EMC Analysis of Buck Converter

The switching power supply effectively controls the output voltage and current by changing the conduction ratio of the switching device. It usually works at a switching frequency of more than tens of kHz. When the switch is turned on, it will flow through the surge current Cdv/dt; when the switch is turned off, a surge voltage Ldi/dt will be generated at both ends, forming a strong electromagnetic interference source. With the continuous development of semiconductor switching devices, the switching frequency will increase to the MHz level, making the electromagnetic interference more serious. Therefore, corresponding measures must be taken to enhance the electromagnetic compatibility (EMC) of the switching power supply.

Electromagnetic compatibility refers to the ability of information and interference to coexist without losing the information contained in the useful signal. In its use environment, power electronic devices are subjected to external electromagnetic interference and also release interference to the surrounding environment. When designing and manufacturing power electronic devices, it should be considered that the electromagnetic disturbance generated by the power electronic device during operation will not have an adverse effect on the operation of other electronic equipment working in the same environment, and at the same time, the electromagnetic interference from the external environment will not affect the operation of the power electronic device.

1 Electromagnetic interference of Buck system

The following is a detailed discussion of the causes and conditions of electromagnetic interference in conjunction with the Buck converter, so as to find out how to suppress and eliminate it. Figure 1 is a schematic diagram of the Buck converter.

Figure 1 Buck structure diagram

The main circuit is mainly composed of power switch tube S, Schottky diode D, filter capacitor C, inductor L, resistive load Ro and non-inductive sampling resistor RL. The basic parameters of this circuit are that the input end is a 36V lead-acid battery, the output requirement is 10A constant current, and the switching frequency is 50kHz. The control chip uses SG3525, and the driver chip uses TLP250. The auxiliary power supply uses flyback. It is an important step to select the appropriate closed-loop parameters for the main circuit. Appropriate closed-loop parameters can make the circuit stable and generate smaller EMD.

Figure 2 is a schematic diagram of the electromagnetic compatibility of the system. Combined with this diagram, we can analyze the electromagnetic environment in which the system is located and its interaction. Obviously, electromagnetic interference can occur both within the system and between systems.

Figure 2 Schematic diagram of electromagnetic compatibility of the system

As can be seen from Figure 2, any EMI consists of three parts: disturbance source, coupling path and victim. The interference generated by the disturbance source enters the victim through the coupling path. If the interference level exceeds the sensitivity of the victim, it will affect its normal operation and constitute interference. Compared with digital circuits, due to the high-speed switching action of the power switch tube of the switching power supply, it generates greater interference intensity; the disturbance source is mainly concentrated on the power switch device and the high-frequency transformer connected to it; the switching frequency is not high, and the main interference forms are conducted interference and near-field interference. Generally, EMD is solved in three aspects: suppressing the disturbance source, cutting off the interference path and improving the anti-interference ability of the victim.

It can be seen that the main interference sources of Buck are the switch tube and the power diode. Due to the high switching frequency and large energy transmission, very high glitches will be generated during the switching process.

Since the geometric size of the designed switching power supply is much smaller than the wavelength corresponding to the 30MHz electromagnetic field, the electromagnetic interference mainly considers the conducted interference. The voltage spikes and oscillations generated by the MOSFET during the switching process mainly interfere with the disturbed body through the parasitic inductance and parasitic capacitance of the wire. The faster the switching process, the higher the spike, the more obvious the oscillation, and the stronger the interference.

2. Measures to suppress electromagnetic interference

The analysis and design are mainly carried out from the aspects of filtering, grounding, absorption and PCB layout.

2.1 Filtering

Since the battery has a certain internal impedance, coupled with the parasitic inductance and internal resistance on the input lead, a series of high-frequency ripples will be caused at the input end. In order to make the input end a constant voltage source that meets the requirements, an EMI filter needs to be added to the circuit input end, which not only suppresses the interference of the outside world to the circuit, but also prevents the circuit from interfering with the battery. Usually, an electrolytic capacitor and a capacitor that filters high-frequency ripples are connected in parallel at the input end. The electrolytic capacitor mainly filters low-frequency ripples, and the capacitor that filters high-frequency ripples uses a CBB capacitor.

Due to the requirements of the output end on the current waveform, the size of the output ripple must be reduced. Therefore, it is also necessary to connect a large-capacity electrolytic capacitor and a large-capacity CBB capacitor to filter the high-frequency ripple in parallel at the output end.

In addition, the decoupling filter capacitors of the integrated chip need to be scientifically configured. Each integrated chip is connected to a decoupling filter capacitor, which is recharged during each switching process to power the chip. The value of the decoupling capacitor is generally 470pF ~ 1000pF, and ceramic or CBB capacitors are used to filter out high-order ripples. The decoupling filter capacitor must be installed close to the integrated circuit, striving for the shortest capacitor lead and the smallest transient current loop area. At the same time, the overall decoupling capacitor should be placed on the PCB board of the entire integrated chip, charged by the power supply, and should be installed where the power bus enters the PCB board.

Figure 3 Sampling current waveform

The current sampling resistor is the most susceptible to interference in the system, and the accuracy of the sampling current will directly affect the output index of the circuit. The waveform of the sampling current is shown in Figure 3. Since the sampling resistor is interfered by the on and off of the switch tube, it is necessary to filter the signal on the sampling resistor. The RC second-order passive filter is used in this circuit, and the circuit is shown in Figure 4 below.

