Mobile phone electromagnetic compatibility issues and solutions

1 Electrostatic discharge immunity test

1.1 Common Problems in Electrostatic Discharge Immunity Test

The problems encountered in electrostatic discharge immunity testing are mainly manifested in the following aspects.

(1) The mobile phone call was interrupted.

(2) Electrostatic discharge causes some functions of the mobile phone to fail, but the failed functions can be restored after the electrostatic discharge process ends or the mobile phone is restarted. These phenomena may be:

The screen display is abnormal, such as white screen, stripes, garbled characters, blurry display, etc.

There are problems with call quality, such as howling or sound disappearance;

Loss of button function or touch screen function;

The software gives false alarms, such as frequently prompting "Charging connected, charger removed" when the charger is not plugged in or out.

(3) The phone automatically shuts down or restarts. This problem may occur during a call or during standby.

(4) Electrostatic discharge may cause the mobile phone to fail or be damaged.

Due to the damage of some components, some functions of the phone cannot be restored after restarting, such as the camera function;

The situation where the device cannot be turned on again after automatically shutting down;

When tested while connected to a charger, the charger may fail, become damaged, or even explode.

1.2 Specific analysis of electrostatic discharge problems

(1) Call interruption: The main reason for call interruption is that electrostatic discharge affects the radio frequency circuit and/or baseband circuit inside the mobile phone, causing a decrease in the communication signal-to-noise ratio and signal synchronization problems, thereby causing call interruption.

An unreasonable structural design may also cause call interruption. The electrostatic discharge test requires a large-area metal horizontal coupling plate, with only a 0.5 mm thick insulating pad placed between the mobile phone and the horizontal coupling plate. When the antenna or a large-area metal component is too close to the horizontal coupling plate, mutual coupling may occur, causing the actual sensitivity of the mobile phone to be greatly reduced. During the electrostatic test, calls are more likely to be interrupted. In severe cases, the mobile phone cannot maintain a call even without electrostatic interference.

(2) Automatic shutdown or restart: The reset circuit of the baseband circuit is interfered by static electricity, causing the mobile phone to shut down or restart by mistake.

(3) Mobile phone failure or damage: High voltage and high current during electrostatic discharge can cause thermal failure or insulation breakdown of the device. The strong electromagnetic field during electrostatic discharge can also cause temporary failure of the device.

(4) Software failure: The electrostatic interference signal is processed as a useful signal, causing the operating system to respond incorrectly.

1.3 Suggestions for improving electrostatic discharge issues

(1) Consider electrostatic discharge in the design solution

Try to choose devices with high electrostatic sensitivity levels;

The device is isolated from the static power source;

Reduce the loop area (the larger the area, the greater the field flow contained, and the greater the induced current). Specific measures may include: the shorter the trace, the better; the closer the power supply and the ground are, the better; when there are multiple sets of power supplies and grounds, connect them in a grid manner; too long signal lines or power lines must be arranged in an interlaced manner with the ground line; the closer the signal line is to the ground line, the better; all components are closer, the better; the closer the devices with the same characteristics are, the better;

Ground plane design: Try to use a complete ground plane on the PCB; the larger the PCB ground area, the better; do not have large gaps;

The ground line of the PCB needs to have low impedance and good isolation;

It is better to layout the power supply and ground in the middle of the board rather than around it;

Place high-frequency bypass capacitors between power supply and ground;

Protect static sensitive components.

(2) Suggestions for rectification after the occurrence of static electricity problems In response to the above-mentioned static electricity discharge problems, the following steps need to be taken to rectify them.

a) Try direct discharge, indirect discharge, air discharge and contact discharge to confirm the coupling path;

b) Discharge from different directions, observe the differences in the phenomena, and determine all the discharge points and discharge paths;

c) Test at different voltages from low to high to determine within which voltage range the phone fails to meet the standards;

d) Test several prototypes, analyze commonalities, and confirm the cause of failure;

e) Determine the relevant sensitive devices based on the coupling path, unqualified phenomenon, and discharge path;

f) Develop solutions for sensitive devices;

g) Verify and correct the solution through experiments. The following measures can be taken during the rectification.

Problems with housing gaps, buttons, and FPCB can be solved by dielectric isolation.

Problems with cameras, microphones, and receivers can be solved by dielectric isolation and enhanced grounding;

Chips with shielding shells can be treated by strengthening the shielding effect and strengthening the grounding of the shielding shell;

Interface circuits and pins of key chips should be protected by using protection devices (such as TVS tubes, ESD protection devices); Software failures can be improved by adding some logical judgments to correctly detect and process alarm information.

2 Electrical fast transient burst immunity test

2.1 Overview of electrical fast transient burst immunity test

The principle of the generation of electrical fast transient pulse groups is as follows: when an inductive load (such as a relay, contactor, etc.) is disconnected, a transient disturbance is generated at the disconnection point due to insulation breakdown of the switch contact gap or contact bounce. When the inductive load is repeatedly switched on and off, the pulse group will be repeated many times with corresponding time intervals. This transient disturbance has a small energy and generally does not cause damage to the equipment, but due to its wide spectrum distribution, it will affect the reliable operation of mobile phones.

