Analysis and Reduction of Radiated EMI of Power Modules

The presented paper introduces a novel type of EMI optimised, balanced power modules with reduced EMI in the frequency range for radiated emissions from 30 MHz to 300 MHz. The comparison of the novel modules to a conventional module layout with same semiconductor chips shows a noise reduction up to 30 dB in the critical frequency region around 30 MHz. For frequencies above 50 MHz the noise reduction is 15 dB to 20 dB. In the frequency range from 30 MHz to 300 MHz the emission is strongly influenced by the module layout in addition to the semiconductor behaviour. Two basic noise sources have to be taken into account. First, stray inductances between DC-Link and power stage create together with semiconductor capacitances differential mode oscillations. Second, the permanent voltage change at the output node drives noise currents through the stray capacitances of the output and of the gates to power ground. The investigations shows that the output capacitances of the module can be minimized by using flip chip technology for the low side IGBT¿s. The gate-stray capacitances can be minimized by layout measures as well. Additionally, an unbalanced layout of the DC-Bus bar promotes the spreading of the noise created by DC-Link oscillations. Therefore a balanced layout is to recommend. The suggested layout measures for power modules are tested by production and measurement of EMI optimised power modules.