As is well known, switching power supplies are widely used due to their unique advantages, namely small size, light weight and high efficiency. However, it also has certain defects, that is, the switching noise is large, which seriously affects the normal operation of the machine and even external devices. The noise of the switching power supply is divided into two types: normal noise (Mornia-modelnoise) and common-mode noise (Common-model noise).
2 Normal noise generation and suppression Normal noise occurs during the turn-on and turn-off of the main switch. The direct influencing factors are the effective inductance (ESL) of the output filter capacitor, the leakage inductance of the transformer, the parasitic capacitance, and the output equivalent capacitance of the switch. Parasitic parameters such as the equivalent capacitance of the rectifier diode. The relationship between the normal noise and the output filter capacitor ESL is the closest, and the electrolytic capacitor with the ESL as small as possible must be selected.
And common mode noise compared to the smaller proportion of normal noise Lu share.
Generally only account for 10% ~ 20% of the noise voltage. Its processing is relatively easy, one of the effective measures to reduce the normal mode noise is added to the output circuit is a differential mode filter, as long as during debugging different order of magnitude of the parallel frequency capacitance at the output is effectively resolved nj, common mode 3 Noise generation and suppression of common-mode noise are noises occurring between the earth and the output. In U), the impedance is Z=l/cC, and the high-frequency region is connected in parallel with ceramic capacitors and polyester film capacitors. better. In (b), the noise through a capacitor to ground lines, such filters should be connected to the ground impedance as small as possible. In (c),, C, C2 asymmetric noise have good filtering effect, c3 symmetric good noise filtering effect, should be connected to the leads of the capacitor and ground line as short as possible. (D) is commonly used in the noise filter circuit, L, L2 presents a high impedance to noise, and C, the noise exhibits a low impedance. When the common mode inductance structure, for symmetrical and asymmetrical noise has a better filtering effect. (e) It is suitable for filtering common mode noise. It should be noted that the grounding impedance should also be as small as possible.
The filter circuit represents the force of the well-inductance and its magnetic flux. It has two equal-numbered windings on the same core. The magnetic flux generated by the power line's round-trip current in the core is reversed. It cancels out. Therefore, it does not have an inductive effect. For common mode noise, a large inductance can be obtained, which exhibits high yaw resistance, so it has a good inhibitory effect on M line.
The common mode inductance and the direction of its magnetic flux are the equivalent circuits of several mode inductors, and its common mode noise is a filter circuit that is effective for both common mode noise and differential mode noise. Among them, LhLC, in order to suppress the differential mode noise loop, L3, C2, C3 constitute a common mode noise suppression loop" L, L2 core should be selected for materials that are not magnetically saturated and the iron 4 common mode noise suppression with excellent Mf characteristics is reduced. The AC side noise filter adds a linear filter to the AC input side. The filter is a low pass filter that allows low frequencies of 50 Hz, 60 Hz, and 400 Hz. For the 20 kHz to 20 MHz band, the attenuation should be 30dB~100dB. The common mode induction in the linear filter has a small distributed capacitance, which is evenly wound around the ring without air gap to reduce the lead inductance. The filter capacitor is generally made of ceramic capacitor, polyester film capacitor or polypropylene capacitor. It is shielded by steel plate and connected to the casing. The grounding of the same incoming line should be good. A practical AC side linear filter is shown in (c).
The filter that reduces the noise on the DC side is on the DC side, and the change in the potential at the P point of the primary side of the transformer (that is, the intermediate point between the two MOSFET tubes) is one of the causes of the common mode noise. Therefore, the most effective method is to convert the transformer. The original secondary side is isolated (plus shielding layer). Common mode absorption circuit. The absorption of the common mode absorption network depends on the high frequency characteristics of the common mode inductor and the common mode capacitor. It should be pointed out that for common-mode noise, parallel capacitance at the output is ineffective.
When the wiring of the switching power supply to the load is long, the common mode noise propagates through the parasitic capacitance, and the output noise increases. If a twisted wire is used, the noise can be greatly reduced. Compared with parallel lines, the noise voltage on the load side is 600mV when transmitted with parallel lines of 1m length and 5cm pitch. When using the twisted line with a pitch of 1cm, the noise voltage on the load side is only 14mV. Choose a reasonable circuit layout. It can also effectively reduce the common mode noise. When assembling the switching power supply, the volume and shape of the extension must be determined according to the requirements of the whole machine. As the output power per unit volume increases, the electrostatic field and electromagnetic field ch/df intensity also increase, and the interference increases. The interference intensity decreases with the square of the distance between the wire and the noise source, that is, when the distance is reduced by half, the noise will be 1/4 of the original. Since the common mode noise is mainly transmitted to the output through the coupled capacitance, A shielding layer should be added between the main switch tube and the heat sink, and the output line and the wiring of the casing should not be too tight. In the actual work, the component distribution diagram shown is often used, which has a good effect of suppressing noise.
