Developing a mobile handheld device with a touch screen man-machine interface is a complex design challenge, especially for projected capacitive touch screen design, which represents the current mainstream technology of multi-touch interface. The projected capacitive touch screen can accurately locate the position where the finger touches the screen. It determines the position of the finger by measuring the small change in capacitance. In this type of touch screen application, a key design issue that needs to be considered is the impact of electromagnetic interference (EMI) on system performance. The performance degradation caused by interference may adversely affect the design of the touch screen. This article will discuss and analyze these interference sources.
Projected capacitive touch screen structure
A typical projected capacitive sensor is installed under a glass or plastic cover. Figure 1 shows a simplified side view of the dual-layer sensor. The transmitting (Tx) and receiving (Rx) electrodes are connected to transparent indium tin oxide (ITO) to form a cross matrix, and each Tx-Rx junction has a characteristic capacitance. The Tx ITO is located under the Rx ITO, separated by a layer of polymer film or optical adhesive (OCA). As shown in the figure, the direction of the Tx electrode is from left to right, and the direction of the Rx electrode is directed from the outside of the paper to the inside of the paper.
Sensor structure reference
How the sensor works
Let us analyze the work of the touch screen without considering interference factors: the operator's hand indicator is at ground potential. Rx is maintained at ground potential by the touch screen controller circuit, while the Tx voltage is variable. The changing Tx voltage causes current to flow through the Tx-Rx capacitor. A carefully balanced Rx integrated circuit isolates and measures the charge entering Rx. The measured charge represents the "mutual capacitance" connecting Tx and Rx.
Sensor status: not touched
When a finger touches the cover plate, magnetic lines of force are formed between the Tx and the finger, and these magnetic lines of force replace a large number of Tx-Rx fringe magnetic fields, as shown in Figure 3. In this way, finger touch reduces the Tx-Rx mutual capacitance. The charge measurement circuit recognizes the changing capacitance (△C), thereby detecting the finger above the Tx-Rx node. Through the △C measurement of all intersections of the Tx-Rx matrix, the touch distribution of the entire panel can be obtained.
Magnetic lines of force under touch
The interference of the projected capacitive touch screen is generated by the imperceptible parasitic path coupling. The term "ground" is usually used to refer to both the reference node of the DC circuit and the low-impedance connection to the earth: the two are not the same term. In fact, for portable touch screen devices, this difference is the root cause of touch coupling interference. To clarify and avoid confusion, we use the following terms to evaluate touch screen interference.
Earth: Connect to the earth, for example, connect to the earth through the earth wire of a 3-hole AC power socket.
Distributed Earth: The capacitive connection of an object to the earth.
DC Ground: The DC reference node of portable equipment.
DC Power: The battery voltage of a portable device. Or the output voltage of a charger connected to a portable device, such as 5V Vbus in a USB interface charger.
DC VCC (direct current VCC power supply): A stable voltage that supplies power to portable equipment electronics (including LCD and touch screen controllers).
Neutral: AC power circuit (nominal at ground potential).
Hot (hot wire): AC power supply voltage, which applies electrical energy to the neutral wire.
LCD Vcom is coupled to the touch screen receiving circuit
The touch screen of the portable device can be installed directly on the LCD display. In a typical LCD architecture, the liquid crystal material is biased by transparent upper and lower electrodes. The lower electrodes determine the multiple single pixels of the display screen; the upper common electrode is a continuous plane covering the entire visible front end of the display screen, which is biased at the voltage Vcom. In a typical low-voltage portable device (such as a mobile phone), the AC Vcom voltage is a square wave that oscillates back and forth between DC ground and 3.3V. The AC Vcom level is usually switched once for each display line. Therefore, the generated AC Vcom frequency is 1/2 of the product of the display frame refresh rate and the number of lines. The AC Vcom frequency of a typical portable device may be 15kHz. Figure 4 is a schematic diagram of LCD Vcom voltage coupling to the touch screen.
In order to reduce the cost of the architecture and obtain better transparency, the single-layer touch screen mounts the Tx and Rx arrays on a single ITO layer, and bridges each array in turn through a separate bridge. Therefore, the Tx array cannot form a shielding layer between the LCD Vcom plane and the sensor Rx electrode. This may cause serious Vcom interference coupling.
