Analysis of EMI Suppression Method for Switching Tube and Diode in Switching Power Supply

Analysis of EMI suppression methods for switching tubes and diodes in switching power supplies
1. Introduction
Electromagnetic interference (EMI) is the lack of electromagnetic compatibility, and it is the process of destructive electromagnetic energy passing from one electronic device to another through conduction or radiation. In recent years, switching power supply has developed rapidly due to its 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 computer, communication, automatic control systems, household appliances and other fields. However, because the switching power supply operates at high frequency and its high di/dt and high dv/dt, the switching power supply has a very prominent disadvantage-it is easy to generate relatively strong electromagnetic interference (EMI) signals. EMI signals not only have a wide frequency range, but also have a certain amplitude, which will pollute the electromagnetic environment through conduction and radiation, and cause interference to communication equipment and electronic products. Therefore, how to reduce or even eliminate the EMI problem in switching power supply has become a problem that switching power supply designers are very concerned about. This article focuses on four methods of suppressing EMI of switching tubes and diodes in switching power supplies.
2. EMI generation mechanism of switch tube and diode
The root cause of the electromagnetic interference generated by the switching power supply itself when the switching tube works under hard switching conditions is that the high-speed switching of the switching tube and the reverse recovery of the rectifier diode during its working process produce high di/dt and high dv/dt, which generate The inrush current and peak voltage form a source of interference. When the switching tube works in hard switching, it will also produce high di/dt and high dv/dt, thus generating large electromagnetic interference. Figure 1 depicts the switching track of the switching tube when the switching tube works under hard switching conditions when connected to an inductive load. The dotted line in the figure is the safe operating area of ​​the bipolar transistor. If the switching conditions of the switching tube are not improved, the switching track It is likely to exceed the safe working area, resulting in damage to the switch tube. Due to the high-speed switching of the switching tube, the inductive load such as the high-frequency transformer or energy storage inductor in the switching power supply will force a large surge current in the primary of the transformer when the switching tube is turned on, which will cause a peak voltage. During the cut-off period of the switch tube, the current mutation caused by the leakage inductance of the high-frequency transformer winding generates a counter electromotive force E=-Ldi/dt, whose value is proportional to the current change rate (di/dt) and proportional to the leakage inductance. The off-voltage peak is superimposed on the off-voltage to form electromagnetic interference. In addition, the reverse recovery characteristics of the anti-parallel diode on the switch tube are not good, or the parameters of the voltage spike absorption circuit are improperly selected, which will 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 disturbances caused by the commutation of the current. It can be seen from Figure 2 that when t0=0, the diode is turned on, and the current of the diode increases rapidly, but the voltage drop of the diode does not drop immediately, but a rapid upshoot occurs. The reason is that during the turn-on process, the long base region of the diode PN junction injects enough minority carriers, and it takes a certain time tr for conductance modulation to occur. This voltage overshoot causes a broadband electromagnetic noise. When it is turned off, a large number of excess minority carriers existing in the long base region of the PN junction need a certain period of time to restore to an equilibrium state, resulting in a large reverse recovery current. When t=t1, the PN junction begins to reverse recovery, and within the time t1-t2, other excess carriers rely on the recombination center to recombine and return to the equilibrium state. At this time, another negative spike appears in the tube pressure drop. Usually t2 < t1, so this spike is a very narrow spike that generates more electromagnetic noise than it was at turn-on. Therefore, the reverse recovery interference of the rectifier diode is also an important source of interference in the switching power supply.


3. EMI suppression method
Di/dt and dv/dt are the key factors for the electromagnetic interference generated by the switching power supply itself, and reducing any of them can reduce the electromagnetic interference in the switching power supply. It can be seen from the above that di/dt and dv/dt are mainly caused by the fast switching of the switching tube and the reverse recovery of the diode. Therefore, if you want to suppress the EMI in the switching power supply, you must solve the problems caused by the fast switching of the switching tube and the reverse recovery of the diode.
3.1 Parallel absorbing device
Adopting absorbing device is a good way to suppress electromagnetic interference. The basic principle of the absorbing circuit is that the switch provides a bypass for the switch when it is turned off, absorbing the energy accumulated in the parasitic distribution parameters, thereby suppressing the occurrence of interference. Commonly used snubber circuits are RC and RCD. The advantages of this type of absorbing circuit are simple structure, cheap price, and easy implementation, so it is a commonly used method for suppressing electromagnetic interference.

