Charger interference

Another potential source of touch screen interference is the power supply of power supply mobile phone charger. The interference is coupled to the touch screen by fingers, as shown in Figure 5. The small cell phone charger usually has AC power line and zero line input, but there is no ground connection. The charger is safe and isolated, so there is no DC link between the power input and the charger secondary coil. However, this will still be generated by switching power supply isolation transformer capacitance coupling. The charger interferes with the finger touch screen to form a return path.

Note: in this case, the charger interference is the external voltage of the device relative to the ground. This interference can be described as a "common mode" interference in the DC power supply and DC ground equivalent. In the charger output of DC power supply and DC ground between the power switch noise, if not been fully filtered, it may affect the normal operation of the touch screen. This kind of power supply rejection ratio (PSRR) problem is another problem, this paper does not discuss.

Charger coupling impedance

Charger switching interference through the transformer primary - secondary winding leakage capacitance (about 20pF) coupling. The weak capacitance coupling effect can be compensated by the parasitic parallel capacitance of the charger cable and the electric equipment itself. When the device is picked up, the shunt capacitance will increase, which is usually sufficient to eliminate the interference of the charger switch and avoid interference effects on the touch operation. When the portable device is connected to the charger and placed on the desktop, and the operator's finger only contact with the touch screen, the charger will appear one of the worst case of interference.

Switching interference component of charger

The typical mobile phone charger adopts flyback (flyback) circuit topology. The interference waveform generated by this charger is more complex, and varies greatly with the charger, it depends on the circuit details and output voltage control strategy. The variation of the amplitude of the disturbance is also great, depending on the design effort and unit cost of the manufacturer on the switch transformer shielding. Typical parameters include:

Including the pulse width modulation waveform: square wave and LC complex ringing waveform. Frequency: under the rated load of 40~150kHz, when the load is light, the pulse frequency or skip cycle operation down to 2kHz. The power supply voltage: up to half of the peak voltage =Vrms/ V 2.

Power supply interference component

At the front of the charger, the AC supply voltage is rectified to generate the charger with high voltage rail. In this way, the switching voltage component of the charger is superimposed on a sine wave of a half of the power supply voltage. Similar to switching interference, the power supply voltage is also formed by switching isolation transformer. In 50Hz or 60Hz, the frequency of the component is much lower than the switching frequency, so its effective coupling impedance is higher. The severity of the power supply voltage disturbance depends on the characteristics of the parallel impedance, and it also depends on the sensitivity of the touch screen controller to the low frequency.

Special power interference: 3 hole plug without grounding

The power adapter rated higher power (such as notebook computer AC adapter), May 3 AC power plug hole configuration. In order to suppress the output of EMI, the main power supply charger may pin connected to the DC output in internal. The Y capacitor is connected between the charger is usually in the firing line and the zero line and the ground, thereby inhibiting the transmission from the power supply line EMI. The connection to that, this kind of connection to the power supply adapter will not PC and USB portable touch screen device caused by interference. Figure 5 shows the dashed box in this configuration.

For PC and its USB connected portable touch screen device, if the PC charger with a 3 hole power input is inserted into the power socket without connection, a special case of charger interference will be generated. The Y capacitor is coupled to the DC output. The relatively large Y capacitance value can be very effectively coupled to the power supply voltage, which makes the larger power frequency voltage through the touch screen of the fingers with a relatively low impedance of the coupling.