Figure 4 Second-order RC passive filter circuit

2.2 Grounding

The grounding of a system mainly includes safety ground, signal ground, chassis ground and shield ground. Here we only discuss the connection method of the common ground of this system.

1) The grounding system must have a very low common impedance to minimize the direct conduction noise voltage generated by the currents in the system through the common impedance.

2) In the case of high-frequency current, ensure that the "signal ground" has a lower common-mode voltage to the "earth" to minimize the radiated noise generated by the "signal ground".

3) Ensure that the current loop formed by the ground wire and the signal line has the smallest area, avoid the "ground loop" formed by the ground wire, and minimize the differential mode interference voltage generated by the external interference magnetic field passing through the loop. At the same time, avoid excessive ground current caused by the ground potential difference through the ground loop, causing conducted interference.

This system uses mixed grounding and floating grounding. The main power uses floating grounding to reduce common impedance and the passage of large current. The control system is first connected to the ground in series, and then connected to the main power ground at a single point. The drive circuit uses optocoupler isolation technology to drive the switch tube on and off.

2.3 Buffer

After taking the above measures, it was found that the glitch during the switching process of the MOSFET was still high. This is because the current flowing through the circuit is relatively large, and a very small parasitic inductance can also cause large glitch.

The purpose of the buffer circuit is to suppress the transient noise generated by the switch tube. The R-C-D network is connected in parallel at both ends of the switch tube to suppress it. It can slow down the voltage rise rate between the drain and source of the switch tube, as shown in Figure 5.

Figure 5 R-C-D absorption circuit

By adding a buffer circuit to the switch tube, a relatively ideal switching waveform can be obtained, as shown in Figure 6.

Figure 6 Switch tube vDS waveform

 During the shutdown process, the power diode will have reverse recovery, which is an important source of interference.

RC snubber circuit is a common method to solve the reverse recovery problem of power diodes. For power diodes working at high frequencies, parasitic parameters must be considered. Figure 7(a) is a circuit model, where D0 is an ideal diode, Lp is the lead inductance, Cp is the junction capacitance, Rp is the parallel resistance (high resistance), and Rs is the lead resistance. As shown in Figure 7(b), capacitor C and resistor R are connected in series and then connected in parallel to the power diode D. When the diode is reversely turned off, the energy in the parasitic inductance charges the parasitic capacitance, and also charges the snubber capacitor C through the snubber resistor R. Under the same energy conditions, the larger the snubber capacitor, the smaller the voltage on it; when the diode is forward-conducting, C discharges through R, and most of the energy is consumed on R.

(a) Equivalent model (b) RC snubber circuit

Figure 7 Power diode equivalent model and RC snubber circuit

By connecting an R-C snubber circuit in parallel to the power diode, a relatively ideal turn-on and reverse turn-off waveform can be obtained, as shown in Figure 8.

Figure 8 Waveform on power diode

2.4 PCB Layout

The placement and wiring design of components on the printed circuit board have a great impact on the EMC performance of the switching power supply. In the high-frequency switching power supply, since the printed board has both low-level small signal control lines and high-voltage and high-current power lines, as well as some high-frequency power switches and magnetic components, how to reasonably arrange the position of components and the reasonable wiring within the limited space of the printed board will directly affect the anti-interference of each component in the circuit and the reliability of the circuit operation.

By analyzing the characteristic impedance of printed conductors, the placement, length, width and overall layout of the wiring can be reasonably selected. The characteristic impedance of a single conductor is composed of DC resistance R and self-inductance L. When designing a printed circuit board, the impedance of the power line and the ground line should be minimized. Because the power line, ground line and other printed lines have inductance, when the power current changes greatly, a large voltage drop will be generated, and the ground line voltage drop is an important factor in the formation of common impedance interference, so the ground line should be shortened as much as possible, and the power line and ground line should be widened as much as possible.

The actual equivalent circuit of the DC power supply system is shown in Figure 9.

Figure 9 Equivalent circuit of DC power supply system

The transient noise voltage generated in the DC power supply circuit originates from the sudden change of the power supply load current ΔiL. The current change is instantaneous, so the amplitude of the transient voltage ΔuL generated is a function of the characteristic impedance Z0 of the power supply transmission line. Reducing the path area is the key to reducing radiation interference. In this Buck circuit, the input capacitor, switch tube, and power diode should be close to each other and the wiring should be compact; at the same time, the output power diode, inductor, output capacitor, and sampling resistor should be close to each other.

In addition, when wiring, make the drain connection of the switch tube as short and thick as possible to reduce the parasitic inductance of the wire. Select a suitable high-frequency filter capacitor (CBB capacitor is used in the prototype) and make it as close to the drain of the MOSFET as possible, and make the capacitor lead as short as possible to reduce the wire inductance.

According to the principles derived from the above analysis, the actual designed prototype main power PCB board is shown in Figure 10.

Figure 10 PCB board of Buck circuit

3 Conclusion

The purpose of electromagnetic compatibility design of switching power supply is to make the designed product not only work properly in a certain electromagnetic interference environment, but also make the electromagnetic disturbance generated by the product itself not affect the operation of other equipment. This paper summarizes the principles of electromagnetic compatibility design of Buck circuit through analysis of various aspects, and manufactures a test prototype based on this principle, thus illustrating the feasibility and correctness of these principles on the basis of practice, and providing good experience for the electromagnetic compatibility design of switching power supply in the future.