EMC testing of high-power switching power supplies and selection of EMI filters

Switching power supplies have the advantages of small size, light weight, and high efficiency, and are widely used in various fields. Due to the inherent characteristics of switching power supplies , the various noises generated by themselves form a strong source of electromagnetic interference. The interference generated increases significantly with the increase of output power, making the harmonic pollution of the entire power grid more serious. It poses a potential threat to the normal operation of electronic equipment. Therefore, solving the electromagnetic interference of switching power supplies is a necessary means to reduce power grid pollution. This article conducts EMC testing on a 15kW switching power supply, analyzes its test results, and introduces how to reasonably and correctly select EMI filters to achieve ideal suppression effects.

1 The mechanism of electromagnetic interference generated by switching power supply

Figure 1 shows the conducted disturbance value of the 15kW switching power supply. It can be seen from the figure that the value is out of tolerance in a large range of 0, 15~15MHz. This is because of the interference noise generated by the switching power supply . The interference noise generated by the switching power supply is divided into differential mode noise and common mode noise.

Figure 1 Conducted disturbance measured without any suppression measures

1.1 Common Mode Noise

Common mode noise is generated by common mode current, IcM, which is characterized by the noise current with the same amplitude and phase flowing back and forth between any power line (L, N) and the ground line. Figure 2 is an electrical schematic diagram of a typical switching power supply common mode noise emission path.

Figure 2 Common mode noise circuit schematic

Due to the high frequency of the switching power supply , there is distributed capacitance between the primary and secondary sides of the switching transformer and the switch tube housing and its heat sink (such as grounding). When the switch tube switches from the on state to the off state, the energy stored in the distributed capacitance (leakage inductance, etc.) of the switching transformer will exchange energy with the distributed capacitance between the collector and the ground of the switch tube, generating attenuated oscillations, causing the voltage between the collector and the emitter of the switch tube to rise rapidly. This pulse beam current operating at the switching frequency returns to any power line through the distributed capacitance between the collector and the ground, generating common-mode noise. 1.2 Differential-mode noise

Differential mode noise is generated by differential mode current IDM, which is characterized by noise currents that travel back and forth between the phase line and the neutral line with opposite phases.

1.2.1 Differential Mode Input Conducted Noise

Figure 3 is a typical electrical schematic diagram of the differential-mode input conduction noise of a switching power supply.

One is that when the switch tube of the switching power supply switches from off to on, the loop capacitor C discharges through the switch tube to form a surge current, and the voltage it generates on the loop impedance is the differential mode noise.

Figure 3 Differential mode input conducted noise circuit diagram

The second is the power frequency differential mode pulsating noise, which is the pulsating current and discharge current generated by the charging and discharging process of the rectifier filter capacitor c during the rising and falling periods of the rectifier voltage. It also contains a large amount of harmonic components that constitute differential mode noise.

The above two types of differential mode noise are returned to the AC power grid at the input end, so they are called input conducted noise. It not only pollutes the power grid, but also causes harm to other electronic and electrical equipment connected to the power grid, and directly leads to a decrease in the input power factor.

1.2.2 Differential-mode output conducted noise

The third type of differential mode noise is output conduction noise, which is caused by the peak voltage generated by the reverse current and the diode junction capacitance and distributed inductance when the diode of the rectifier output part changes from forward bias to reverse bias. Figure 4 is a typical half-wave rectifier filter circuit:

Figure 4 Differential mode output conducted noise schematic diagram 2 Correct selection of EMI filter

EMI filter is a reflective low-pass filter that conducts at power frequency. Insertion loss and impedance characteristics are important technical indicators. EMI filter is in a mismatched state during normal operation, because in practical applications, it cannot achieve matching. For example, the impedance of the filter input end (grid impedance) changes with the amount of power consumption. The impedance of the filter output end (power supply impedance) changes with the size of the load. In order to obtain the best EMI suppression effect, the circuit structure and parameters of the EMI filter must be selected according to the source impedance characteristics and load impedance characteristics to which the two ends of the filter are connected, that is, follow the input and output impedance mismatch principle.

