The switching power supply operates by turning on and off the semiconductor device, and controls the output voltage by changing the ratio of the on and off times.
It usually operates at a switching frequency above 20 kHz. When the switch is turned off, a surge voltage L (di/dt) is generated across the switch. When the switch is turned on, a surge current C (di/) flows through it. Dt), forming a strong source of electromagnetic interference. As switching power supplies move toward modularity, the switching frequency will increase by the order of magnitude and electromagnetic interference will be more severe. Therefore, measures must be taken to improve the electromagnetic compatibility (EMC) of switching power supplies.
1 Switching power supply noise interference causes and propagation path Switching power supply noise interference is high-frequency oscillation noise and surge noise, and its conduction mode is characterized by normal mode noise and common mode noise, and also radiates noise to the surrounding space. Combined with the block diagram of the switching power supply circuit, the main causes of noise interference and the propagation path of noise interference are analyzed.
1.1 Switching Circuit Noise Interference The switching circuit is the main source of high frequency noise interference. The switch tube T operates in a high-frequency on-off state. Due to the charge of the pulse transformer B, the primary coil L1 and the parasitic capacitance stored in the switch tube, a surge voltage is generated across the switch tube, a surge current flows, and the TL1C5 constitutes a high The frequency current loop will generate electromagnetic radiation noise interference, and the high frequency current will pass through the primary rectification loop to mix the common mode noise into the AC power supply. In order to reduce the tube temperature and improve the efficiency, the switch tube needs to rely on a large radiator. An insulating material is added between the switch tube and the heat sink, and noise interference generated by the switch circuit is transmitted through the distributed capacitor C0 to the heat sink. The presence of the primary and secondary distributed capacitance Ci of the pulse transformer makes high-frequency noise interference easily transmitted between the primary and secondary. On the one hand, the high-frequency side high-frequency noise is directly coupled to the rectification side, causing common-mode noise at the DC output. On the other hand, the noise interference generated by the secondary high-frequency rectifier diode is coupled to the input high-voltage side, causing the AC grid to be subject to noise interference, as shown.
1.2 Secondary rectification loop noise Interference The high-frequency rectifier diode D5 operates in the high-frequency on-off state, and the high-order harmonic current forms a high-frequency current loop through the pulse transformer primary winding L2, the rectifier diode D5, and the filter capacitor C6. On the one hand, electromagnetic radiation noise interference will occur, and on the other hand, high frequency harmonic current will interfere with the DC output with common mode noise. Since the switch tube works in the high-frequency on-off state, the rectifier diode D5 and the freewheeling diode D6 are also in the high frequency. 1 Author: Introduction: Yang Zhimin, Shaanxi, Westerner\Engineer! Engaged in circuit design application research gHouse.Allrightsreserved, http:// on-off status. Due to the reverse recovery time of the diode, it will easily cause a short circuit of a large current transient, forming a source of noise interference. In the high current rectified forward switching power supply, the freewheeling diode is one of the most influential. When the energy storage inductor L releases energy through the freewheeling diode, diode D6 should be completely turned off before the forward voltage of the pulse transformer arrives to ensure a normal output path. When the freewheeling diode flows through a large current and the reverse voltage is applied, D6 cannot be instantaneously turned off due to the charge storage effect. It takes a period of recovery time to enter the off state, and a pulse transformer secondary short circuit is formed during the remote recovery time. D5, D6 flow a large short-circuit current /, while the primary and the switching tube generate current spikes. As shown. The network can slow down the rate of voltage rise between the collector and the emitter. As shown.
(3) Serially connect the saturable coil. The freewheeling diode D6 is connected in series with the saturable core coil. When D6 flows through the reverse current, the core coil generates a back EMF to prevent the reverse current from rising and prevents the pulse transformer from being short-circuited.
The RCD network 22 attenuates the propagation factor to destroy and attenuate the interference noise propagation path and the propagation medium, and can effectively attenuate the propagation factor.
Attenuate noise interference caused by distributed capacitive coupling. Use isolation. An electrostatic isolation layer is inserted between the switch tube and the heat sink, and between the primary and secondary of the pulse transformer, so that the original distributed capacitance is reduced by nearly half. The isolation layer must be connected to the common ground of the DC input.
At this time, the switching tube noise basically passes through the C01 (Ci1) loop, and no longer passes through the 1.3-second rectification loop. The control loop noise interferes with the rectification loop. When the rectified pulse voltage exceeds the charging voltage across the capacitor C1, the current is input from the power supply. The side inflow causes the voltage waveform at the input end of the rectifier circuit to be distorted to form interference noise.