(1) Connect RC circuit in parallel:
add RC absorbing circuit at both ends of the switching tube T, as shown in Figure 3. Add an RC absorbing circuit at both ends of the rectifying diode D in the secondary rectifying circuit, as shown in Figure 5, to suppress the surge current.
(2) Connect RCD circuit
in parallel Add RCD absorbing circuit at both ends of switch tube T, as shown in Figure 4.
3.2 Saturable magnetic core coil connected
in series In the secondary rectification circuit, the coil of saturable magnetic core 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 is passed, and the inductance is very small, which will not affect the normal operation of the circuit. Once the current is going to be reversed, the magnetic core coil will generate a large counter electromotive force to prevent the rise of the reverse current. Therefore, connecting it in series with the diode D can effectively suppress the reverse surge current of the diode D.
3.3 Traditional quasi-resonant technology
Generally speaking, soft switching technology can be used to solve the problem of switching tubes, as shown in Figure 6. Figure 6 shows the switching track of the switching tube working under soft switching conditions. The 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 the soft switching technologies, quasi-resonance suppresses the electromagnetic interference on the switching tube better, so this article takes the quasi-resonant technology as an example to introduce the 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, fully controls the occurrence of current surges when the switch is turned on and voltage surges when it is turned off. In this way, not only can the switching loss be reduced to a minimum, but also the noise can be reduced. Valley switching requires that the energy stored in the switch during the off time must be released when the switch is on. Its average loss is: It

can be seen from this formula that the reduction will lead to a large reduction, thereby reducing the stress on the switch, improving efficiency, reducing dv/dt, that is, reducing EMI.



Figure 8 shows the topology of the LLC series resonance. It can be seen from the figure that the two main switches Ql and Q2 form a half-bridge structure, and its driving signal is a complementary signal with a fixed 50% duty cycle. The inductance Ls, the capacitance Cs and the excitation inductance Lm of the transformer form an LLC resonant network . In the LLC series resonant converter, since the excitation inductance 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 only needs to be higher than the intrinsic resonant frequency fm of the LLC to realize zero switching of the main switch. The voltage is turned on. Therefore, the LLC series resonance can reduce the EMI on the main switch tube and minimize the electromagnetic radiation interference (EMI). In the LLC resonant topology, as long as the resonant current has not dropped to zero, the adjustment trend of the frequency to the output voltage will not change, that is, the output voltage will continue to rise as the frequency decreases. The zero-voltage turn-on condition of the main switch 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 fm<f<fs, the main switch on the primary side works under the condition of zero voltage turn-on, and does not depend on the magnitude of the load current. At the same time, when the rectifier diode on the secondary side works in discontinuous or critical discontinuous state, the rectifier diode can be turned off under the condition of zero current, the problem of its reverse recovery is solved, and there is no voltage spike anymore.
4. Comparative analysis of suppression methods
The parallel RC snubber circuit and the series saturable magnetic core coil are simple and commonly used methods, mainly to suppress high voltage and surge current, play the role of absorption and buffer, and their suppression effect on EMI is comparable Inferior than quasi-resonant technology and LLC series resonant technology. The following focuses on the comparative analysis of the quasi-resonant technology and the LLC series resonant technology. An RCD snubber circuit is added to the quasi-resonance, that is, a peak voltage absorption circuit composed of diodes, capacitors and resistors. Its main function is to absorb the rising edge peak voltage energy generated by the MOSFET power switch tube when it is turned off, and reduce the peak voltage amplitude. value, to prevent overvoltage breakdown of the power switch tube. However, this will increase losses, and since the diode is used in the snubber circuit, it will also increase the reverse recovery problem of the diode. It can be seen from the above analysis 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 be satisfactory. The LLC series resonant topology suppresses EMI better than the quasi-resonant one. Its advantages have been analyzed above.
5. Conclusion
With the continuous development of switching power supply technology, its volume is getting smaller and smaller, and its power density is getting higher and higher. EMI has become a key factor for the stability of switching power supply. The internal switching tube and diode of the switching power supply are the main sources of EMI. This paper mainly introduces four methods of suppressing EMI of switching tubes and diodes, analyzes and compares them, and aims to find a more effective method of suppressing EMI. Through analysis and comparison, it can be concluded that the suppression effect of LLC series resonance technology is better, and its efficiency increases with the increase of voltage, and its operating frequency varies greatly with voltage, but the change with load is small.