This test is a test that couples a pulse group consisting of many fast transient pulses to the power port of a mobile phone. The characteristics of the test pulses are: short transient pulse rise time, repeated occurrence, and low energy. The purpose of this test is to test the performance of the mobile phone when it is affected by this type of transient interference. It is generally believed that the reason why electrical fast transient pulse groups cause malfunctions of mobile phones is that the pulse group charges the semiconductor junction capacitance in the circuit. When the energy on the junction capacitance accumulates to a certain level, it will cause malfunctions of the mobile phone. Specifically, during the test, the mobile phone may experience communication interruption, freeze, software alarm, loss of control and storage functions, etc.

2.2 Analysis of Common Problems in Electrical Fast Transient Burst Immunity Test

The fast transient pulse waveform is directly transmitted into the mobile phone through the charger, resulting in excessive noise voltage on the mainboard circuit. When injecting into the live wire or neutral wire alone, although the injection is done in a common mode to the ground, there is differential mode interference between the live wire and the neutral wire, and this differential mode voltage will appear at the DC output end of the charger. When injecting into the live wire and the neutral wire at the same time, there is common mode interference, but it has little effect on the output of the charger. The reasons for the problems of the mobile phone during the test are complex, and are specifically manifested in the following aspects.

The electrical fast transient pulse group suppression function was not considered in the early design, no relevant filtering components were added, and the cable isolation was not paid attention to during the PCB design and integrated wiring. The motherboard grounding design did not meet the specifications, and no shielding protection measures were taken for key components.

The manufacturer did not select reliable key components from component suppliers, which resulted in component aging or failure during the test, making it susceptible to interference from electrical fast transient pulses;

During the production and assembly process of the whole machine, problems with the processing technology and assembly level may lead to poor product consistency, and some mobile phones submitted for inspection have quality problems;

During the detection process, rectification is required due to problems with other test items. The choice of rectification plan may affect the failure of the electrical fast transient pulse group test.

2.3 Suggestions for improvement of issues related to electrical fast transient pulse group immunity test

To solve the problems in the test of electrical fast pulse group interference, filtering and absorption can be used to suppress electrical fast transient pulses.

(1) In the early stages of mobile phone design, the focus should be on suppressing the interference of electrical fast transient pulse groups.

The power input position of the PCB layer should be filtered. Usually, a combination of large and small capacitors is used. According to the actual situation, a first-level magnetic bead can be added to filter out high-frequency signals. Surface packaging should be used as much as possible.

Try to reduce the common impedance value of the PCB ground line;

PCB layout should keep interference sources away from sensitive circuits as much as possible;

All kinds of PCB traces should be as short as possible;

Reduce loop area;

When doing integrated wiring, attention should be paid to the isolation of strong and weak current wiring, and the isolation of signal lines and power lines. Integrated wiring is a very important design component of the system. A bad integrated wiring pattern is likely to ruin the stability of a well-designed PCB.

Key sensitive chips need to be shielded;

The software should correctly detect and process alarm information and restore the product status in a timely manner.

(2) Reliable quality chips should be used when selecting components. It is best to have undergone chip-level electromagnetic compatibility simulation tests. The selection of reliable quality chargers, data cables and batteries can improve the ability to suppress electrical fast transient pulse signals;

(3) Manufacturers should strictly control quality during the assembly and production process, do a good job in production process control, try to ensure the consistency of product quality, and reduce the phenomenon of test failures caused by quality problems of individual mobile phones;

(4) If problems occur during the EFT test, you can add a magnetic ring or an electrical fast transient pulse group filter to the charger for rectification. The smaller the inner diameter, the larger the outer diameter, and the longer the length of the magnetic bead, the better. The rectification method of adding a TVS tube has limited effect;

(5) According to the latest GB/T 17626.4-2008 standard, the repetition frequency will increase to 100 kHz, which will be more stringent than 5 kHz. Manufacturers should pay attention to the relevant electrical fast transient pulse group test protection work as soon as possible.

3 Radiated disturbance and conducted disturbance

3.1 Specific situations of issues related to radiated disturbance and conducted disturbance

Radiated interference tests are prone to failure in the frequency ranges of 30 MHz-100 MHz and 200 MHz-900 MHz, while conducted interference tests are prone to failure in the frequency range of 5 MHz-30 MHz.

3.2 Analysis of issues related to radiated disturbance and conducted disturbance

The radiated disturbance and conducted disturbance test is an electromagnetic compatibility test conducted when the phone is charged by a charger and the phone is in communication mode and at maximum transmission power. The test result is the test result when the phone and charger work together. The reason for failure may be caused by the charger, the phone itself, or the poor compatibility of the phone and charger when working together.

The problem may be caused by the following reasons.