The control loop will form a source of noise interference due to the generation of the pulse control signal.
2 switching power supply EMC design switching power supply noise interference source is impossible to eliminate, EMC design requirements must meet the inequality: interference source intensity X propagation attenuation due to equipment anti-interference ability. Therefore, the switching power supply EMC design adopts a way to reduce the intensity of the interference source and attenuate the propagation factor.
2.1 Attenuating the source of the interference The suppression of the surge can effectively reduce the intensity of the source of interference.
(1) Adopt an absorption loop. In the dotted line, the main switch tube T and the absorption circuit switch T1 repeatedly interact with each other, and when the switch tube is disconnected, the residual magnetic energy in the circuit accumulates in the capacitor and is released through the output side with the switch being turned on. The absorption loop is connected across the output side diode to suppress the inrush current generated by the diode charge storage effect. Specialized devices have been developed. The RC absorption loop is usually used, and the circuit is simple and can effectively suppress the surge. However, the loss in resistance causes the converter efficiency to drop.
C02 (Ci2) is transmitted outward. As shown.
Suppresses high frequency harmonic radiation and causes noise interference. Use shielding. The pulse transformer, choke coil, etc. are first shielded inside the switching power supply, and then the entire switching power supply is shielded. Double shielding, pay attention to heat and ventilation problems and electromagnetic continuity.
Destroy the norm, common mode noise interference propagation path. Filtering is used. The AC side filter circuit is as shown. L, C1, C2 filter out common mode noise interference, C3, C4 filter out normal mode noise interference. The DC side filter circuit is as shown. L3 inductance value is small, then L4 release energy does not have to all pass D5, D6 back to L4. It can be designed by C7 return switching power supply electromagnetic compatibility (EMI), except that the corresponding circuit is used to suppress noise interference, printed board Manufacturing processes, device selection and installation locations, wiring methods, etc. will also affect electromagnetic compatibility.
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-- Fast charging speed. When using lithium batteries for your electronic device, you can save time for charging and improve working efficiency.
-- Long service life. The lithium batteries provides up to 2000 cycle life with proper use. The features of less maintenance and longevity makes it a perfect option for power supply.
-- Wide temperature range. The lithium batteries can withstand extreme temperature. It can work in a cold winter or hot environment without battery failure. It is suitable for outdoor applications.
It usually operates at a switching frequency above 20 kHz. When the switch is turned off, a surge voltage L (di/dt) is generated across the switch. When the switch is turned on, a surge current C (di/) flows through it. Dt), forming a strong source of electromagnetic interference. As switching power supplies move toward modularity, the switching frequency will increase by the order of magnitude and electromagnetic interference will be more severe. Therefore, measures must be taken to improve the electromagnetic compatibility (EMC) of switching power supplies.
1 Switching power supply noise interference causes and propagation path Switching power supply noise interference is high-frequency oscillation noise and surge noise, and its conduction mode is characterized by normal mode noise and common mode noise, and also radiates noise to the surrounding space. Combined with the block diagram of the switching power supply circuit, the main causes of noise interference and the propagation path of noise interference are analyzed.
1.1 Switching Circuit Noise Interference The switching circuit is the main source of high frequency noise interference. The switch tube T operates in a high-frequency on-off state. Due to the charge of the pulse transformer B, the primary coil L1 and the parasitic capacitance stored in the switch tube, a surge voltage is generated across the switch tube, a surge current flows, and the TL1C5 constitutes a high The frequency current loop will generate electromagnetic radiation noise interference, and the high frequency current will pass through the primary rectification loop to mix the common mode noise into the AC power supply. In order to reduce the tube temperature and improve the efficiency, the switch tube needs to rely on a large radiator. An insulating material is added between the switch tube and the heat sink, and noise interference generated by the switch circuit is transmitted through the distributed capacitor C0 to the heat sink. The presence of the primary and secondary distributed capacitance Ci of the pulse transformer makes high-frequency noise interference easily transmitted between the primary and secondary. On the one hand, the high-frequency side high-frequency noise is directly coupled to the rectification side, causing common-mode noise at the DC output. On the other hand, the noise interference generated by the secondary high-frequency rectifier diode is coupled to the input high-voltage side, causing the AC grid to be subject to noise interference, as shown.
