Resonance Mechanism in Power Electronic Products for Automobiles and its Relationship to EMC Performance

Research Objective. This chapter explains the resonance mechanism in automotive power electronic products and how resonance affects EMC performance. Power electronic products consist of a printed circuit board (PCB) and some connecting parts, such as bus bars, all enclosed in a metal case. The PCB pattern and connection parts have self-inductance (L). Stray capacitance (C) is formed between the PCB and metal case. These L and C form an unintended resonance circuit that lowers EMC performance. This chapter describes the resonance mechanism in motor controllers, and how it affects both immunity and emission tests. Methodology. A product controlling motor rotation speed was tested at 1–400 MHz based on bulk current injection (BCI) test standards. The frequency at which the abnormality occurred and the motor rotation fluctuation were recorded. Next, the resonance frequency was measured at the connector using a network analyzer. Comparing the BCI test result with the measured resonance frequency showed a relationship. Then, by measuring the magnetic field of the PCB surface, the resonance location was identified. It was assumed that the PCB pattern, bus bar, and metal case formed an unintended resonance circuit. Therefore, we modified the PCB pattern shape to change the stray capacitance between the PCB and case, confirming changes in the resonance frequency and BCI result. Finally, a dumping resister was added to suppress the resonance and its effect was confirmed. In addition to BCI, we investigated conducted emissions and confirmed the relationship between resonance and emitted noise from a product. Results. The investigated product was abnormal at 50–58 MHz on the BCI test, and the measured resonance frequency matched at 56 MHz. Meanwhile, the resonance location measured magnetically was around the PCB ground pattern and bus bars. The parts’ self-inductance was estimated at 70 nH. Similarly, the stray capacitance between the PCB and case was estimated at 100 pF, indicating a resonance of around 60 MHz. Supporting this was an unintended resonance circuit, which affected the BCI result. By adding a dumping resister between the ground pattern and case, the resonance was dissolved and the BCI abnormality disappeared. In addition to BCI, we tested conducted emission using another product. Even though the switching devices had no ringing waveform, a resonance of 120 MHz was observed at the connector. This worsened the emission level. We improved the emission level by adding a dumping resister. Conclusion: The self-inductance of the pattern of PCB, bus bars and stray capacitance between the PCB and metal case generates unintended resonance, creating a close relationship to the EMC test results. In BCI, the ground impedance increased in resonance and caused instability in the products. With conducted emission, the harmonic noise from the switching device was amplified by the resonance. As mentioned above, the unintended resonance circuit lowers EMC performance. Diminishing the resonance with a dumping resister improves the EMC performance of the products. It is important to strive for a design that creates no resonance.