Switching power supply electromagnetic interference suppression technology and design method

1 switching power supply electromagnetic interference generation

The switching power supply usually rectifies the commercial frequency alternating current into direct current, and then turns it into a high frequency through the control of the switching tube, and then outputs through the rectifying and filtering circuit to obtain a stable direct current voltage. Power frequency rectification filtering uses high-capacity capacitor charging and discharging, high-frequency switching of the switching tube, reverse recovery of the output rectifier diode, etc., which produces extremely high di/dt and du/dt, forming a strong inrush current. And spike voltage, which is the most basic cause of electromagnetic interference from switching power supplies. In addition, the driving waveform of the switching transistor, the MOSFET drain-source waveform, and the like are periodic waves close to a rectangular wave shape. Therefore, the frequency is in the MHz level, and these high-frequency signals interfere with the basic signals of the switching power supply, especially the signals of the control circuit.

1.1 Input Harmonic Interference of Rectifier Circuit

The switching power supply input terminal usually adopts a bridge rectifier and capacitor filter circuit. The rectifier bridge can only be turned on when the ripple voltage exceeds the voltage on the input filter capacitor. The current is input from the mains supply and the filter capacitor is charged. Once the voltage across the filter capacitor is higher than the instantaneous voltage of the mains supply, the rectifier is turned off. Therefore, the current of the input circuit is pulsed and has a high efficiency of harmonic current. This is because of the nonlinear characteristics of the rectifier circuit, and the current on the AC side of the rectifier bridge is severely distorted.

The number of harmonics on the DC side is n times. Therefore, the high-frequency harmonic current on the DC side of the rectifier circuit not only causes the circuit to generate power, but also increases the reactive power of the circuit, and the high-frequency harmonics generate conducted interference and radiation interference along the transmission line.

1.2 Interference generated by the switching circuit

Switching circuits play a key role in switching power supplies and are one of the main sources of interference. The switch tube load is the primary coil of the high frequency transformer and is an inductive load. At the moment of conduction, the primary coil generates a large inrush current and a high surge spike voltage appears at both ends of the primary coil; at the moment of disconnection, due to the leakage flux of the primary coil, a portion of the energy is not from the primary coil Transmitted to the secondary coil, the energy stored in the inductor will form a peak with attenuated oscillations of the capacitor and resistor in the collector circuit, superimposed on the turn-off voltage to form a turn-off voltage spike. If the spike has a high enough amplitude, it is very likely that the switch will break down.

1.3 Common mode conducted disturbance caused by high frequency transformer

High-frequency transformer is an important component in energy storage, isolation, output and voltage conversion in switching power supply. Its leakage inductance and distributed capacitance have a great influence on the electromagnetic compatibility of the circuit. Since the primary coil has leakage flux, a part of the energy is not transmitted to the secondary coil, but a decaying oscillation with a peak formed by a capacitor and a resistor in the collector circuit is superimposed on the turn-off voltage to form a turn-off voltage spike. Produces the same magnetizing inrush current transient as when the primary coil is turned on. This noise is transmitted to the input and output terminals to form conducted disturbances, which may break through the switching tube. In addition, the high frequency switching current loop formed by the high frequency transformer primary coil, the switching tube and the filter capacitor may generate large space radiation to form radiated disturbance.

There is a distributed capacitance between the primary and secondary of the FM transformer of the switching power supply. Interference channels are formed by using a device capacitor (the distributed capacitance of the device to ground) to be equivalent to the entire switching power supply. The common mode interference passes through the coupling capacitance of the transformer, and then returns to the ground through the device capacitor, and a voltage divider composed of the transformer coupling capacitor and the device capacitor is obtained. The high-frequency switching current loop formed by the primary coil, the switching tube and the filter capacitor of the pulse transformer may generate large space radiation and form radiated disturbance.