Another potential source of touch screen interference is the switching power supply of power-powered mobile phone chargers. The interference is coupled to the touch screen through fingers, as shown in Figure 5. Small cell phone chargers usually have AC power live and neutral input, but no ground connection. The charger is safely isolated, so there is no DC connection between the power input and the charger's secondary coil. However, this still produces capacitive coupling through the isolation transformer of the switching power supply. The charger interference forms a return path by touching the screen with a finger.
Charger interference coupling model
Note: In this case, charger interference refers to the voltage applied to the device relative to ground. This interference may be described as "common mode" interference because of its equivalent value on the DC power supply and DC ground. If the power switching noise generated between the DC power supply output by the charger and the DC ground is not sufficiently filtered, it may affect the normal operation of the touch screen. This kind of power supply rejection ratio (PSRR) issue is another issue, which is not discussed in this article.
Charger coupling impedance
The charger switch interference is generated by the coupling of the transformer primary-secondary winding leakage capacitance (approximately 20pF). This kind of weak capacitive coupling can be compensated by the relatively distributed parallel capacitance of the charger cable and the power receiving device itself. When the device is picked up, the parallel capacitance will increase, which is usually enough to eliminate the interference of the charger switch and prevent the interference from affecting the touch operation. When the portable device is connected to the charger and placed on the desktop, and the operator's finger is only in contact with the touch screen, there will be a worst-case interference produced by the charger.
Charger switch interference component
A typical mobile phone charger uses a flyback circuit topology. The interference waveform generated by this charger is more complicated and varies greatly from charger to charger. It depends on the circuit details and output voltage control strategy. The interference amplitude also varies greatly, depending on the design effort and unit cost invested by the manufacturer in the shielding of the switching transformer. Typical parameters include:
Waveform: Including complex pulse width modulation square wave and LC ringing waveform. Frequency: 40~150kHz under rated load. When the load is very light, the pulse frequency or skip cycle operation drops below 2kHz. Voltage: up to half of the peak voltage of the power supply=Vrms/√2.
Charger power interference component
At the front end of the charger, the AC power supply voltage is rectified to generate the charger high-voltage rail. In this way, the switching voltage component of the charger is superimposed on a sine wave that is half of the power supply voltage. Similar to switching interference, this power supply voltage is also coupled through a switching isolation transformer. At 50Hz or 60Hz, the frequency of this component is much lower than the switching frequency, so its effective coupling impedance is correspondingly higher. The severity of power supply voltage interference depends on the characteristics of the parallel impedance to the ground, and also depends on the sensitivity of the touch screen controller to low frequencies.
The special case of power interference: a 3-hole plug without grounding, and a power adapter with a higher rated power (such as a laptop AC adapter) may be equipped with a 3-hole AC power plug. In order to suppress EMI at the output, the charger may internally connect the ground pin of the main power supply to the output DC ground. This type of charger usually connects a Y capacitor between the live wire and the neutral wire and ground to suppress the conducted EMI from the power wire. Assuming that the ground connection exists intentionally, this type of adapter will not cause interference to the portable touch screen device that is powered by the PC and USB connection. The dashed box in Figure 5 illustrates this configuration.
For a PC and its USB-connected portable touch screen device, if a PC charger with a 3-hole power input is plugged into a power socket that is not connected to the ground, a special case of charger interference will occur. The Y capacitor couples the AC power supply to the DC ground output. A relatively large Y capacitance value can couple the power supply voltage very effectively, which enables a large power supply frequency voltage to be coupled with a relatively low impedance through a finger on the touch screen.
Summary of this article
Projected capacitive touch screens, which are widely used in portable devices today, are susceptible to electromagnetic interference. Interference voltage from internal or external sources will be capacitively coupled to the touch screen device. These interference voltages will cause the charge movement in the touch screen, which may cause confusion in the charge movement measurement when the finger touches the screen. Therefore, the effective design and optimization of the touch screen system depends on the understanding of the interference coupling path and the reduction or compensation as much as possible.
The interference coupling path involves parasitic effects, such as transformer winding capacitance and finger-device capacitance. Proper modeling of these effects can fully recognize the source and magnitude of interference.
For many portable devices, battery chargers constitute the main source of interference for touch screens. When the operator's finger touches the touch screen, the generated capacitance causes the charger interference coupling circuit to be closed. The quality of the internal shielding design of the charger and the proper grounding design of the charger are the key factors that affect the interference coupling of the charger.