Discussion on the electromagnetic interference of LED

LED TV general lighting fixtures usually use pulse width modulation (PWM) technology to achieve constant current power supply and dimming functions of LED, which will generate a certain degree of radio interference and harmonics. This article tests and studies some LED lamps to grasp the relevant data on electromagnetic interference that may be generated by LED lamps during use, and proposes corresponding solutions.

1 Principle and purpose

LED light source is a low-voltage DC device. Therefore, LED lamps need a power supply that can convert 220 V AC into low-voltage DC that meets the working requirements of LED. However, stage and film lamps need to have dimming functions, so their power supply is more complicated. The mainstream power supply devices for studio LED lamps are:

(1) The front stage uses a common pulse width modulation (PWM) switching power supply as a low-voltage DC power supply, and the switching frequency is usually tens of kilohertz.

(2) The latter stage uses chopping to perform low-frequency pulse width modulation, and adjusts the brightness of the lamp by adjusting the duty cycle. This part of the circuit is usually developed by the lamp manufacturer itself, and the switching frequency varies greatly from factory to factory. It usually ranges from hundreds of hertz to tens of kilohertz.

In the pulse width modulation mode, the power supply device will produce high-order harmonics when it chops the current, which will cause radio interference to the audio, video system and other weak current systems in theaters and studios, and may affect the normal operation of other equipment. In addition, since the rectifier circuit and capacitor filter circuit in the power supply device will distort the power frequency current, the low-order harmonics generated will pollute the power grid and affect the operation of other electrical equipment.

Therefore, it is necessary to conduct a series of tests on LED lamps: test according to the national standard GB17743-2007 "Limits and measurement methods for radio disturbance characteristics of electrical lighting and similar equipment" to measure whether their radio disturbance characteristics meet the requirements of this standard; test according to the national standard GB 17625.1-2003 "Electromagnetic compatibility limits harmonic current emission limits (equipment each phase input current ≤ 16A)" to test whether their harmonic current meets the requirements of this standard.

Through the experiments and research of this topic, we can understand the relevant data on electromagnetic interference that may be generated by LED lamps when used in the studio, and propose corresponding solutions.

2 Experimental content and methods

2.1 Experimental conditions

This study requires various professional electromagnetic compatibility test equipment, such as power analyzers, artificial power networks, conducted test receivers, radiated interference receivers and three-loop antennas. Some tests also need to be carried out in a radio darkroom. Therefore, it is necessary to entrust a professional testing agency approved by relevant national departments to carry out the testing.

2.2 Test lamps

There are 2 categories and 5 models of test lamps. There is 1 lamp for each model, a total of 5 lamps. See Table 1 for details.

Table 1 Types and numbers of test lamps

2.3 Experimental content

(1) According to the requirements of the national standard GB 17743-2007, find the maximum interference point generated during the dimming process and measure the radio interference value at this point.

(2) According to the national standard GB 17625.1-2003, the lamps are divided into five equal levels between the minimum power and the maximum power, and the harmonic components of each level are tested.

2.4 Experimental methods and procedures

First age the lamp to be tested for 100 hours, and then send it for inspection after the parameters of the lamp are stable.

2.4.1 Radio disturbance test

2.4.1.1 Conducted disturbance voltage test

Tests are carried out at the power supply and control ends of the lamps according to the requirements of the national standard GB 17743-2007.

(1) The lamp power supply end test is carried out in the following five frequency bands: 9 kHz to 50 kHz; 50 kHz to 150 kHz; 150 kHz to 0.5 MHz; 0.5 MHz to 5.0 MHz; 5.0 MHz to 30 MHz.

(2) The lamp control terminal test is carried out in the following two frequency bands: 150 kHz ~ 0.5 MHz; 0.5 MHz ~ 30 MHz.

2.4.1.2 Radiated electromagnetic disturbance test

The test is carried out according to the requirements of national standard GB 17743-2007.

The test is carried out according to the requirements of national standard GB 17743-2007.

