Investigating CM Voltage and Its Measurement for AC/DC Power Adapters to Meet Touchscreen Immunity Requirement

The common-mode (CM) noise at the output of ac/dc power adapters can interfere with touchscreen's normal operation in consumer electronic products. Existing power adapter's output CM voltage measurement used in power electronics industry does not align with the touchscreen's sensitivity test used by touchscreen manufacturers. This paper first reviews existing measurement setups and identifies the technical issues in the tests. The CM noise source impedance of ac/dc power adapters and the characteristics of touchscreens are investigated. A new measurement setup is proposed based on both the characteristics of CM noise source of power adapters and touchscreens. A CM voltage limit is also proposed with industry companies. A measurement device is developed for the proposed measurement setup and procedure. The experimental results validate the proposed techniques.