Abstract
In isolated power converters, the transformer is a key part of voltage transformation and isolation. Since common-mode (CM) noise is rather difficult to suppress compared with different-mode (DM) noise, more and more scholars are paying attention to the characteristics of CM noise, especially in high-frequency CM noise behaviors. CM noise can be further divided into conducted CM noise and radiated CM noise, and the main focus of this paper is on conducted CM noise. The CM coupling capacitance of the transformer is one of the main contributors of CM noise, which has been verified in many previous studies. Hence, eliminating the CM noise in a transformer coupling path can significantly lower the whole CM noise level of the converter. Professional conducted electromagnetic interference (EMI) testing instruments are quite expensive. In this paper, a general transformer evaluation technique for CM noise behavior is proposed. Only a signal generator and oscilloscope can achieve transformer CM noise behavior evaluation. PCB planar flyback transformers are designed, and a series of noise spectrums and voltage waveforms can verify the effectiveness of the proposed transformer evaluation method. The flyback adapter porotype can pass the EMI standard limited line EN55022 class B by the proposed evaluation method.
Highlights
Switch-mode power supplies (SMPSs) have been widely used in both high power applications and low power applications
A signal generator and an oscilloscope can effectively evaluate the CM noise behavior of the transformer in the low-frequency range, which is easy to be implemented without the expensive cost of the professional conduction emission (CE) electromagnetic interference (EMI) testing instruments
If ips is equal to isp, the CM current flowing through transformer will have the opportunity to be totally canceled due to their same amplitudes and opposite phases
Summary
Switch-mode power supplies (SMPSs) have been widely used in both high power applications and low power applications. Reference [12] proposed that the parasitic capacitance of a transformer with two windings can can be be modeled with six capacitances to apply the energy conservation. This model is too complex to achieve circuit analysis. A signal generator and an oscilloscope can effectively evaluate the CM noise behavior of the transformer in the low-frequency range, which is easy to be implemented without the expensive cost of the professional CE EMI testing instruments. The CM noise flowing characteristic in the transformer coupling path is analyzed under different circuit configurations.
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