2 Normal noise generation and suppression Normal noise occurs during the turn-on and turn-off of the main switch. The direct influencing factors are the effective inductance (ESL) of the output filter capacitor, the leakage inductance of the transformer, the parasitic capacitance, and the output equivalent capacitance of the switch. Parasitic parameters such as the equivalent capacitance of the rectifier diode. The relationship between the normal noise and the output filter capacitor ESL is the closest, and the electrolytic capacitor with the ESL as small as possible must be selected.
And common mode noise compared to the smaller proportion of normal noise Lu share.
Generally only account for 10% ~ 20% of the noise voltage. Its processing is relatively easy, one of the effective measures to reduce the normal mode noise is added to the output circuit is a differential mode filter, as long as during debugging different order of magnitude of the parallel frequency capacitance at the output is effectively resolved nj, common mode 3 Noise generation and suppression of common-mode noise are noises occurring between the earth and the output. In U), the impedance is Z=l/cC, and the high-frequency region is connected in parallel with ceramic capacitors and polyester film capacitors. better. In (b), the noise through a capacitor to ground lines, such filters should be connected to the ground impedance as small as possible. In (c),, C, C2 asymmetric noise have good filtering effect, c3 symmetric good noise filtering effect, should be connected to the leads of the capacitor and ground line as short as possible. (D) is commonly used in the noise filter circuit, L, L2 presents a high impedance to noise, and C, the noise exhibits a low impedance. When the common mode inductance structure, for symmetrical and asymmetrical noise has a better filtering effect. (e) It is suitable for filtering common mode noise. It should be noted that the grounding impedance should also be as small as possible.
The filter circuit represents the force of the well-inductance and its magnetic flux. It has two equal-numbered windings on the same core. The magnetic flux generated by the power line's round-trip current in the core is reversed. It cancels out. Therefore, it does not have an inductive effect. For common mode noise, a large inductance can be obtained, which exhibits high yaw resistance, so it has a good inhibitory effect on M line.
The common mode inductance and the direction of its magnetic flux are the equivalent circuits of several mode inductors, and its common mode noise is a filter circuit that is effective for both common mode noise and differential mode noise. Among them, LhLC, in order to suppress the differential mode noise loop, L3, C2, C3 constitute a common mode noise suppression loop" L, L2 core should be selected for materials that are not magnetically saturated and the iron 4 common mode noise suppression with excellent Mf characteristics is reduced. The AC side noise filter adds a linear filter to the AC input side. The filter is a low pass filter that allows low frequencies of 50 Hz, 60 Hz, and 400 Hz. For the 20 kHz to 20 MHz band, the attenuation should be 30dB~100dB. The common mode induction in the linear filter has a small distributed capacitance, which is evenly wound around the ring without air gap to reduce the lead inductance. The filter capacitor is generally made of ceramic capacitor, polyester film capacitor or polypropylene capacitor. It is shielded by steel plate and connected to the casing. The grounding of the same incoming line should be good. A practical AC side linear filter is shown in (c).
The filter that reduces the noise on the DC side is on the DC side, and the change in the potential at the P point of the primary side of the transformer (that is, the intermediate point between the two MOSFET tubes) is one of the causes of the common mode noise. Therefore, the most effective method is to convert the transformer. The original secondary side is isolated (plus shielding layer). Common mode absorption circuit. The absorption of the common mode absorption network depends on the high frequency characteristics of the common mode inductor and the common mode capacitor. It should be pointed out that for common-mode noise, parallel capacitance at the output is ineffective.
When the wiring of the switching power supply to the load is long, the common mode noise propagates through the parasitic capacitance, and the output noise increases. If a twisted wire is used, the noise can be greatly reduced. Compared with parallel lines, the noise voltage on the load side is 600mV when transmitted with parallel lines of 1m length and 5cm pitch. When using the twisted line with a pitch of 1cm, the noise voltage on the load side is only 14mV. Choose a reasonable circuit layout. It can also effectively reduce the common mode noise. When assembling the switching power supply, the volume and shape of the extension must be determined according to the requirements of the whole machine. As the output power per unit volume increases, the electrostatic field and electromagnetic field ch/df intensity also increase, and the interference increases. The interference intensity decreases with the square of the distance between the wire and the noise source, that is, when the distance is reduced by half, the noise will be 1/4 of the original. Since the common mode noise is mainly transmitted to the output through the coupled capacitance, A shielding layer should be added between the main switch tube and the heat sink, and the output line and the wiring of the casing should not be too tight. In the actual work, the component distribution diagram shown is often used, which has a good effect of suppressing noise.
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