Conducted common-mode EMI suppression of forward converter based on compensation principle

1 Introduction
With the widespread application of power electronic devices, the switching frequency of power converters is getting higher and higher, and the structure is getting more and more compact, making the electromagnetic interference (EMI) problem more and more serious. EMI signals not only have a wide frequency range, but also have a certain amplitude. Through conduction and radiation, they will pollute the electromagnetic environment and cause interference to communication equipment and electronic products. Therefore, how to reduce or even eliminate the EMI problem in switching power supplies has become an important issue in the design of switching power supplies. The usual method to suppress common-mode interference is to connect a larger magnetic choke coil in series on the input line, but this destroys the overall appearance structure of the converter. This paper first analyzes the generation mechanism of common-mode EMI in switching power supplies and traditional common-mode EMI suppression technology. On this basis, for forward converters, a common-mode EMI suppression method based on the compensation principle is introduced. That is, a method of using a compensation transformer winding and a compensation capacitor to eliminate common-mode electromagnetic interference is adopted. Compared with other methods, the structure is simpler and passive, and only a small winding and a small capacitor need to be added.

2 Causes and propagation of common-mode EMI in switching power supplies
When the switching power supply operates at high frequency, due to the high du/dt, the parasitic capacitance Ct between the transformer coils and the parasitic capacitance Cq between the switch tube and the heat sink are excited to generate noise current, thereby generating common-mode interference, as shown in Figure 1. The common-mode interference current starts from the switch tube with high du/dt, flows through the grounded heat sink and the ground wire, and then flows back to the input line through the high-frequency LISN network (equivalent to two 50Ω resistors).


3 Traditional common-mode EMI suppression circuit
The traditional common-mode interference suppression circuit is shown in Figure 2. In order to keep the leakage current flowing into the ground by the filter capacitor Cy within a safe range, the value of Cy is small, and the corresponding choke coil inductance Lcm becomes larger, especially because Lcm has to transmit all the power, its loss, volume and weight will increase. The application of passive compensation technology can better suppress the common-mode interference of the circuit without affecting the operation of the main circuit, and can reduce the volume and weight of Lcm to save costs.



4 Compensation principle and its application
4.1 Compensation principle
In power electronic devices, common-mode interference is generated by the current Cdu/dt to ground generated by the switch tube device when it is turned on and off at high frequency. Therefore, an additional transformer winding is used to generate a 180° reverse voltage on the compensation capacitor, and the generated compensation current is superimposed on the interference current on the parasitic capacitor to eliminate the interference. This is the principle of passive compensation.

4.2 Analysis of common-mode EMI coupling channel model


[page] Figure 3 is a schematic diagram of the common-mode EMI transmission channel of the forward converter. The LISN module is a standard circuit for measuring conducted emissions (equivalent to two 50Ω resistors). Capacitors Cpara, Ct and Cout are distributed capacitances represented by dotted lines, where Cpara represents the distributed capacitance to ground of the primary side of the transformer, the power switch tube and the heat sink, Ct represents the coupling capacitance between the primary side and the secondary side, and Cout represents the distributed capacitance to ground of the load. The switch tube generates a potential jump when it is turned on and off, and the changing potential forms a common-mode noise current through the coupling capacitance to ground, as shown in Figure 3. The common-mode current flows to the ground in two ways, one through the heat sink and the ground capacitance Cpara of the switch tube, and the other through the transformer coupling capacitor Ct and the secondary side ground capacitance Cout.
Figure 4 is the equivalent circuit of the primary side common-mode noise. Since the secondary side output voltage is low, the common-mode noise generated by it can be ignored. Since the contact area between the switch tube and the heat sink is large and the spacing is small, the distributed capacitance between the two is as high as several 10nF. If the heat sink is directly grounded, the common-mode noise will form a loop along this distributed capacitance and the LISN impedance, causing serious common-mode noise.


When the parasitic capacitance Cpara of the switch tube is much larger than the coupling capacitance Ct of the primary and secondary sides, the parasitic capacitance Cpara is the main propagation channel of the noise. The simplified circuit is shown in Figure 5. According to the path of the primary side common-mode noise current, it can be seen that as long as a voltage opposite to the noise source is added to the original circuit, the same magnitude of reverse current is generated through the compensation capacitor, and the common-mode EMI can be suppressed.

4.3 External compensation capacitor suppresses common-mode noise
Figure 6 is a forward converter circuit with a compensation circuit added. The most prominent feature of this circuit is that it can use its own magnetic reset coil as a compensation coil. The number of turns is the same as that of the primary winding. With an external compensation capacitor, compensation can be easily achieved. The size of the compensation capacitor Ccomp is the same as that of the parasitic capacitor Cpara. When working, Nc causes Ccomp to generate a compensation current with the same magnitude and opposite direction as the interference current on Cpara. After superposition, the interference current is eliminated. The compensation current does not flow through all the power, but only transmits the interference current. The compensation circuit is very simple.


Since the forward converter has its own demagnetization winding, the external compensation coil is omitted. Only an external compensation capacitor is needed to improve and suppress common-mode EMI. Figure 7 is a simplified circuit of common-mode current flow with the addition of a compensation capacitor.


It should be pointed out that passive compensation technology has certain application conditions, which are affected by factors such as the rise and fall time of the switching current and voltage, and the structure of the transformer. Especially for this compensation circuit, the application conditions are limited. When the line coupling capacitance of the transformer is much larger than the parasitic capacitance of the switch tube, the interference current does not pass through the compensation capacitor but directly enters the ground through the transformer coupling capacitor. At this time, the suppression effect is not very ideal. 5 Conclusion This paper analyzes the common-mode EMI propagation coupling mode of typical switching power supplies, traditional common-mode EMI suppression methods, and the common-mode EMI coupling channel model of forward converters, and proposes a method to suppress the common-mode EMI of forward converters using the passive compensation principle. This method is subject to certain application conditions, but the compensation circuit structure is simple, easy to implement, and has certain guiding significance. References [1] Qian Zhaoming, Cheng Zhaoji. Fundamentals of electromagnetic compatibility design and interference suppression technology for power electronic systems. Zhejiang University Press, 2000. [2] Zhou Zhimin, Zhou Jihai, Ji Aihua. Monolithic switching power supply: application circuit, electromagnetic compatibility, PCB wiring. Electronic Industry Press, 2007, (7). [3] Himanshu K. Patel. Flyback Power Supply EMI Signature and Suppression Techniques. IEEE, 2008. [4] Zhou Weiping, Xia Li, Hou Xinguo, Wu Zhengguo. Suppression of electromagnetic interference in power electronic systems. Journal of Zhongyuan University of Technology, 2003, (8).