Study on Conducted Electromagnetic Interference of DC SSR Based on Synthesizing Modeling Method

In a DC SSR (Solid State Relay) with a transformer isolation circuit, the transformer works at self-oscillating state and the high frequency signal would be transmitted from original side to secondary side through the magnet core. The high dv/dt and di/dt signals would produce high conducted EMI (Electro-Magnetic Interference), which may lead to a performance downgrade or even failure of other electronic devices. In this paper, a certain type of DC SSR was investigated. An accurate time-domain circuit model used for analyzing conducted EMI of the SSR was proposed based on the high frequency model of the power diode and MOSFET, the equivalent circuit of the dual-winding transformer and the parasitic parameters of the SSR. The conducted emitting energy at the input port power line of the SSR was simulated by Pspice software and the simulation results agreed well with the experiment. In order to restrain the EMI, an EMI filter was proposed and connected to the input port in series. The experimental results show that the conducted EMI after optimization decreased and conformed with the regulations of MIL-STD-461E, and the EMC (Electro-Magnetic Compatibility) performance of the SSR is improved. The synthesizing modeling method of SSR and the analysis method of conducted EMI proposed in this paper is of great value for EMC prediction of relay's performance.