How to improve the electromagnetic compatibility of mobile phones
1. Electrostatic discharge immunity test
1.1, electrostatic discharge immunity test common problems
The problems in the electrostatic discharge immunity test are mainly reflected in the following aspects:
(1) The mobile phone call is interrupted.
(2) Electrostatic discharge causes some functions of the mobile phone to fail, but the function of failure after the end of the electrostatic discharge process or after restarting the mobile phone can be restored. These phenomena may be: the screen display is abnormal, such as the screen display is white, streaks appear, the display is garbled, the display is blurred, etc.;
There is a problem with the call effect, such as howling or the sound disappears;
Loss of key function or touch screen function;
The software has a false alarm. For example, if the charger is not plugged in, the prompt “Charging is connected and the charger has been removed” is frequently displayed.
(3), the phone automatically shuts down or restarts. This problem can occur both during a call and during standby.
(4) Electrostatic discharge causes the mobile phone to fail or be damaged.
Some of the features of the phone cannot be recovered after restarting due to some device damage, such as camera function;
Can not be turned on again after automatic shutdown;
When testing with a charger, the charger may also experience problems such as failure, damage, or explosion.
1.2, the specific analysis of electrostatic discharge problems
(1) Interrupted call: The main reason for the interruption of the call is that the electrostatic discharge affects the RF circuit and/or the baseband circuit inside the mobile phone, causing a decrease in the communication signal-to-noise ratio and a problem in the signal synchronization, resulting in a call interruption.
Unreasonable structural design can also result in interruption of calls. In the ESD test, a large-area metal horizontal coupling plate is required, and only one insulating pad with a thickness of 0.5 mm is placed between the mobile phone and the horizontal coupling plate. When the distance between the antenna or the large-area metal component is too close to the horizontal coupling plate, mutual coupling may occur, and the sensitivity actually achieved by the mobile phone is greatly reduced. The call is more easily interrupted during the electrostatic test, and even if static electricity is not applied in severe cases Interfering with the mobile phone can not keep the call.
(2), automatic shutdown or restart: the reset circuit of the baseband circuit is affected by static electricity, causing the mobile phone to be shut down or restarted by mistake.
(3), mobile phone failure or damage: high voltage and high current during electrostatic discharge lead to thermal failure or insulation breakdown of the device. It may also be affected by a strong electromagnetic field during electrostatic discharge, causing temporary failure of the device.
(4) Software failure: The electrostatic interference signal is treated as a useful signal, causing the operating system to respond incorrectly.
1.3, suggestions for improvement of electrostatic discharge problems
(1) Consider the electrostatic discharge problem in the design scheme
Try to choose a device with a high level of static sensitivity;
The device is isolated from the static power supply;
Reduce the loop area (the larger the area, the larger the field flow involved, the greater the induced current). Specific measures may include: the shorter the trace, the better; the closer the power supply is to the ground, the better; when there are multiple sets of power and ground, connect in a grid; too long signal lines or power lines must be staggered with the ground; The closer the line is to the ground line, the better; the closer the components are, the better; the closer the features are, the better;
Ground plane design: Try to use the complete ground plane on the PCB; the larger the PCB ground contact area, the better; do not have large gaps;
The ground wire of the PCB needs low impedance and good isolation;
The power supply and ground layout are better in the middle of the board than in the surrounding area;
Place a high frequency bypass capacitor between the power supply and ground;
Protect static sensitive components.
(2) Rectification suggestions after the occurrence of static electricity problems For the above electrostatic discharge problems, the following steps need to be taken to rectify.
a) Try direct discharge and indirect discharge, air discharge and contact discharge to confirm the coupling path;
b) discharge from different directions, observe the difference between the phenomena, determine all discharge points and discharge paths;
c) From low to high, test at different voltages to determine which voltage range of the mobile phone is unqualified;
d) test several prototypes, analyze commonalities, and confirm the cause of failure;
e) judging relevant sensitive devices according to the coupling path, the unqualified phenomenon, and the discharge path;
f) develop solutions for sensitive devices;
g) Verify and correct the solution through testing. The following measures can be specifically adopted in the rectification.
The problem of chassis gaps, buttons, and FPCBs can be handled by media isolation;
Problems such as camera, microphone, and earpiece can be handled by media isolation, enhanced grounding, etc.
The chip with the shielding shell can be processed by strengthening the shielding effect and strengthening the grounding of the shielding shell;
For the interface circuit and the key of the key chip, it should be protected by using protection devices (such as TVS tube, ESD protection device); for software failure, it can be improved by adding some logic judgment to correctly detect and process the alarm information. .
2. Electrical fast transient pulse group immunity test
2.1. Overview of Electrical Fast Transient Burst Immunity Test
The principle of electrical fast transient burst generation is as follows: when an inductive load (such as a relay, contactor, etc.) is disconnected, due to insulation breakdown of the switch contact gap or contact bounce, etc. Transient harassment. When the inductive load repeats the switch multiple times, the pulse group will repeat repeatedly in the corresponding time gap. This kind of transient disturbance energy is small and generally does not cause damage to the equipment, but due to its wide spectral distribution, it will affect the reliable operation of the mobile phone.
