Honggang Sheng

Techniques of the modeling, measurement and reduction of common mode noise for a multi-winding switching transformer

The reduction of common mode (CM) noise for isolated converters has been a popular research topic. This paper proposed a generalized two-capacitor modeling technique for switching transformers with multiple windings and complicated winding structures. Based on the proposed model, a Flyback transformer with multiple winding structures is investigated. A new measurement technique to characterize and evaluate switching transformers CM noise performance is developed. Only a signal generator and an oscilloscope are needed for the measurement and no any in-circuit tests are needed. The technique can help to quickly design and evaluate transformers for CM noise reduction in mass testing and production. CM noise reduction techniques including balance capacitor technique, core shielding technique and winding design technique are developed based on the developed two-capacitor model and measurement technique. Experimental results verify the proposed model and the developed CM noise reduction techniques.

Investigating the interference of common mode noises of AC/DC power adapters to the touchscreens of consumer electronics

The common mode (CM) noise of power adapters tends to interfere the performance of touchscreens of consumer electronics. However, theres a lack of literature illustrating and modeling the mechanism. This paper investigates the CM noise source impedances of the AD/DC adapters and the characteristics of touchscreens. Based on this understanding, the relationship of CM voltage of an adapter and the CM noise that will influence the performance of touchscreen is disclosed. Also, a noise sensitivity test is performed to investigate a touchscreens sensitivity to CM noise. The paper also discusses how to agree this test with adapter CM voltage test.

Investigation and reduction of line frequency common mode voltages at the outputs of AC/DC power converters

Although existing standards such as IEC62684 and EN 301 489-34 limit the line frequency common mode (CM) voltages at the output of AC/DC converters, the mechanism and characteristics of line frequency CM voltages at the outputs of AC/DC converters are unclear and the technique to reduce it is not well investigated and developed. In this paper, the line frequency CM voltages at the outputs of AC/DC converters are analyzed and a modeling technique is developed. Simulations and experiments were conducted to validate the developed technique. An AC/DC Flyback converter is taken as a study example. The paper also analyzed the effect of the start-up circuit at the primary side on the CM voltages. The relationship between leakage current and CM voltage is also discussed. The techniques to reduce the output line frequency CM voltages is also investigated and developed.