(1) The test in the frequency range of 9 kHz to 30 MHz is carried out in the following four frequency bands: 9 kHz to 70 kHz; 70 kHz to 150 kHz; 150 kHz to 3.0 MHz; 3.0 MHz to 30 MHz. Figure 1 is a schematic diagram of the test device.

Figure 1 Schematic diagram of the radiated electromagnetic disturbance test device in the frequency range of 9 kHz to 30 MHz

(2) In the anechoic chamber, the test of the frequency range of 30 MHz to 300 MHz is carried out in the following two frequency bands: 30 MHz to 230 MHz; 230 MHz to 300 MHz. Figure 2 is a schematic diagram of the radiated electromagnetic disturbance test device in the frequency range of 30 MHz to 300 MHz.

Figure 2 Schematic diagram of the radiated electromagnetic disturbance test device in the frequency range of 30 MHz to 300 MHz

The radio disturbance characteristic limits for the above tests are given in GB 17743-2007.

2.4.2 Harmonic test

According to the requirements of GB 17625.1-2003 for Class C equipment and built-in dimmer lamps, the harmonic components of the lamps are tested. There are 5 equal levels between the minimum power and the maximum power, that is, each harmonic component is tested under 20%, 40%, 60%, 80% and 100% power conditions. The range of the test harmonic order n is:

n = 2, 3, 5, 7, 9 and 11 ≤ n ≤ 39 (odd harmonics only)

FIG3 is a schematic diagram of a harmonic current testing device.

Figure 3 Schematic diagram of harmonic current test device

The harmonic current emission limits for the above tests are specified in GB 17625.1-2003.

3 Experimental Results and Analysis

Lamps 1#, 3# and 4# were selected from the lamp samples 1# to 5# in this study and sent to Beijing Household Electrical Appliances Quality Supervision and Inspection Station for testing. However, the test results of each lamp exceeded the limit values ​​specified in the standards to varying degrees. For this reason, the lamp factory made technical improvements to the lamps, and the improved lamp 1# was re-tested and passed the tests of both standards.

The 5# and 2# lamps were sent to relevant testing institutions in Guangzhou for testing, and both have passed the tests of two standards.

The following is an analysis of the main test parameters of three of the lamps.

3.1 1# lamp test results

3.1.1 Measurement of radio disturbance characteristics

(1) Conducted disturbance voltage test

A. The power supply frequency of the lamp is in the range of 9 kHz to 30 MHz, and the conducted disturbance voltage (quasi-peak value) is shown in Figure 4.

Figure 4 Conducted disturbance voltage (quasi-peak value) in the range of 9 kHz to 30 MHz at the power supply end

B. The conducted disturbance voltage (average value) at the power supply end of the lamp is in the range of 9 kHz to 30 MHz. See Figure 5.

Figure 5 Conducted disturbance voltage (average value) in the range of 9 kHz to 30 MHz at the power supply end

(2) Radiated electromagnetic disturbance test

A. Radiated electromagnetic disturbance in the frequency range of 9 kHz to 30 MHz According to GB 17743-2007, the lamp should be tested in three mutually orthogonal loop antenna systems, and each loop antenna is tested for radiated electromagnetic disturbance. Figure 6 shows the radiated electromagnetic disturbance value of the horizontal loop antenna test.

Figure 6 Radiated electromagnetic disturbance value of horizontal loop antenna test

B. Radiated electromagnetic disturbance within the frequency range of 30 MHz to 300 MHz is tested using the CDN method. The limit value should comply with the corresponding requirements of GB 17743-2007. The test results are shown in Figure 7.

Figure 7 Radiated electromagnetic disturbance tested by CDN method in the frequency range of 30 MHz to 300 MHz

The above test data is based on the test unit's information that it is about 42% of the full power (the control data is hexadecimal 6C, the maximum value is FF) and the radio interference is at its maximum. According to the above test data, the conducted interference voltage and radiated electromagnetic interference at the power supply end of the 1# lamp meet the requirements of GB 17743-2007.

Note: This test does not test the conducted interference voltage at the control end.

