induced current spikes simulated without and with package model differ by as much as 11.75 times. Factors that lead to the increased cross-turn-on current will be identified. Asymmetrical and symmetrical modules following commercial layouts are tested by a double pulse tester that switches the low side on and off while maintaining the high side off. Cross-turn-on currents inside a module increase the high-side switching energy and total switching energy by 44% and 20%, respectively, under nominal condition (800 V / 300 A and 2.5 Ω gate resistance). Terminal currents, peak cross-turn-on currents, low-side, high-side, and total switching energies of the asymmetrical module and symmetrical module corresponding to various input voltages (200 V to 800 V), low-side gate resistances (2.5 Ω to 25 Ω), and load currents (60 A to 360 A) are normalized for comparison. Peak cross-turn-on current of symmetrical module is always lower and only 16% of that of asymmetrical module under nominal condition. Symmetrical layout greatly decreases the cross-turn-on currents without increasing the total switching energy. One-Die, two-die, four-die, and six-die modules are modeled on the asymmetrical layout to explore how number of dice influences cross-turn-on and switching energy. Gate resistance and load current of each die of the modules are kept the same. Peak cross-turn-on current and total switching energy which are divided by the number of dice for fair comparison increase with the number of dice. The peak cross-turn-on current and switching energy of the six-die module are 134% and 36% higher than those of the one-die module, respectively. Severity of cross-turn-on soars as number of dice increases. Besides symmetrical layout, paralleling SiC Schottky diode is effective to reduce cross-turn-on current to 48% and total switching energy to 65% of the original values. Decreasing the gate-loop inductance to half, which is difficult in reality due to areas occupied by the dice for bond-wire structure, has little influence on the problem and increasing external gate resistance suppresses cross-turn-on at the expense of higher switching energy.
Zichen Miao, Graduate research assistant
Virginia Tech
Blacksburg, Virginia
USA


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