The effectiveness of electromagnetic compatibility rectification in 3C certification

Since August 1, 2003, China's electronic product quality compulsory certification system (3C certification) has been in place for nearly three years. The state has taken corresponding compulsory measures, and products without 3C certification marks are not allowed to be sold. At present, most electronic products have been included in the compulsory certification product catalog, and regular sales merchants have also responded positively. Electronic products that have not passed the 3C certification have basically been cleared out of the market, and the consumer safety of the general public has been legally protected.

However, in the annual certification review and market sampling inspection, there are still a considerable number of products that have been affixed with the 3C mark but still found to be unqualified. Among them, the inconsistency between the electromagnetic compatibility project and the certification sample inspection results is the highest. At present, the inconsistency between the 3C certification samples and the electromagnetic compatibility of products in daily production is an important factor affecting the effectiveness of 3C certification. After analysis and research, we found a common phenomenon that the consistency of the 3C review and market sampling inspection results of samples that passed the 3C certification test once is high, and the consistency of the 3C review and market sampling inspection results of samples that have undergone repeated rectification of safety or electromagnetic compatibility items during the 3C certification test is low. Therefore, it can be said that the current low effectiveness of electromagnetic compatibility rectification is a key factor affecting the effectiveness of 3C certification. This article analyzes in detail the characteristics that affect the effectiveness of the electromagnetic compatibility rectification process, and explains in detail how to use effective electromagnetic compatibility means to solve problems and choose the best solution in each rectification process.

(I) Electromagnetic compatibility rectification affects safety performance

When doing electromagnetic compatibility rectification of the power supply part, it often affects the safety performance of the product, especially when doing conducted interference rectification. We often increase the capacity of the filter capacitor, because the larger the capacitor, the better the suppression effect on low-frequency interference. At the same time, the leakage current through the capacitor will increase, and the residual power on the capacitor will increase, resulting in failure of the leakage current and residual voltage test of the power plug. Electromagnetic compatibility rectification that affects safety performance is invalid. Therefore, it is required that the electromagnetic compatibility rectification engineer must re-test the safety performance after rectifying the safety parts such as X capacitors and Y capacitors. In addition, the following empirical rules can be referred to, otherwise the effectiveness of electromagnetic compatibility rectification cannot be guaranteed.

1. The relationship between the capacitance of the X capacitor and the safety standard charging and discharging voltage:

1. The charging and discharging voltage constant = the bleeder resistor (megaohm) * the capacitance of the X capacitor (uF).
2. The charging and discharging voltage constant must be controlled to be less than 1 to meet the safety requirements. )
3. When the X capacitor is added for the requirements of electromagnetic compatibility, the bleeder resistor must be increased or the bleeder resistor resistance must be appropriately reduced.

2. The relationship between the capacitance of the Y capacitor and the safety standard leakage current:

1. Leakage current = input voltage/Y capacitor capacitance (capacitance * 2π?).
2. The safety standard requires that the leakage current of 3-plug grounded products is less than 3.5mA.
3. The safety standard requires that the leakage current of 2-plug non-grounded products is less than 0.25mA. 4.
The safety standard requires that the leakage current of handheld products is less than 0.75mA.
5. Pay attention to the relationship between the total capacity and leakage current of the Y capacitors of the above different categories of products. The following simple algorithm can be used:
1000pF=0.1mA

. Effective electromagnetic compatibility rectification should not affect the safety performance of the product.

(II) Electromagnetic compatibility rectification affects the performance indicators of electrical appliances.

When we suppress interference signals on power lines and signal lines, we often take the approach of adding ferrite beads, winding magnetic rings on the lines, and adding filter capacitors and RC circuits on the signal lines. These methods can effectively suppress the transmission of interference signals on the signal lines, but also cause some attenuation and distortion to the useful signals transmitted on the signal lines. When different signal lines pass through the same magnetic ring and are wound together, different signals will be coupled with each other through the magnetic ring to produce stringing. Improper use of these rectification measures will reduce the performance indicators of the product or even make it unqualified.

