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Projectile capacitive touch screen electromagnetic interference problem solution

作者:品触光电 来源: 日期:2015-12-31 9:52:41

作者: Multitouch     时间:2014-03-04     源于:51touch    总点击:629

Develop mobile handheld devices with the touch screen man-machine interface design is a complex challenge, especially for the projected capacitive touch screen design, it represents the current mainstream technology of multi-touch interface. The projectile capacitive touch screen can pinpoint the position of a finger's light touch screen, and it can determine the position of the finger by measuring the small changes in the capacitance. One of the key design issues to consider in such touch screen applications is the impact of EMI on system performance. The performance degradation caused by interference may adversely affect the design of the touch screen, and this article will discuss and analyze the interference sources.

Projection capacitive touch screen structure

The typical projection capacitance sensor is mounted under glass or plastic cover. Figure 1 shows a simplified side view of a two-layer sensor. The emission (Tx) and reception (Rx) electrodes are connected to the transparent indium tin (ITO), forming the cross matrix, and each tx-rx node has a characteristic capacitance. Tx ITO is located below 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 points from the outside of the paper to the paper.


Figure1Sensor structure reference

   

The sensor works

Let's not consider the interference factor for a while, to analyze the work of the touch screen: the operator's finger mark is in the ground potential. Rx is kept on the ground by the touch screen controller circuit, while the Tx voltage is variable. The changing Tx voltage enables the current to pass through the tx-rx capacitance. A carefully balanced Rx integrated circuit isolates and measures the charge into the Rx, and the measured charge represents the "mutual capacitance" that connects Tx and Rx.

Sensor status: untouchable  Figure 2 shows the schematic diagram of the magnetic force line under the untouchable state. With no finger touch, the tx-rx  magnetic line takes up a considerable amount of space in the cover plate. The edge magnetic field is projected beyond the electrode structure, hence the term "projectile capacitance".


Figure2The magnetic force line in the untouchable state

     

Sensor status: touch

When the fingers touch the cover, the Tx creates a magnetic force between the fingers, which replaces a large number of tx-rx edge magnetic fields, as shown in figure 3. In this way, finger touch reduces the tx-rx mutual capacitance. The charge measurement circuit identifies the changing capacitance (delta C) to detect the fingers above the tx-rx node. Through the measurement of delta C at all intersections of the tx-rx matrix, the whole panel can be obtained by the touch distribution map.

Figure 3 also shows another important effect: capacitive coupling between fingers and Rx electrodes. Through this path, the electrical interference may be coupled to Rx. Some degree of finger - Rx coupling is inevitable.


Figure3The magnetic force line in the touchable state

     

Special terms

The interference of the projectile capacitive touch screen is generated by the undetectable parasitic path coupling. The term "ground" is usually used to refer to the reference node of the dc circuit, and can be used to refer to the low impedance connection to the earth: the two are not the same terms. 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 (Earth) : connected to the Earth, for example, by connecting the ground wire of a power outlet to the Earth through a 3-hole ac power outlet.

• Distributed Earth (Distributed field) : the capacitance of the object to the Earth.

• DC Ground (DC) : DC reference node for portable devices.

• DC Power (DC Power) : battery voltage of portable devices. Or a charger that is connected to a portable device, such as 5V Vbus in a USB charger.

• DC VCC (DC VCC power supply) : stable voltage for portable device electronics (including LCD and touch screen controllers).

• Neutral (zero line) : ac power circuit (nominal position).

• Hot (fire line) : ac power supply voltage, and electrical energy relative to zero.

LCD Vcom is coupled to the touch screen receiver line

The portable device touch screen can be installed directly onto the LCD display screen. In a typical LCD structure, the liquid crystal is offset by a transparent upper and lower electrode. The multiple electrodes below determine the display's multiple single pixels; The upper part of the public electrode is the continuous plane covering the entire visual front of the display screen, which is biased against voltage Vcom. In a typical low-voltage portable device, such as a cell phone, ac Vcom voltage is a square wave that oscillates back and forth between dc and 3.3 V. Ac Vcom level usually switches every display line, so the generated communication Vcom frequency is 1/2 of the product of the display frame refresh rate and line number. A typical communication Vcom with a portable device may be 15kHz.


Figure 4 : LCD Vcom voltage coupling to the touch screen.

     

The double-layer touch screen consists of the separation ITO layer, which is full of Tx array and Rx array. The Tx line occupies the entire width of the Tx array spacing, which is separated only by the minimum spacing between the lines. This architecture is known as the self-shielding type, because the Tx array shields the Rx array from the LCD Vcom. However, coupling can still occur through interspace between Tx bands.

