Dealing with the noise of switching power supply from three aspects[Copy link]
Noise comes from three aspects: PCB design/circuit oscillation/magnetic components: 1) Circuit oscillation, the power supply output has a large low-frequency oscillation. It is mostly caused by insufficient circuit stability margin. In theory, the frequency domain method/time domain method or Routh criterion in system control theory can be used for theoretical analysis. Now; the circuit stability can be easily verified by computer simulation method to avoid self-excited oscillation. There are many software available. For the circuit that has been made, it can be improved by adding output filter capacitors or inductors/changing the signal feedback position/increasing the integral capacitor of PI regulation/reducing the open-loop gain. 2) PCB design A) It is mainly caused by EMI noise. The RF noise adjusts the PI regulator so that the output error signal contains disturbances. Mainly check whether the high-frequency capacitor is too far away from the relevant components, whether there is a large C-shaped surrounding wiring, etc. B) The PCB line of the control circuit shares at least two points with the power circuit. The PCB copper wire is not an ideal conductor. It can always be equivalent to an inductor or resistor. When the power current flows through the PCB line shared with the control circuit, a voltage drop is generated on the PCB. When the nodes of the control circuit are scattered in different positions, the voltage drop caused by the power current disturbs the control network and makes the circuit noise. This phenomenon often occurs on the power ground line. Pay attention to single-point grounding to improve it. 3) Magnetic components Magnetic materials have the characteristics of magnetism to strain. Enameled wires will also be affected by electromotive force in the leakage magnetic field. Under the combined effect of these factors, overtones or 1/N frequency resonance will occur locally. Changing the switching frequency and varnishing the magnetic components can improve it. This is my usual little experience, try it. I don't know if the noise you are talking about refers to the noise of mechanical vibration or the high-frequency AC component in the output voltage? Both types of noise are often encountered in switching power supplies. Mechanical noise is mostly caused by abnormal electrical oscillations in the circuit. When the frequency is lower than 20K, the sound emitted on the magnetic core of the transformer, inductor, etc. can be heard by the human ear. The solution is to adjust the compensation, reduce the input impedance of the amplifier, and add absorption circuits in places where interference is sensitive. The output ripple noise is mainly due to the peak voltage caused by the leakage inductance of the transformer and the line inductance at the moment when the switch tube is turned off. It is the cause of the output ripple noise, but the frequency of the switching power supplies we make is generally very high, much greater than 20K, so if there is no abnormal circuit oscillation, we cannot hear the sound. After the AC power is input into the switching power supply, it is rectified by the bridge rectifier V1~V4 into a DC voltage Vi and applied to the primary L1 of the high-frequency transformer and the switch tube V5. The base of the switch tube V5 inputs a high-frequency rectangular wave of tens to hundreds of kilohertz, and its repetition frequency and duty cycle are determined by the requirements of the output DC voltage VO. The pulse current amplified by the switch tube is coupled to the secondary circuit by the high-frequency transformer. The ratio of the primary and secondary turns of the high-frequency transformer is also determined by the requirements of the output DC voltage VO. The high-frequency pulse current is rectified by the diode V6 and filtered by C2 to become the DC output voltage VO. Therefore, the switching power supply will generate noise in the following links, forming electromagnetic interference. (1) The high-frequency switching current loop composed of the primary L1 of the high-frequency transformer, the switch tube V5 and the filter capacitor C1 may generate large spatial radiation. If the capacitor filtering is insufficient, the high-frequency current will also be transmitted to the input AC power supply in a differential mode. (2) The high-frequency transformer secondary L2, rectifier diode V6, and filter capacitor C2 also form a high-frequency switching current loop that will generate spatial radiation. If the capacitor filtering is insufficient, the high-frequency current will be mixed with the output DC voltage in the form of differential mode and conducted outward. (3) There is a distributed capacitor Cd between the primary and secondary of the high-frequency transformer. The high-frequency voltage of the primary will be directly coupled to the secondary through these distributed capacitors, generating common-mode noise of the same phase on the two output DC power lines of the secondary. If the impedance of the two lines to the ground is unbalanced, it will also be converted into differential mode noise. (4) The output rectifier diode V6 will generate a reverse surge current. When the diode is forward-conducted, the charge in the PN junction accumulates. When the diode is reverse-voltage-applied, the accumulated charge will disappear and generate a reverse current. Because the switching current needs to be rectified by the diode, the time for the diode to change from on to off is very short. In a short time, the stored charge needs to disappear, which generates a reverse current surge. Due to the distributed inductance and distributed capacitance in the DC output line, the surge causes high-frequency attenuation