Case 1: The DVD produced by a factory was identified as an unqualified product in the market sampling inspection conducted by the Municipal Product Quality Supervision and Inspection Institute. The factory was required to rectify the product within a time limit. We accepted the factory's commission to rectify the product.

The unqualified items are: electromagnetic compatibility and electrical performance as shown in the following table

No. Test item Unit Standard Requirement Test result Single evaluation
1 Disturbance power dBpW 30~300MHz
Quasi-peak limit: 45~55
Average value limit: 35~45 AV line
81 42.8 42.7*
108 40.6 39.4*
135 42.0 40.0* Unqualified
2 Audio signal-to-noise ratio dB ≥80 58 59 Unqualified
3 Audio distortion plus noise dB ≤-60 -51 -51 Unqualified
4 Crosstalk (fundamental wave) dB ≥70 (1KHz) 65 65 Unqualified

According to our experience, the performance indicators of DVDs mainly depend on the decoding scheme adopted. At present, the performance parameters of the decoding boards provided by the decoding solution suppliers can meet the standard requirements. As long as DVD manufacturers assemble according to the reference circuit provided by the decoding solution suppliers, there will generally be no performance indicator problems. However, the electromagnetic compatibility of each decoding solution supplier is quite different. At the same time, the choice of power supply circuit by DVD manufacturers, whether the design of input/output port circuit is well matched with the decoding circuit and the installation process of the whole machine are important factors affecting the electromagnetic compatibility of the whole machine.

We disassembled the prototype and saw that many electromagnetic compatibility rectification measures had been taken. The connection wires from the decoding board to the control board, input/output board, and power board were all covered with magnetic rings. The grounding installation of the decoding board used a plastic card installation. A long wire was connected from the ground point to the power ground. The input/output port circuit was grounded and had some 103P filter capacitors. The power circuit and the input/output port circuit shared a PCB board. The AV terminal and the metal shell were grounded, but the shell had insulating paint and the AV terminal ground was not reliably connected to the metal shell ground.

We preliminarily concluded that the performance indicators were unqualified due to the electromagnetic compatibility rectification, and at the same time, some rectification measures were not in place, so the electromagnetic compatibility rectification did not achieve the expected effect. First, we removed the added magnetic ring and filter capacitor, separated the power circuit and the input/output port circuit, and tested the signal-to-noise ratio to 85 dB, and the interference power exceeded the standard even more.

After analysis, we determined that the reason for the failure of the prototype was that the 27MHz clock harmonic radiation of the decoder board was large, which was the main reason for the failure of electromagnetic compatibility. The poor decoupling of the power circuit and the poor isolation of the AV input/output port circuit, the added magnetic ring on the AV signal line and the filter capacitor on the AV port caused the AV signal to be distorted and strung together, which was the main reason for the failure of performance indicators.

After the test, we made the following adjustments to the electromagnetic compatibility rectification plan: the crystal oscillator ground of the decoder board is connected to the metal shell with a ground wire nearby, the grounding screws between the AV terminal and the metal shell are replaced with sawtooth contact surfaces, which can pierce the insulating paint and reliably connect to the metal shell when tightened, and the 101P filter capacitor is connected to the ground of the AV board input signal line, and the decoupling circuit composed of 10mH inductor and 103P capacitor is added to each voltage output of the power board. The magnetic ring on the connection line from the decoder board to the control board and the power board is retained, and the magnetic ring on the AV signal line is removed. After the above rectification plan is adjusted, the interference power is tested to be qualified, the signal-to-noise ratio is 85 dB, and the prototype is sent to the Municipal Product Quality Supervision and Inspection Institute for review and all test items are qualified.

Through this case, we have learned a lesson that electromagnetic compatibility rectification that affects the performance indicators of electrical appliances is invalid, and effective electromagnetic compatibility rectification should not affect the performance indicators of the product.