The charger and mobile phone did not fully consider the electromagnetic compatibility performance in the initial design stage;

During the design, no design was made for electromagnetic compatibility with radiated and conducted disturbances and no corresponding countermeasures were taken;

The electromagnetic compatibility of the components used in the charger and mobile phone is poor or the quality does not meet the requirements;

When choosing a charger for a mobile phone, the electromagnetic compatibility and matching between the mobile phone and the charger are not fully considered. The mobile phone is a nonlinear load. When ringing or talking, if the battery is low and charging is required, a lot of energy is consumed and there will be a large impact current. If the selected charger is not matched or the output current is too small, the charger will work at full load or overload during the test, causing electromagnetic compatibility problems, and even more seriously, safety problems. In addition, if the charging is not normal, it will also cause abnormal operation of the mobile phone components and cause electromagnetic compatibility problems. Mutual interference between the charger and the mobile phone will also cause the test results to exceed the standard;

Before the test, the mobile phone and charger were not tested together for electromagnetic compatibility. The charger may have been tested for electromagnetic compatibility using only the load, and the test results cannot reflect the results of the joint test with the mobile phone.

3.3 Suggestions for improvement of issues related to radiated disturbance and conducted disturbance

(1) During the design phase, full consideration should be given to the electromagnetic compatibility characteristics and the grounding design of the circuit board should be reasonably considered. The grounding loop should be kept as small as possible, grid grounding should be used, and the signal line or power line should be as close to the ground line as possible. During the design process, filtering measures should be taken for the charger and the charging port of the mobile phone, and shielding measures should be taken for components sensitive to radiation emission, and shielding covers should be added.

(2) Select components with good quality and good electromagnetic compatibility characteristics.

(3) Optimize the location, layout, and routing of devices. Device layout has always grouped components according to function and device type. For example, for a circuit board with both analog circuits and digital devices, the devices can be grouped and laid out according to operating voltage and frequency; for a given product series or power supply voltage, the devices can be grouped by function. After the device grouping layout is completed, the power supply layer must be arranged under each device group according to the difference in the power supply voltage of the component group. If there are multiple layers of ground, the digital ground layer must be close to the digital power layer, the analog ground must be close to the analog power layer, and the analog ground and digital ground must have a common location. Usually, there are A/D or D/A devices in the circuit, and these conversion devices are powered by both analog and digital power supplies, so the converter should be placed between the analog power supply and the digital power supply. If the digital ground and analog ground are separate, they will converge at the converter. When the circuit board is grouped according to the device series and power supply voltage, the transmission of the signal within the group cannot cross other device groups. If the signal crosses the boundary, it cannot be closely coupled with its return path, which will increase the loop area of ​​the circuit, thereby increasing the inductance and reducing the capacitance, which will lead to an increase in common-mode and differential-mode interference. Various isolation zones should be avoided during the circuit board design process. Although a row of vias that are very close to each other does not violate the design rules, too many vias on the power layer and the ground layer are sometimes equivalent to opening an isolation zone. Avoid wiring in this area. For example, if a 3 ns signal loop deviates from its signal source path by 0.40 inches, the overshoot/undershoot and induced crosstalk will increase significantly, enough to cause abnormal circuit operation and increase differential-mode and common-mode interference at the same time.

(4) Fully consider the compatibility and matching between the charger and the mobile phone. The output current of the charger should be greater than the peak current of the mobile phone. Before selecting a matching charger, the corresponding charger should be used with the mobile phone to perform a pre-test of radiated interference and conducted interference to verify the electromagnetic compatibility characteristics between the two, and select a charger with good electromagnetic compatibility characteristics.

(5) Post-correction measures

Analyze the test results and listen to the advice of the electromagnetic compatibility test engineer. For the radiated interference test, confirm through the test whether the charger has a greater impact on the test results or the mobile phone has a greater impact. Generally, if the low frequency exceeds the limit, the charger has a greater impact, and if it is high frequency, the mobile phone may have a greater impact; the conducted interference test also needs to confirm which influence is the main factor.

If the influence of the charger is the main factor, first confirm whether the various components of the charger are working properly; if there is a problem with a certain component, replace the corresponding component before testing. Adding filter capacitors or improving the corresponding filter circuit will improve both radiated interference and conducted interference.

If it is confirmed that the problem is with the mobile phone, determine the source of the excess frequency and shield the corresponding components: strengthen the shielding characteristics; improve the grounding of the shield; add corresponding filter capacitors or adjust the filter circuit; improve the corresponding matching circuit to reduce harmonic or mixing interference; strengthen the filtering and grounding of the mobile phone's charging circuit, etc.

Use a good charging cable. It is recommended to use a shielded cable that can be grounded at both ends.

Adding a ferrite ring on the phone side or charger side may improve the radiated interference to a certain extent, but sometimes has little effect on the conducted interference. The corresponding frequency of the ring should be selected according to the test frequency.

In summary, the following principles should be followed for radiated disturbance and conducted disturbance:

a) Pay attention to electromagnetic compatibility design in the design stage;

b) Pay attention to the matching between the charger and the mobile phone;

c) Select high-quality components.

4 Conclusion

The electromagnetic compatibility performance of a mobile phone is directly related to all the performance of the mobile phone. Ensuring the electromagnetic compatibility performance of a mobile phone is an important part of ensuring the quality of the mobile phone. Therefore, the electromagnetic compatibility testing and design of the mobile phone cannot be ignored.