Low source impedance and low load impedance: select the (T)n filter structure; (2) High source impedance and high load impedance: select the (π)n“ filter structure; (3) Low source impedance and high load impedance: select the (LC)n“ filter structure; (4) High source impedance and low load impedance: select the (CL) filter structure.

If the impedance mismatch principle cannot be met, the insertion loss performance of the filter will be affected, and in severe cases it will even cause resonance and interference amplification at certain frequencies. Therefore, the impedance mismatch connection principle is a principle that must be followed when applying EMI filters.

As for the conducted disturbance value measured in Figure 1, it can be seen that it is seriously out of tolerance in the range of 0.15 to 15MHz, with the maximum value exceeding the limit by nearly 40dB, and the peaks are relatively dense. This indicates that the surge voltage and surge current generated by the power supply are large, that is, the du/dt and di/dt of the power supply are large, that is, the generated disturbance energy is large. The common-mode noise equivalent circuit of the switching power supply is high-impedance capacitive, while the differential-mode equivalent circuit has high and low impedance at the same time. In view of this situation, the circuit structure of the EMI filter is selected as a secondary common-mode inductor and a separate differential-mode inductor type, which can filter out both common-mode noise and differential-mode noise. The insertion loss is 40dB, and the measured conducted disturbance value is shown in Figure 5.

Figure 5 Conducted disturbance measured after adding EMI filter

As can be seen from Figure 5, the conducted disturbance value is still out of tolerance in some frequency bands, and the effect is not very ideal. This is because the conducted disturbance value measured by the conducted receiver is a comprehensive parameter. It cannot determine which is more important, the common mode interference or the differential mode interference, in the frequency range of 0.15-15MHz. Generally speaking: the differential mode interference component is large at the low end of 0.15-0.5MHz, the common mode interference and differential mode interference exist at the same time at 0.5-5MHz, and the common mode component is large between 5-30MHz. The second reason is that the inductance and capacitance components of the filter are affected by their distributed parameters, and the higher the frequency, the greater the impact. The assembly process and grounding quality of the filter's internal inductance and capacitance will also have a great impact on the insertion loss. The third reason is that since the filter inductance will be affected by the current surge, its peak current is about twice the rated current. When it is overloaded and fully loaded, the differential mode inductance is prone to magnetic saturation, causing the inductance to drop rapidly, resulting in a deterioration in the insertion loss performance. 3 More ideal solutions

In view of the above situation, an inductor with a certain value is connected in series at the front end of the EMI filter. In the AC circuit, the value of the inductor is X = wL = 2πrfL. The inductor is a reactor, so this inductor is also called a line reactor. From X = 2πrfL, it can be seen that its inductive reactance is proportional to the frequency. For low-frequency current, it can pass through the line reactor unimpeded, and for high-frequency current, the line reactor presents high impedance and high voltage drop. Therefore, the line reactor can be used as a low-pass (high-impedance) filter for current.

Moreover, most of the harmonic voltage generated by the switching power supply is dropped on the incoming line reactor. Therefore, the series connection of the incoming line reactor not only reduces the overall conducted disturbance value, but also improves the voltage harmonics. When the inductance value is selected as 6mH, the suppression effect is shown in Figure 6. Therefore, for the finalized high-power switching power supply, choosing the incoming line reactor + EMI filter is an ideal way to solve its electromagnetic disturbance.

Figure 6 Conducted disturbance measured after line reactor + EMI filter

4 Conclusion

The electromagnetic interference generated by high-power switching power supplies is a complex issue. The harm of electromagnetic interference and conducted interference generated by power supplies is particularly serious. According to the mechanism of electromagnetic interference generation, the correct selection of EMI filters is the key to effectively suppressing conducted interference. Its purpose is to effectively suppress the conducted interference of the switching power supply to the power grid, and to reduce the conducted interference introduced from the power grid, so that the electromagnetic compatibility of the switching power supply meets the limit requirements specified by national standards.