Electronic enthusiast network: This article mainly describes the electromagnetic disturbance suppression technology (EMC) of switching power supply. The measures to suppress the electromagnetic disturbance of the switching power supply The suppression measures taken by the EMC generated by the switching power supply are mainly from two aspects: one is to reduce the disturbance intensity of the disturbance source; the other is to cut off the disturbance propagation. In order to achieve this goal, we mainly start from the following aspects: choose the appropriate switching power supply working mode and working frequency; choose the appropriate circuit components; use the correct shielding method, grounding, filtering measures, and use reasonable component layout.
1 Reduce the intensity of disturbance from the source of disturbance
Choosing the right switching power supply works differently, and the electromagnetic disturbance intensity generated by them and the electromagnetic disturbance control generated are different. For example, the self-excited switching power supply not only changes the pulse width when the load is light or heavy, but also changes the switching frequency greatly. This makes it difficult to overcome the switching pulse disturbance and control its propagation; Change, it changes the pulse width to keep the output stable. Obviously, his exciting switching power supply makes it easier to control electromagnetic disturbances. Isolated switching power supplies are less irritating than isolated switching power supplies. Harmony generated by bridge rectification is less than that caused by other rectification methods. Optocoupler isolation is easier to control than transformer isolation. The isolated switching power supply resonant type is much smaller than the polarity inverting type.
The operating frequency of the switching power supply is also closely related to the disturbance intensity. The low switching power supply operating frequency can not only reduce the high frequency component of the disturbance, but also greatly reduce the propagation efficiency of the conducted disturbance and the radiation disturbance. In the actual design, when we choose the working mode, we comprehensively consider the performance of the electromagnetic capacity, so that we can often get the effect of the work. As for the working frequency, of course, the smaller the better, without increasing the cost and affecting the work efficiency.
Choosing the right circuit components
The switching power supply circuit is the most direct and primary source of electromagnetic disturbance generated by the switching power supply. In the switching circuit, the switching tube is the core. In our actual design and testing, we found that we used the different types of switch tubes with the same current capacity to conduct radiated disturbance tests. The maximum and maximum disturbance of the overall disturbance is 15-20dB.
At the high end of the frequency of conducted disturbances, we also found the same phenomenon (the low-end of the frequency of conducted disturbances is not as high-end obvious). This is related to whether the switch tube is designed with electromagnetic capacity in mind. A good switching tube is designed to take into account the high frequency suppression of the switching moment and the conversion efficiency. This switch tube cost may be higher. Another key component in the switching circuit is the pulse transformer. The pulse transformer has two effects on the electromagnetic capacitance. One is to add an electrostatic screen between the primary coil and the secondary coil and lead to the ground. The grounding wire is as close as possible to the switching tube. The emitter is connected to the 0V ground (thermal ground) of the DC input, which can greatly reduce the distributed capacitance cd, thereby reducing the coupling disturbance of the primary and secondary electric fields.
In order to reduce the leakage flux of the pulse transformer, a closed core (such as a ring) may be selected, and the closed core has a smaller leakage magnetic flux than the open core. It is not possible to suppress the leakage flux by wrapping the high magnetic permeability screen material in the pulse transformer, thereby reducing the disturbance through the magnetic flux leakage. The choice of C1 in the switching loop is also critical. Select a capacitor with good high-frequency characteristics or connect a high-frequency capacitor in parallel to reduce the high-frequency impedance. This can reduce the high-frequency current and conduct it in differential mode to the AC power supply to form conducted disturbance. . In the secondary rectification loop, the rectifier diode D2 is often critical. In the low-voltage and high-current rectification loop, the fast recovery Schottky is a better choice. Other fast recovery diodes or diodes with soft recovery characteristics can be used for the high voltage output circuit.
Harassment absorption loop
The RC absorption back can be connected in parallel with the switch T of the switching circuit as shown in Fig. 3(b), or the RC absorption circuit can be connected in parallel with the switching tube T as shown in Fig. 3(a), or in the RC/DRC circuit. Absorbing the high surge spike voltage generated when the switch tube T is turned on and off, reducing the disturbance of the switch circuit. As shown in Fig. 3(c), a coil with a saturable core or a microcrystalline magnetic bead (co system) sc1 and sc2 are connected in series to the rectifier diodes D2 and D1 of the output terminal. The saturable core coil/microcrystalline magnetic bead is saturated when passing normal current, and the inductance is small, which will not affect the normal operation of the circuit. Once the current flows backward, it will generate a large back EMF and prevent the reverse. The current is applied to the current, so that it can effectively suppress the reverse surge current of the diode D2 by connecting it in series with the diodes D1 and D2. Huijing magnetic beads can be directly placed on the leads of the diodes, which is convenient to use and better than the RC absorption circuit. In addition, the filter network composed of D1, L, and C2 can better filter the high-frequency ripple in the output DC and reduce the high-frequency differential mode disturbance at the output.
PFC network in a rectification loop
For a rectification loop, the most significant disturbance is the waveform distortion of the power supply network caused by the selective power take-off of the AC grid by the rectifier loop, and the power factor is low. To solve this problem, an off-the-shelf PFC (Power Factor Control) template can be added to the rectifier circuit. The module is divided into active and passive, and the active template has better effect, but the circuit is complicated and the cost is high. In order to be more suitable for the designed products, there are also companies that provide PFC circuits with a power factor from 0.4 to above 0.9. The designed switching power supply can pass the current harmonic test of GB176250.1-1998 smoothly.
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