Analysis of the causes of electromagnetic radiation interference generated by switching regulated power supplies

The operating frequency of the switching regulated power supply is mostly 20-100kHz, which belongs to the super-audio range. As the switching adjustment device of the power supply, the transistor or field effect transistor works in the on and off state at the corresponding frequency. The oscillation waveform is similar to a square wave (there is also overshoot). According to the Fourier analysis method, it can be decomposed to obtain the DC component, fundamental wave and high-order harmonics. The fundamental wave has the largest energy, followed by the third, fifth, seventh... and so on.

2. Relationship between radio broadcasting and electromagnetic interference

As we all know, radio broadcasting uses modulation to transmit information. The audio signal is modulated by amplitude modulation (AM) and frequency modulation (FM) on the high frequency carrier, and then the modulated wave is radiated in the form of electromagnetic waves through the transmitting antenna. The radio receiving device receives them through the receiving antenna, and then selects, converts, amplifies and demodulates them to restore them to audio signals. Finally, the sound is released by the speaker through the low-frequency amplifier. If there is only a high-frequency carrier without a modulated signal in the audio range, no matter how large its energy is, the radio receiving device will not restore any sound information through the speaker. From this, it can be imagined that if there is only super-audio interference (the lower limit frequency of super-audio oscillation is 15k! Felt, which is beyond the audible range of the human ear), the high-order harmonics generated will not become interference signals in the audio range that we hear through the radio. In fact, this interference is sometimes very serious, and strong noise may appear in the entire medium wave and short wave range. So where does the interference come from? The switching power supply has interference signals in the audio range, and modulates the harmonics of super-audio oscillation, interfering with radios and other equipment in a way similar to radio broadcasting, which is an important source of interference.

3. Characteristics of direct rectifier capacitor filter circuit

A high-frequency switching power supply that directly rectifies and filters the AC power supply has an AC input voltage of 220V, 50Hz. A bridge rectifier circuit is generally used, and then filtered by an electrolytic capacitor to become a 100Hz sawtooth pulsating DC voltage, which is provided to the switching power supply.

As can be seen from the figure, the function of the AC power supply is to charge the electrolytic capacitor once every Wms to replenish the energy consumed by the switching circuit. The voltage waveform is sawtooth-shaped, but the current waveform is pulse-shaped. They are all non-sinusoidal periodic quantities, containing rich harmonics, and most of them are in the audio range (20-20000Hz), among which the 100Hz component is likely to be an important source of audio interference.

Similarly, according to Fourier analysis, this non-sinusoidal periodic function can be decomposed into fundamental wave and higher harmonics, whose energy is mainly concentrated near the fundamental wave, and the energy of higher harmonics is very small. Since its peak-to-peak value is small (only a few volts to a few carbovolts), if it can be transmitted in the form of electromagnetic waves through certain circuit elements (such as LC networks, etc.), the interference to radio and other radio receiving equipment is relatively small. Taking the 1000kHz frequency in the medium wave band (531-1611kHz) as an example, the 1kHz harmonic in the 1∞Hz pulse-shaped current resonates with it, and the energy at this frequency is almost zero. Unless the radio is placed close to it, there will be no interference. But in fact, many radios can still have interference at a distance from the switching power supply. What is the reason? The following analysis can be used to draw a conclusion. Using a low-frequency signal to modulate the amplitude of the high-frequency carrier so that the overall envelope of the high-frequency carrier has the same change law as the low-frequency signal is amplitude modulation, and the corresponding modulated wave is the amplitude modulated wave. If the low-frequency signal is a single-frequency sine wave, and the high-frequency carrier is also a single-frequency sine wave, the two can be modulated to obtain an amplitude modulated wave. According to the principle of amplitude modulation, when the amplitude ratio of the low-frequency signal to the high-frequency carrier signal is less than 1:3, the envelope of the carrier is basically similar to the low-frequency signal. If the low-frequency signal is multiple frequencies, the spectrum of the amplitude modulated wave is the carrier and the upper and lower sidebands. The low-frequency pulse amplitude of the switching power supply is small, while the amplitude of the high-frequency switching pulse is large. This amplitude modulation effect will inevitably occur inside the switching power supply. Therefore, the electromagnetic radiation generated by the power supply is an amplitude modulated interference wave, because both contain rich fundamental waves and harmonics. As a result of cross-modulation, the interference is spread over a wide frequency range above the super-audio frequency, so strong interference sounds can be heard in the medium and short wave ranges, and this interference is almost spread over the entire band.

In order to further verify the existence of the aforementioned problem, a DC regulated power supply was used instead of the AC power supply to directly power the switching power supply. The test results were just as expected, that is, since there was no 100Hz charging pulse, no noise could be heard from the radio no matter where it was placed.

Based on the above test, we can draw the following conclusions: The interference of the switching power supply to the radio broadcast mainly comes from the 100Hz low-frequency charging pulse and its harmonics modulating the high-frequency square wave to form a complex amplitude modulation wave. Since the high-frequency switching square wave is strong and difficult to suppress, the capacity of the high-frequency filter capacitor should not be blindly increased.

This reminded us of the problem of fluorescent lamp electronic ballasts polluting the electromagnetic environment, so corresponding tests were also conducted on fluorescent lamp electronic ballasts. Although the fluorescent lamps could be lit, the problem that followed was that it was almost impossible to receive broadcasts on the radio under the fluorescent lamps, as the radio signals were drowned out by the noise. In addition, since the load of the electronic ballast was a large fluorescent lamp exposed in space, the interference electromagnetic waves radiated by it were very strong and the noise was very serious.

4. Anti-interference measures

In summary, the electromagnetic radiation interference of the switching regulated power supply mainly comes from the amplitude modulation of the super-audio oscillation square wave and its harmonics by the low-frequency 100Hz charging pulse and its harmonics. The main ways to reduce it are as follows:

(1) Use filter electrolytic capacitors of appropriate capacity. The main considerations should be anti-interference and performance-price ratio.
62); font-family: Tahoma, Arial, sans-serif; font-size: 14px; text-align: justify; ">(2) Where possible, use appropriate methods to reduce the overshoot and oscillation of the ultrasonic oscillation square wave waveform. The purpose of this is mainly to reduce the high-order harmonic energy of the ultrasonic oscillation, thereby reducing the degree of radio interference.

(3) Use a relatively complete filter network to reduce the interference of the switching power supply to the AC mains. There are many forms of this filter network, and the circuit form can be selected based on the specific power supply and load.

(4) Rationally design the printed circuit board, adopt large-area grounding measures, and strengthen shielding. This can effectively reduce the intensity of electromagnetic radiation caused by distributed capacitance and also improve the reliability of the power supply.