3.1.2 Harmonic current measurement

According to the requirements of GB 17625.1-2003, the power range between the minimum power and the maximum power should be divided into 5 equal stages, that is, each harmonic component should be tested under 20%, 40%, 60%, 80% and 100% power conditions. The test unit only provides test data when the interference is maximum at 20% and other conditions.

(1) At 20% of the full power, i.e. around 24 W, the test data is shown in Figure 8.

Figure 8 Harmonic current at 20% of full power

(2) Except for 20% of full power, the test data at the maximum harmonics at other powers are shown in Figure 9.

Figure 9 Harmonic currents at maximum harmonics at powers other than 20% of full power

According to the above test data, the harmonic current of 1# lamp meets the requirements of GB17625.1-2003. At low power (20%), although there are relatively large even harmonics, GB 17625.1-2003 does not limit even harmonics other than the 2nd order.

This is also the reason for the large distortion of the current waveform (see Figure 10), and should be taken seriously.

Figure 10 Current waveform of 1# lamp

3.2 5# lamp test results

3.2.1 Measurement of radio disturbance characteristics

(1) Conducted disturbance voltage test.

A. The frequency of the power supply end of the lamp is in the range of 9 kHz to 30 MHz for the conducted disturbance voltage (quasi-peak value), see Figure 11.

Figure 11 Conducted disturbance voltage (quasi-peak value) at the power supply end of the lamp in the frequency range of 9 kHz to 30 MHz

B. The control terminal frequency of the lamp is within the range of 150 kHz to 30 MHz. See Figure 12.

Figure 12 Conducted disturbance voltage (quasi-peak value) at the lamp control end in the frequency range of 150 kHz to 30 MHz

(2) Radiated electromagnetic disturbance test

A. The lamp radiates electromagnetic disturbance in the frequency range of 9 kHz to 30 MHz.

According to GB 17743-2007, the lamp should be tested in three mutually orthogonal loop antenna systems, and each loop antenna should be tested for radiated electromagnetic disturbance. Figure 13 shows the radiated electromagnetic disturbance value of one of the loop antenna tests.

Figure 13 Radiated electromagnetic disturbance in the frequency range of 9 kHz to 30 MHz

B. The lamp radiates electromagnetic disturbance in the frequency range of 30 MHz to 300 MHz.

According to the above test data, the harmonic current of the 5# lamp all meets the requirements of GB17625.1-2003. In the five power tests, the 11th harmonic is relatively large, but all below the limit. And the current waveform distortion of this lamp is relatively small. See Figure 20.

Figure 20 Current waveform of 5# lamp

3.3 Test results of 3# lamp

3.3.1 Measurement of radio disturbance characteristics

The power supply frequency of the lamp is in the range of 9 kHz to 30 MHz, and the conducted disturbance voltage (quasi-peak value) is shown in Figure 21.

Figure 21 Conducted disturbance voltage (quasi-peak value) at power supply end with a frequency range of 9 kHz to 30 MHz

The above data show that the disturbance voltage of 3# lamp at 631.5 kHz and 771 kHz exceeds the limit of GB 17743-2007.

3.3.2 Harmonic current measurement

The lamp was tested for harmonics at full power (50 W). The test results are shown in Table 3.

Table 3 Results of harmonic test of 3# lamp at full power (50 W)

From Table 3, we can see that most of the harmonic currents exceed the requirements of GB17625.1-2003. The "Fail" in the table means failure. In addition, the power factor of the lamp is extremely low, only 0.455.

Note: This lamp is only tested at full power. It is not divided into 5 equal stages between minimum power and maximum power as required by GB17625.1-2003, that is, the harmonic components are not tested at 20%, 40%, 60%, 80% and 100% power conditions.

4 Conclusions and Recommendations

4.1 Conclusion

LED lamps will generate a certain degree of radio interference and harmonic waves when working, but as long as the lamps are equipped with a power supply device with good electromagnetic compatibility performance, the electromagnetic interference can be controlled within the range permitted by the standards. Lamps that meet the relevant standards will not affect the normal operation of other equipment in the studio and on the stage.