oscillation, which is a differential mode noise. (5) The load of the switch tube V5 is the primary coil L1 of the high-frequency transformer, which is an inductive load. Therefore, a high surge peak voltage will appear at both ends of the tube when the switch is turned on and off. This noise will be transmitted to the input and output ends. (6) There is a distributed capacitance CI between the collector of the switch tube V5 and the heat sink K. Therefore, the high-frequency switching current will flow to the heat sink K through CI, then flow to the chassis ground, and finally flow to the protective ground wire PE of the AC power line connected to the chassis ground, thereby generating common mode radiation. The power lines L and N have a certain impedance to PE. If the impedance is unbalanced, the common mode noise will also be converted into differential mode noise. From the above analysis, we can know that there are many noise interference sources in the switching power supply, and the interference channels are diverse. The noise interference sources with greater influence can be summarized into the following three types: (1) Interference caused by the reverse recovery time of the diode. (2) Harmonic interference generated when the switch tube is working When the power switch tube is turned on, a large pulse current flows through it. During the cut-off period, the current mutation caused by the leakage inductance of the high-frequency transformer winding will also produce spike interference. (3) Interference generated by the AC input circuit The rectifier tube at the input end of the switching power supply will also cause high-frequency attenuation oscillation and interference during the reverse recovery period. Generally, a relatively large filter capacitor is always connected behind the rectifier circuit, so the conduction angle of the rectifier tube is small, which will cause a large charging current, causing the AC current on the AC input side to be distorted, affecting the power supply quality of the power grid. In addition, the equivalent series inductance of the filter capacitor also has a great influence on the interference. All these interferences can be divided into two categories according to the propagation path: conducted interference and radiated interference. The interference formed by the peak interference and harmonic interference energy generated by the switching power supply through the input and output lines of the switching power supply is called conducted interference. When the energy of harmonics and parasitic oscillations propagates through the input and output lines, electric and magnetic fields are generated in space. These interferences generated by electromagnetic radiation are called radiated interference. Because the switching power supply itself is a strong interference source, in addition to taking measures in the circuit to suppress its electromagnetic interference, the switching power supply should also be effectively electromagnetically shielded, filtered and grounded. Methods for suppressing switching power supply noise The three elements that form electromagnetic interference are interference source, propagation path and disturbed equipment. Therefore, electromagnetic interference suppression should also start from these three aspects. First, the interference source should be suppressed and the cause of the interference should be eliminated directly; second, the coupling and radiation between the interference source and the affected equipment should be eliminated to cut off the propagation path of the electromagnetic interference; third, the anti-interference ability of the affected equipment should be improved to reduce its sensitivity to noise. The third point is not within the scope of this article. The use of power factor correction (PFC) technology and soft switching power conversion technology can greatly reduce the noise amplitude. (1) Circuit measures The main reason for the electromagnetic interference generated by the switching power supply is the rapid change of voltage and current. Therefore, it is necessary to reduce the rate of change of voltage and current in the circuit as much as possible (du/dt, di/dt). The use of absorption circuit is also a good way to suppress electromagnetic interference. The basic principle of the absorption circuit is to provide a bypass for the switch when the switch is turned off, absorb the energy accumulated in the parasitic distributed parameters, and thus suppress the occurrence of interference. Commonly used absorption circuits include RC, RCD, LC passive absorption networks and active absorption networks. Filtering is a good way to suppress conducted interference. For example, connecting a filter to the power input can suppress the interference generated by the switching power supply and fed back to the power grid, and can also suppress the noise from the power grid from invading the power supply itself. In the filter circuit, many special filter components are also used, such as through-hole capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filtering characteristics of the circuit. Proper design or selection of filters and correct installation of filters are important components of anti-interference technology. The specific measures are as follows: a. Install a power filter at the AC input end, and the circuit type of the filter is shown in Figure 2. Among them, LD and CD are used to suppress differential mode noise. Generally, LD takes 100~700μH and CD takes 1~10μF, which is more effective for 10~150KHz. LC and CC are used to suppress common mode noise. Generally, LC takes 1~3μH and CC takes 2000~6800pF, which is effective for suppressing 150KHz. The above common mode noise is effective. The parameters of the above devices should be adjusted in practice.