(III) Electromagnetic compatibility rectification affects installation processability

In electromagnetic compatibility rectification, grounding and shielding measures are often taken, and components such as resistors and capacitors are connected in parallel or in series in the circuit. However, if the installation position of these rectification measures is not designed in the original PCB board, it will definitely affect the installation processability, and the efficiency of the production line will decrease. Some rectification measures cannot even be implemented on the production line. For example, the method of sticking shielding paper inside the machine to suppress radiation interference is difficult to do during production, and it cannot maintain repeatability. When these rectification measures conflict with production, manufacturers generally choose to give up electromagnetic compatibility and ensure the processability of production, so this rectification becomes a decoration that only copes with 3C certification testing.

In the above case, we used the measure of grounding the decoder board nearby. One end of the grounding was welded to the ground next to the 27MHz crystal oscillator with enough welding area. When considering the installation process, we drilled a mounting hole on the bottom plate at the other end, fixed the welding piece firmly on the bottom plate with self-tapping screws, and welded the other end of the ground wire. At the same time, we changed the original screws of the AV terminal to serrated screws, which was also considered to achieve a good grounding effect without changing the installation process. The electromagnetic compatibility rectification that affects the installation process is invalid. Effective electromagnetic compatibility rectification should not affect the installation process of product production.

(IV) Electromagnetic compatibility rectification affects product reliability .

The filtering measures taken in the electromagnetic compatibility rectification are most likely to affect the reliability of the product. The more filter capacitors are added, the greater the probability of circuit distortion and short circuit caused by capacitor failure. Therefore, when rectifying, it is not the case that the more filter elements are added, the better. Instead, the location and propagation path of the disturbance should be accurately found. When selecting filter elements, the parameters such as withstand voltage and current should be considered to meet the principle of high reliability. After adding rectification elements, the adverse effects on the reliability of the original circuit should also be considered. For example, adding filter capacitors to the switch tube of the switching power supply or increasing the capacitance value of the absorption circuit can effectively reduce the electromagnetic radiation of the switch tube, but it also increases the loss of the switch tube and the heat generation. Therefore, it is also necessary to consider whether the original heat sink area is sufficient.

Here is a case analyzed in the article "Analysis of Case Studies on Solutions to Harmonic Current in Televisions" published in the October 2005 issue of this magazine. The rectification of harmonic current in CRT televisions generally adopts a low-frequency choke harmonic current suppression filtering solution because this solution is simple, practical and cheap. Televisions that fail the 3C certification test for harmonic current will definitely pass the test as long as this harmonic choke is added. However, there is no time to consider whether they can be produced or if there are other problems during long-term use during the 3C test. The article "Analysis of Case Studies on Solutions to Harmonic Current in Televisions" describes that the addition of this harmonic choke produces switching shocks, reduced surge resistance, and reduced drop resistance, which seriously affect the reliability of the entire machine. If these problems are not discovered and solved during certification and production, there may be greater quality problems in the hands of customers. Some manufacturers have passed the 3C certification test with this solution, but have not used it much in production. This is also due to this consideration. Therefore, electromagnetic compatibility rectification that affects product reliability is invalid. Effective electromagnetic compatibility rectification should not affect product reliability.

When manufacturers find that these EMC rectification measures are ineffective during product production, they will not hesitate to remove or adopt other alternatives without the opportunity to retest with the testing agency, which leads to inconsistency between the rectified certified samples and the produced products. The characteristics of the effectiveness of EMC rectification discussed above: no impact on safety, electrical performance indicators, installation process, and reliability are the basic characteristics we have summarized based on actual experience. Of course, there will be other characteristics for different products, such as the lowest cost, etc. The highest level of EMC rectification is to integrate the rectification into the entire product design plan, which can not only reduce costs but also improve product performance. This topic is specially discussed in another article. In fact, the effectiveness of EMC rectification has directly affected the effectiveness of 3C certification!