Analysis and Research on EMI Suppression Methods

Research direction:

* Four methods to suppress EMI of switch tubes and diodes

Solution:

* Parallel RC absorption circuit and series saturable magnetic core coil mainly suppress high voltage and surge current

* Quasi-resonance technology mainly reduces the switching loss of the switch tube and suppresses its electromagnetic interference

* LLC series resonance technology can suppress EMI of switch tubes and diodes

Abstract: With the continuous advancement of electronic technology, switching power supplies are developing rapidly towards high frequency and high efficiency. EMI suppression has become an important indicator in the design of switching power supplies. This article combines the EMI generation mechanism of the switch tube and diode in the switching power supply, lists four methods of EMI suppression: parallel absorption circuit, series saturable magnetic core coil, traditional quasi-resonance technology, and LLC series resonance technology, and compares and analyzes their suppression effects.

1 Introduction

Electromagnetic interference (EMI) is the lack of electromagnetic compatibility, which is the process of destructive electromagnetic energy from one electronic device to another through conduction or radiation. In recent years, switching power supplies have developed rapidly with their advantages of high frequency, high efficiency, small size, and stable output. Switching power supplies have gradually replaced linear regulated power supplies and are widely used in computers, communications, automatic control systems, household appliances and other fields. However, due to the high frequency and high di/dt and high dv/dt of switching power supplies, switching power supplies have a very prominent disadvantage - they are prone to generate relatively strong electromagnetic interference (EMI) signals. 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 interfere with communication equipment and electronic products. Therefore, how to reduce or even eliminate the EMI problem in switching power supplies has become a very concerned issue for switching power supply designers. This article focuses on four methods to suppress EMI of switching tubes and diodes in switching power supplies.

2 Switching tube and diode EMI generation mechanism

The fundamental reason why the switching power supply itself generates electromagnetic interference when the switch tube works under hard switching conditions is that the high-speed switching of the switch tube and the reverse recovery of the rectifier diode during its operation generate high di/dt and high dv/dt, and the surge current and peak voltage generated by them form interference sources. The switch tube will also generate high di/dt and high dv/dt when working under hard switching, thereby generating large electromagnetic interference. Figure 1 depicts the switching trajectory of the switch tube when the switch tube works under hard switching conditions when connected to an inductive load. The dotted line in the figure is the safe working area of ​​the bipolar transistor. If the switching conditions of the switch tube are not improved, its switching trajectory is likely to exceed the safe working area, resulting in damage to the switch tube. Due to the high-speed switching of the switch tube, the inductive load such as the high-frequency transformer or energy storage inductor in the switching power supply is forced to have a large surge current in the primary of the transformer at the moment when the switch tube is turned on, which will cause a peak voltage. During the cut-off period of the switch tube, the leakage inductance of the high-frequency transformer winding causes a sudden change in current, thereby generating a reverse electromotive force E=-Ldi/dt, whose value is proportional to the current change rate (di/dt) and the leakage inductance. It is superimposed on the turn-off voltage to form a turn-off voltage spike, thereby forming electromagnetic interference. In addition, the reverse recovery characteristics of the reverse parallel diode on the switch tube are not good, or the parameters of the voltage spike absorption circuit are improperly selected, which can also cause electromagnetic interference. There are two sources of interference caused by the reverse recovery of the rectifier diode, which are the input rectifier diode and the output rectifier diode. They are all interference caused by the commutation of the current. As shown in Figure 2, when t0=0, the diode is turned on, and the current of the diode increases rapidly, but its tube voltage drop does not drop immediately, but a rapid overshoot will appear. The reason is that during the opening process, the long base region of the diode PN junction injects enough minority carriers, and it takes a certain time tr for conductivity modulation to occur. This voltage overshoot will cause a broadband electromagnetic noise. When the switch is turned off, the large number of excess minority carriers in the long base region of the PN junction need a certain amount of time to recover to the equilibrium state, resulting in a large reverse recovery current. When t=t1, the PN junction begins to reversely recover. During the time t1-t2, other excess carriers rely on the recombination center to recombine and return to the equilibrium state. At this time, a negative spike appears in the tube voltage drop. Usually t2

3 EMI Suppression Methods

Di/dt and dv/dt are the key factors for the electromagnetic interference generated by the switching power supply itself. Reducing any of them can reduce the electromagnetic interference in the switching power supply. From the above, it can be seen that di/dt and dv/dt are mainly caused by the fast switching of the switch tube and the reverse recovery of the diode. Therefore, if you want to suppress EMI in the switching power supply, you must solve the problems caused by the fast switching of the switch tube and the reverse recovery of the diode.

3.1 Parallel absorption device

Adopting an absorption device is a good way to suppress electromagnetic interference. The basic principle of the absorption circuit is to provide a bypass for the switch when it is disconnected, absorb the energy accumulated in the parasitic distributed parameters, and thus suppress the interference. Common absorption circuits include RC and RCD. The advantages of this type of absorption circuit are simple structure, low price, and easy implementation, so it is a commonly used method to suppress electromagnetic interference.