In addition, the following should be noted when installing the power filter: The power filter must be grounded when installed. Except for the filter that the manufacturer specifically states that it can be used without grounding, all power filters must be grounded, because the common mode bypass capacitor of the filter must be grounded to work. The general grounding method is to connect the filter to the metal casing and connect the filter housing to the grounding point of the equipment with a thicker wire. The lower the grounding impedance, the better the filtering effect. Install as close to the power inlet as possible. When installing, keep the input/output ends of the filter as far away as possible to avoid interference signals from the input end directly coupling to the output end. If necessary, use a shielding partition to separate them. b. Install a common-mode noise filter at the output end of the power supply. Put a ferrite ring on the output line to make a conjugate choke, and then install a high-frequency capacitor, which can suppress some common-mode noise. Increasing the inductance of the output filter inductor and the capacitance of the filter capacitor can suppress differential mode noise, and multiple capacitors in parallel will have a better effect. c. The output rectifier diode uses multiple diodes in parallel to share the load current, selects rectifier diodes with soft reverse recovery current characteristics, appropriately reduces the turn-on rate of the switch tube, reduces the leakage inductance of the high-frequency transformer and ensures that it is not saturated. These are all effective means to suppress noise. d. Install an RC absorption network between the primary side, secondary side, CE pole of the switch tube, and the output rectifier diode. Suppress voltage spikes and current surges. Connect a saturable amorphous magnetic ring in series in the output rectifier diode branch to suppress the reverse surge current of the diode, which has a better effect. e. When arranging the printed circuit board, try to reduce the area of the high-frequency loop and shorten the high-frequency signal line. When wiring the whole machine, you should also pay attention to: do not put the input AC power line and the output DC power line of the switching power supply close together, let alone bundle them together, and keep them as far away from noise sources as possible. It is best to use twisted pair cables for the output DC power line, at least they should be routed close together. The input and output power lines of the power supply should be as far away from the signal lines in the control and drive circuits as possible. f. Try to reduce the distributed capacitance CI between the collector of the switching tube and the heat sink. You can use an insulating pad with a low dielectric constant and appropriately thicken the thickness of the pad. If necessary, insert a thin copper plate between the insulating pads for electrostatic shielding. g.Grounding One purpose of power grounding is for safety, and the other purpose is to consider electromagnetic compatibility issues. A good grounding system can play a great role in reducing electromagnetic interference. Grounding for safety reasons is generally called safety ground, which connects the metal shell of the power supply to the earth. When considering electromagnetic compatibility issues, you must first understand the concepts of signal ground and ground loop interference. Signal ground: A low impedance path for signal current to flow back to the signal source. Ground loop interference: When a large current flows through the ground line, a voltage drop will occur because the impedance of the ground line is not zero. This voltage will generate current on the connecting cable of the two circuits. Due to the imbalance of the circuit, the current on each wire is different, so a differential mode voltage will be generated, causing interference to the circuit. This interference is caused by the current generated in the ground loop, so it is called ground loop interference. The main methods to solve the grounding problem are: 1) Minimize the high-frequency impedance caused by the wire inductance. 2) Increase the impedance of the ground loop, use a shielded isolation transformer or optocoupler between the primary and secondary to transmit the signal to reduce the ground loop interference. 3) It is best not to share the same power supply and the same ground wire for two unit circuits. Amplifier shielding shell, transformer shielding layer good grounding, etc. (2) Structural measures: shielding Shielding is one of the important means to solve electromagnetic compatibility problems, the purpose is to cut off the propagation path of electromagnetic waves. Most electromagnetic compatibility problems can be solved by electromagnetic shielding. The biggest advantage of using electromagnetic shielding to solve electromagnetic interference problems is that it will not affect the normal operation of the circuit. Shielding is divided into electrical shielding, magnetic shielding and electromagnetic shielding. For switching power supplies, it is mainly necessary to do a good job of shielding the casing, high-frequency transformer, switch tube and rectifier diode, and control and drive circuits, and to improve the shielding effectiveness through various methods.
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