1.2 Secondary rectification loop noise Interference The high-frequency rectifier diode D5 operates in the high-frequency on-off state, and the high-order harmonic current forms a high-frequency current loop through the pulse transformer primary winding L2, the rectifier diode D5, and the filter capacitor C6. On the one hand, electromagnetic radiation noise interference will occur, and on the other hand, high frequency harmonic current will interfere with the DC output with common mode noise. Since the switch tube works in the high-frequency on-off state, the rectifier diode D5 and the freewheeling diode D6 are also in the high frequency. 1 Author: Introduction: Yang Zhimin, Shaanxi, Westerner\Engineer! Engaged in circuit design application research gHouse.Allrightsreserved, http:// on-off status. Due to the reverse recovery time of the diode, it will easily cause a short circuit of a large current transient, forming a source of noise interference. In the high current rectified forward switching power supply, the freewheeling diode is one of the most influential. When the energy storage inductor L releases energy through the freewheeling diode, diode D6 should be completely turned off before the forward voltage of the pulse transformer arrives to ensure a normal output path. When the freewheeling diode flows through a large current and the reverse voltage is applied, D6 cannot be instantaneously turned off due to the charge storage effect. It takes a period of recovery time to enter the off state, and a pulse transformer secondary short circuit is formed during the remote recovery time. D5, D6 flow a large short-circuit current /, while the primary and the switching tube generate current spikes. As shown. The network can slow down the rate of voltage rise between the collector and the emitter. As shown.
(3) Serially connect the saturable coil. The freewheeling diode D6 is connected in series with the saturable core coil. When D6 flows through the reverse current, the core coil generates a back EMF to prevent the reverse current from rising and prevents the pulse transformer from being short-circuited.
The RCD network 22 attenuates the propagation factor to destroy and attenuate the interference noise propagation path and the propagation medium, and can effectively attenuate the propagation factor.
Attenuate noise interference caused by distributed capacitive coupling. Use isolation. An electrostatic isolation layer is inserted between the switch tube and the heat sink, and between the primary and secondary of the pulse transformer, so that the original distributed capacitance is reduced by nearly half. The isolation layer must be connected to the common ground of the DC input.
At this time, the switching tube noise basically passes through the C01 (Ci1) loop, and no longer passes through the 1.3-second rectification loop. The control loop noise interferes with the rectification loop. When the rectified pulse voltage exceeds the charging voltage across the capacitor C1, the current is input from the power supply. The side inflow causes the voltage waveform at the input end of the rectifier circuit to be distorted to form interference noise.
The control loop will form a source of noise interference due to the generation of the pulse control signal.
2 switching power supply EMC design switching power supply noise interference source is impossible to eliminate, EMC design requirements must meet the inequality: interference source intensity X propagation attenuation due to equipment anti-interference ability. Therefore, the switching power supply EMC design adopts a way to reduce the intensity of the interference source and attenuate the propagation factor.
2.1 Attenuating the source of the interference The suppression of the surge can effectively reduce the intensity of the source of interference.
(1) Adopt an absorption loop. In the dotted line, the main switch tube T and the absorption circuit switch T1 repeatedly interact with each other, and when the switch tube is disconnected, the residual magnetic energy in the circuit accumulates in the capacitor and is released through the output side with the switch being turned on. The absorption loop is connected across the output side diode to suppress the inrush current generated by the diode charge storage effect. Specialized devices have been developed. The RC absorption loop is usually used, and the circuit is simple and can effectively suppress the surge. However, the loss in resistance causes the converter efficiency to drop.
C02 (Ci2) is transmitted outward. As shown.
Suppresses high frequency harmonic radiation and causes noise interference. Use shielding. The pulse transformer, choke coil, etc. are first shielded inside the switching power supply, and then the entire switching power supply is shielded. Double shielding, pay attention to heat and ventilation problems and electromagnetic continuity.
Destroy the norm, common mode noise interference propagation path. Filtering is used. The AC side filter circuit is as shown. L, C1, C2 filter out common mode noise interference, C3, C4 filter out normal mode noise interference. The DC side filter circuit is as shown. L3 inductance value is small, then L4 release energy does not have to all pass D5, D6 back to L4. It can be designed by C7 return switching power supply electromagnetic compatibility (EMI), except that the corresponding circuit is used to suppress noise interference, printed board Manufacturing processes, device selection and installation locations, wiring methods, etc. will also affect electromagnetic compatibility.
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-- Fast charging speed. When using lithium batteries for your electronic device, you can save time for charging and improve working efficiency.
-- Long service life. The lithium batteries provides up to 2000 cycle life with proper use. The features of less maintenance and longevity makes it a perfect option for power supply.
-- Wide temperature range. The lithium batteries can withstand extreme temperature. It can work in a cold winter or hot environment without battery failure. It is suitable for outdoor applications.
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