The test is a test that couples a burst of pulses consisting of many fast transient pulses to the power port of a mobile phone. The characteristics of the test pulse are: transient pulse rise time is short, repeated occurrence, low energy. The purpose of the test was to test the performance of the mobile phone in the event of such transient harassment. It is generally believed that the reason why the electric fast transient pulse group causes the malfunction of the mobile phone is that the pulse group charges the semiconductor junction capacitance in the line, and when the energy on the junction capacitance accumulates to a certain extent, the mobile phone may be mishandled. Specifically, the mobile phone communication interruption, crash, software alarm, control and storage function loss during the test.
2.2. Analysis of Common Problems in Electrical Fast Transient Burst Immunity Test
The electrical fast transient pulse waveform is directly transmitted into the handset through the charger, causing excessive noise voltage on the motherboard circuit. When the live or neutral line is injected separately, despite the common mode injection to ground, there is differential mode interference between the live and neutral lines, which will appear at the DC output of the charger. When the hot and neutral lines are injected at the same time, there is common mode interference, but the output of the charger is not greatly affected. The reasons for the problems caused by the mobile phone during the testing process are complicated, and the specific aspects are as follows.
In the early design, the electric fast transient burst suppression function was not considered. No related filter components were added. The PCB design integrated wiring also did not pay attention to cable isolation. The motherboard grounding design also did not meet the specifications, and the key components did not. Take shielding protection measures, etc.;
The manufacturer does not select key components with reliable performance in the selection of component suppliers, resulting in aging of the device or device failure during the test, which is easily interfered by electrical fast transient pulses;
In the process of assembly and assembly of the whole machine, the problems in the processing technology and assembly level may lead to poor product consistency, and there are quality problems in the individual inspection mobile phones;
During the detection process, due to problems in other test items, the rectification may be caused, and the selection of the rectification plan may affect the failure of the electric fast transient burst test.
2.3. Suggestions for improvement of related problems in electrical fast transient burst immunity test
For the problems of electric fast burst group interference test, filtering and absorption methods can be adopted to achieve the suppression of electrical fast transient pulses.
(1) In the initial stage of mobile phone design, it is important to consider the design of suppressing fast transient bursts.
In the PCB layer power input position to filter, usually the combination of size and capacitance, according to the actual situation, you can add a first-order magnetic beads to filter high-frequency signals, as far as possible to use surface packaging;
Minimize the common ground impedance of the PCB;
The PCB layout tries to keep the interference source away from the sensitive circuit;
PCB traces should be as short as possible;
Reduce the loop area;
Pay attention to the isolation of strong and weak wiring and the isolation of signal lines and power lines during integrated wiring. Integrated wiring is a very important design component of the system. A poor 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 the alarm information and restore the status of the product in time.
(2), the selection of components should use reliable quality chips, it is best to do chip-level electromagnetic compatibility simulation test, reliable quality charger, data line and battery selection can improve the fast transient pulse signal Inhibition ability
(3) In the assembly and production process, the manufacturer should strictly control the quality, control the production process, ensure the consistency of product quality, and reduce the test failure caused by the quality problems of individual mobile phones;
(4) If there is a problem during the EFT test, the motor can be rectified by adding a magnetic ring or an electric fast transient pulse group filter. The smaller the inner diameter of the magnetic bead, the larger the outer diameter and the longer the length. The better. The rectification method using the TVS tube has a limited effect;
(5) According to the latest GB/T 17626.4-2008 standard, the repetition frequency will increase the 100 kHz option, which will be more severe than 5 kHz. Manufacturers should pay attention to the related electrical fast transient burst test protection work as early as possible.
3. Radiation disturbance and conducted harassment
3.1, the specific situation of radiation harassment, conduction harassment related issues
Radiated disturbance tests are mainly unacceptable in the frequency range of 30 MHz-100 MHz and 200 MHz-900 MHz, and conducted disturbances are easily unacceptable in the frequency range of 5 MHz-30 MHz.
3.2. Analysis of related problems of radiated disturbance and conduction disturbance
The radiated disturbance and conducted disturbance test is an electromagnetic compatibility test conducted when the charger is used to charge the mobile phone while the mobile phone maintains the communication state and the maximum transmission power. The result of the test is the test result in the case where the mobile phone and the charger work together. The reason for the failure may be caused by the charger, or it may be caused by the mobile phone itself, or it may be that the compatibility between the mobile phone and the charger is not good.
The reasons for the problem may be as follows.