(1) Parallel RC circuit

An RC absorption circuit is added at both ends of the switch tube T, as shown in Figure 3. An RC absorption circuit is added at both ends of the rectifier diode D in the secondary rectifier circuit, as shown in Figure 5, to suppress surge current.

(2) Connecting to RCD circuit

Add RCD absorption circuit at both ends of the switch tube T, as shown in Figure 4.

3.2 Series connection of saturable magnetic core coil

In the secondary rectification circuit, the saturable magnetic core coil is connected in series with the rectifier diode D, as shown in Figure 5. The saturable magnetic core coil is saturated when the normal current passes through it, and the inductance is very small, which will not affect the normal operation of the circuit. Once the current is about to reverse, the magnetic core coil will generate a large back electromotive force to prevent the reverse current from rising. Therefore, connecting it in series with the diode D can effectively suppress the reverse surge current of the diode D.

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3.3 Traditional Quasi-Resonant Technology

Generally speaking, soft switching technology can be used to solve the problem of switch tubes, as shown in Figure 6. Figure 6 shows the switching trajectory of the switch tube working under soft switching conditions. Soft switching technology mainly reduces the switching loss on the switch tube and can also suppress the electromagnetic interference on the switch tube. Among all soft switching technologies, quasi-resonance has a better effect in suppressing electromagnetic interference on the switch tube, so this article takes quasi-resonance technology as an example to introduce soft switching technology to suppress EMI. The so-called quasi-resonance means that the switch tube is turned on at the bottom of the voltage, as shown in Figure 7. The parasitic inductance and capacitance in the switch as part of the resonant element can completely control the occurrence of current surge when the switch is turned on and voltage surge when it is turned off. This method can not only reduce the switching loss to a very small level, but also reduce noise. Valley switching requires that the energy stored in during the off time must be released when the switch is turned on. Its average loss is, and it can be seen from this formula that reducing will greatly reduce, thereby reducing the stress on the switch, improving efficiency, and reducing dv/dt, that is, reducing EMI.

3.4 LLC Series Resonance Technology

FIG8 is the topological structure of LLC series resonance. As can be seen from the figure, the two main switches Q1 and Q2 form a half-bridge structure, and the driving signal is a complementary signal with a fixed 50% duty cycle. The inductor Ls, the capacitor Cs and the excitation inductor Lm of the transformer form an LLC resonant network. In the LLC series resonant converter, since the excitation inductor Lm is connected in series in the resonant circuit, the switching frequency can be lower than the intrinsic resonant frequency fs of the LC, and the zero voltage turn-on of the main switch can be achieved by only being higher than the intrinsic resonant frequency fm of the LLC. Therefore, LLC series resonance can reduce the EMI on the main switch tube and minimize electromagnetic radiation interference (EMI). In the LLC resonant topology, as long as the resonant current has not dropped to zero, the frequency adjustment trend of the output voltage will not change, that is, the output voltage will continue to rise as the frequency decreases. At the same time, due to the existence of the resonant current, the zero voltage turn-on condition of the upper and lower main switches of the half bridge is guaranteed. Therefore, the operating frequency of the LLC resonant converter has a lower limit, that is, the series resonant frequency fm of Cs and Ls and Lm. In the operating frequency range of fm

4 Comparative analysis of inhibition methods

The use of parallel RC absorption circuit and series saturable magnetic core coil are both simple and commonly used methods. They are mainly used to suppress high voltage and surge current, and play an absorption and buffering role. Their EMI suppression effect is poorer than that of quasi-resonance technology and LLC series resonance technology. The following focuses on the comparative analysis of quasi-resonance technology and LLC series resonance technology. Adding RCD snubber circuit to quasi-resonance, that is, a spike voltage absorption circuit composed of diodes, capacitors and resistors, its main function is to absorb the rising edge spike voltage energy generated by the MOSFET power switch tube when it is turned off, reduce the spike voltage amplitude, and prevent the power switch tube from overvoltage breakdown. However, this will increase the loss, and because a diode is used in the snubber circuit, the reverse recovery problem of the diode will also increase. From the above analysis, it can be seen that the quasi-resonance technology mainly reduces the switching loss on the switch tube and can also suppress the electromagnetic interference on the switch tube, but it cannot suppress the electromagnetic interference on the diode, and when the input voltage increases, the frequency increases; when the output load increases, the frequency decreases, so its suppression effect is not very good, and generally cannot achieve the desired results. Therefore, if you want to get a better suppression effect, you must solve the reverse recovery problem on the diode, so that the suppression effect can satisfy people. The LLC series resonant topology is better than the quasi-resonant topology in suppressing EMI. Its advantages have been analyzed above.

5 Conclusion

With the continuous development of switching power supply technology, its size is getting smaller and smaller, and its power density is getting higher and higher. EMI has become a key factor in the stability of switching power supplies. The internal switch tube and diode of the switching power supply are the main sources of EMI. This article mainly introduces four methods of suppressing EMI of switch tubes and diodes and analyzes and compares them, with the aim of finding a more effective method to suppress EMI. Through analysis and comparison, it is concluded that LLC series resonance technology has a better suppression effect, and its efficiency increases with increasing voltage. Its operating frequency changes greatly with voltage, but changes less with load.