Studies found that, during the charging process, the common mode noise and surge voltage generated by the AC-DC charger enters the smartphone's capacitive screen when touched. This seriously affects the touch performance, and often leads to inaccurate touch readings or false touch signals. Therefore, mobile phone manufacturers joined forces and formulated the IEC 62684 regulation, which standardizes the noise spectrum specifications of battery chargers.

▲Touch-Screen Position Detected Error Caused by Common-Mode Noise 
(Reference: Study of Common-Mode Voltage Measurements for IEC62684; Publisher: IEEE)

For safety reasons, mobile phone chargers are designed with transformers for primary and secondary isolation. When the phone is connected to the charger, the parasitic capacitance in the charger's isolation transformer will form a voltage division with the parasitic capacitance of the phone's chassis to the ground conductor, and a common-mode voltage is generated on the mobile phone. If the common-mode voltage is too high, it will affect the mobile phone and cause problems such as touch malfunctioning. Charger manufacturers are seeking fast and stable test solutions for common-mode noise production, as to ensure that the electromagnetic interference characteristics of each product meet the specifications.

The IEC 62684 MoU defines the common-mode noise elements, specification limits, and measurement setting recommendations for mobile chargers. After continuous testing, verification, and adjustment, Chroma ATE offers a solution that meets these requirements and measures the common-mode noise swiftly and steadily. Chroma 8020 Adapter/Charger ATS applies to sensitive common-mode noise measurement, effectively filtering out unstable, discontinuous, and other unexpected interference surges, and precisely measures common-mode noise. Repeated experiments have confirmed that the difference of each value measurement is less than 20mV, which is much smaller than the 100mV~200mV value fluctuation when using an oscilloscope.

In addition, the differential mode measurement architecture of the Chroma Timing/Noise Analyzer is different from the common ground architecture of the oscilloscope. The test device can so avoid the mutual influence of noise characteristics caused by the output common to ground of the DUTs during multi-DUT testing. With Chroma 8020, it takes less than 2 seconds to measure the common-mode noise of 4 DUTs at once. Chroma 8020 Adapter/Charger ATS offers global test coverage that fits the rapid mass production of the modern production line.

▲Charger Common Mode Noise Test Architecture