Chargers and mobile phones did not adequately consider electromagnetic compatibility during the initial design phase;
At the time of design, no electromagnetic compatibility for radiated disturbance and conducted disturbance is designed and countermeasures are taken;
The electromagnetic compatibility of the components selected by the charger and the mobile phone is not good or the quality is not up to standard;
When the mobile phone is selected, the electromagnetic compatibility between the mobile phone and the charger and the matching between the mobile phone and the charger are not fully considered. The mobile phone is a non-linear load, and when charging and talking, if the battery is insufficient, the charging is performed. The energy is very large, there will be a large inrush current, so if the selected charger does not match or the output current is too small, the test will cause the charger to work at full load or overload and cause electromagnetic compatibility problems, even more serious. There will be security issues. In addition, if the charging is not normal, it will cause the mobile phone device to work abnormally and cause electromagnetic compatibility problems. Mutual interference between the charger and the mobile phone will also cause the test result to exceed the standard;
Before the test, the mobile phone and the charger did not cooperate with the electromagnetic compatibility pre-test. The charger may use the load alone to do the electromagnetic compatibility test, and the test result cannot reflect the result of the joint test with the mobile phone.
3.3. Suggestions for improvement of radiation disturbance harassment related problems
(1) In the design stage, the electromagnetic compatibility characteristics should be fully considered, and the grounding design of the circuit board should be properly considered. The grounding loop should be kept as small as possible. Use the grid grounding, and the signal line or power line should be as close as possible to the ground. During the design process, filtering measures are taken on the charging ports of the charger and the mobile phone, shielding measures are taken for the radiation-emitting sensitive components, and the shielding cover is added.
(2) Select components with good quality and good electromagnetic compatibility.
(3) Optimize the position, layout and wiring of the device. Device layouts are always grouped by function and device type. For example, for boards with both analog and digital devices, the devices can be grouped by operating voltage and frequency; for a given product line or The supply voltage, which groups the devices by function. After the device grouping is completed, the power layer must be placed under each device group according to the difference in the power supply voltage of the component group. If there are multiple layers, then the digital ground plane must be placed close to the digital power layer, and the analog power layer should be closely placed in analog, and the analog ground and the digital ground should have a common location. Typically, there are A/D or D/A devices in the circuit that are powered by both analog and digital supplies, so place the converter between the analog and digital supplies. If the digital ground and analog ground are separate, they will merge at the converter. When the board is grouped according to the device family and the supply voltage, the signals in the group cannot be transmitted across the other device group. If the signal crosses the limit, it cannot be tightly coupled to its return path, which increases the loop area of the circuit. The inductance increases and the capacitance decreases, which in turn leads to an increase in common mode and differential mode interference. Avoid various isolation bands during board design. Although a row of through holes that are close together does not violate the design rules, too many vias on the power plane and the ground plane are sometimes equivalent to opening an isolation strip, to avoid wiring in this area, for example, a 3 ns If the signal loop deviates from its source path by 0.40 inches, the overshoot/undershoot and induced crosstalk will increase significantly enough to cause abnormal operation of the circuit and increase differential mode and common mode interference.
(4) Fully consider the compatibility and matching of the charger and the mobile phone. The output current of the charger should be greater than the peak current of the phone. Before selecting a matching charger, the corresponding charger should be used with the mobile phone for pre-test of radiated disturbance and conducted disturbance, verify the electromagnetic compatibility between the two, and select a charger with good electromagnetic compatibility.
(5), late rectification measures
Analyze the test results and listen to the recommendations of the EMC test engineers. For the radiation disturbance test, it is confirmed by experiments that the influence of the charger on the test result is large or the influence of the mobile phone is large. Generally, if the low frequency exceeds the limit, the influence of the charger is greater. If it is high frequency, the influence of the mobile phone may be large; the conducted disturbance test also confirms which influence is the main factor.
If the influence of the charger is the main factor, first confirm whether the various devices of the charger are working properly; if there is a problem with a certain device, replace the corresponding device before testing. Increasing the filter capacitance or improving the corresponding filter circuit will improve both radiated and conducted disturbances.
If it is confirmed that the problem is the mobile phone, determine the source beyond the frequency, and shield the corresponding device: strengthen the shielding characteristics; improve the grounding of the shielding; increase the corresponding filter capacitor or adjust the filter circuit; improve the corresponding matching circuit to reduce the harmonics Or mixing interference; enhancing the filtering and grounding of the charging circuit of the mobile phone, and the like.
With a good charging cable, it is recommended to use a shielded cable that can be grounded at both ends.
Adding a ferrite magnetic ring on the mobile phone side or the charger side may have some improvement for radiated disturbance, and sometimes has little effect on the conducted disturbance. According to the frequency of the test, the corresponding frequency of the magnetic ring is selected.
In summary, for radiation disturbance and conducted disturbance, the following principles should be grasped:
a) focus on the EMC design of the design phase;
b) pay attention to the matching of charger and mobile phone;
c) Choose good components.
The electromagnetic compatibility of mobile phones is directly related to the performance of mobile phones. Ensuring the electromagnetic compatibility of mobile phones is an important part of ensuring the quality of mobile phones. Therefore, the electromagnetic compatibility test and design of mobile phones cannot be ignored.