To reduce the cost of architecture and get better transparency, the single-layer touch screen installs Tx and Rx arrays on a single ITO layer and spans the array through individual Bridges. Therefore, the Tx array cannot form a shielding layer between the LCD Vcom plane and the sensor Rx electrode. This could have serious Vcom interference coupling.

Charger interference

Another potential source of touch screen interference is the switching power of the power supply phone charger. The interference is coupled to the touch screen by finger, as shown in figure 5. Small cell phone chargers usually have ac power lines and zero input, but no ground connections. The charger is safely isolated, so there is no dc connection between the power input and the charger secondary coil. However, this will still generate capacitive coupling through the switch power isolation transformer. The charger interferes with the finger touch screen and forms the return path.


Figure 5: chargers interference coupling model.

Note: in this case, the charger interference refers to the additional voltage of the equipment relative to the ground. This interference may be described as "common mode" interference due to its equivalent in dc and dc. The power switch noise generated between the dc power supply of the charger and the dc field, if not filtered properly, may affect the normal operation of the touch screen. This power control ratio (PSRR) problem is another problem, which is not discussed in this article.

Charger coupling impedance

The charger switch interferes with the coupling generation of the transformer primary - secondary winding leakage capacity (about 20pF). This weak capacitance coupling can be found in the charger cable and the electrical equipment itself relatively distributed parasitic and uncoupling capacitance compensation. When picking up the device, the shunt capacitance will increase, which is usually sufficient to eliminate the switch interference of the charger and prevent interference affecting the touch operation. When a portable device is connected to the charger and placed on the desktop, and the operator's finger is only touched with the touch screen, there will be a worst-case interference from the charger.

Charger switch interference component

The typical cell phone charger is a flyback circuit topology. The interference waveform of this charger is more complex, and varies greatly depending on the charger. It depends on the circuit details and the output voltage control strategy. The interference amplitude varies greatly depending on the design effort and unit cost of the manufacturer's input on the switch transformer shielding. Typical parameters include: waveform: complex pulse width modulation and LC ringing waveform. Frequency: 40 ~ 150kHz under rated load, when load is very light, pulse frequency or jump cycle operation falls below 2kHz. Voltage: half of the power supply peak voltage = Vrms/square root 2.

Charger power supply interference component

At the front of the charger, ac power supply voltage rectification generates charger high voltage rail. Thus, the switch voltage component of the charger is superimposed on the sine wave of half the power supply voltage. Similar to the switch interference, this power supply voltage is coupled by 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 magnitude of the power supply voltage interference depends on the characteristics of the impedance of the ground parallel, and also depends on the sensitivity of the controller to the low frequency.


Figure 6: example of the charger waveform.

   

Special case of power interference: 3 hole plug without grounding

A power adapter with a high rated power (such as a laptop ac adapter) may be equipped with a three-hole ac power plug. In order to suppress the output of EMI, the charger may connect the ground pin of the main power source to the lost dc. Such chargers usually connect the Y capacitance between the fire line and the zero line, thereby inhibiting the transmission of EMI from the power line. Assuming an intentional connection exists, these adapters do not interfere with portable touch-screen devices connected to the power supply PC and USB connection.

The dotted line in figure 5 illustrates this configuration.

For PC and the USB connection of the portable touchscreen devices, if you have 3 holes power input PC charger into the land of no power socket connection, a special case will produce charger interference. The Y capacitance will coupling ac power to dc output. The relatively large Y capacitance value can be very effective in coupling the power supply voltage, which enables the larger power frequency voltage to be coupled to a relatively low impedance through the fingers on the touch screen.

This article summary

The projective capacitive touch screens widely used in portable devices are susceptible to electromagnetic interference, and interference voltages from inside or outside can be coupled to the touch screen equipment by capacitance. These interference voltages cause charge movements within the touch screen, which may confuse the charge motion measurements on the touch screen. Therefore, the effective design and optimization of the touch screen system depends on the understanding of the interference coupling path, as well as the possible reduction or compensation to it. 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 size of the interference.

For many portable devices, battery chargers constitute the main source of interference for touch screens. When the operator fingers touch the touch screen, the resulting capacitance causes the charger to interfere with the coupling circuit to close. The quality of the internal shield design and the proper grounding design of the charger are the key factors affecting the coupling of the charger.


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