1.4 Distribution and parasitic parameters caused by switching power supply noise

The distributed parameter of the switching power supply is the internal factor of most interference. The distributed capacitance between the switching power supply and the heat sink, the distributed capacitance between the primary and secondary of the transformer, and the leakage inductance of the primary and secondary sides are all noise sources. Common mode interference is transmitted through the distributed capacitance between the primary and secondary of the transformer and the distributed capacitance between the switching power supply and the heat sink. The distributed capacitance of the transformer winding is related to the structure and manufacturing process of the high-frequency transformer winding. The distributed capacitance between the switching power supply and the heat sink is related to the structure of the switch tube and the way the switch tube is mounted. The use of shielded insulating gaskets reduces the distributed capacitance between the switch and the heat sink.

Components that operate at high frequencies have high-frequency parasitic characteristics that affect their operating conditions. When the high frequency is working, the wire becomes the emission line, the capacitance becomes the inductance, the inductance becomes the capacitance, and the resistance becomes the resonance circuit. When the frequency is too high, the frequency characteristics of each element undergo a considerable change. In order to ensure the stability of the switching power supply during high-frequency operation, the design of the switching power supply should fully consider the characteristics of the components during high-frequency operation, and choose to use components with better high-frequency characteristics. In addition, at high frequencies, the inductive reactance of the parasitic inductance of the wire is significantly increased. Due to the uncontrollability of the inductor, it eventually becomes a single emission line, which becomes a source of radiation interference in the switching power supply.

2 measures to suppress electromagnetic interference

Switching power supplies have two forms of electromagnetic interference: common mode interference and differential mode interference. According to the electromagnetic interference sources analyzed above, combined with their coupling paths, interference can be suppressed from EMI filters, absorption circuits, grounding, and shielding to attenuate electromagnetic interference to within the allowable limits.

2.1 AC input EMI filter

Filtering is a method of suppressing conducted interference. Connecting a filter at the input end of the power supply can suppress the noise from the power grid from invading the power supply itself, and can also suppress the interference generated by the switching power supply and fed back to the power grid. As an important unit to suppress the conducted interference of power lines, the power supply filter plays an extremely important role in the electromagnetic compatibility design of equipment or systems. The power input terminal usually uses the EMI filter circuit as shown in Figure 1. The circuit can effectively suppress low frequency differential mode disturbance and high frequency mode common mode disturbance at the input end of the AC power source. In the circuit, the differential mode capacitors Cx1 and Cx2 (also called X capacitors) across the power supply are used to filter out the differential mode interference signal. Generally, a ceramic capacitor or a polyester film capacitor is used. The capacitance value is usually 0.1~0.44F. . The common mode capacitors Cy1 and Cy2 (also known as Y capacitor) grounded in the middle of the line are used to short-circuit the common mode noise current, which is usually C1=C2 # 2 200 pF. The suppression inductors L1 and L2 are usually taken as 100~130H. The common mode choke L is composed of two coils which are equivalent and wound in the same direction on one core. The inductance L#15~25 mH is usually required. When the load current crosses the common mode choke, the magnetic lines of force generated by the coils connected in series on the live line are opposite to the lines of magnetic force generated by the coils connected in series on the zero line, which cancel each other out in the core. Therefore, even in the case of a large load current, the core does not saturate. For the common mode interference current, the magnetic fields generated by the two coils are in the same direction, which will exhibit a large inductance, thereby attenuating the common mode interference signal.

2.2 using absorption circuit

The main cause of EMI in switching power supplies is the sharp change in voltage and current, so it is necessary to reduce the rate of change of voltage and current in the circuit (du/dt and di/dt) as much as possible. The absorption circuit can suppress EMI. The basic principle is to provide a bypass when the switch is turned off, and absorb the energy accumulated in the parasitic distribution parameters, thereby suppressing the occurrence of interference. The RC snubber circuit as shown in Figure 2(a) can be connected in parallel across the switch. During the turn-on and turn-off of the switch or diode, the reverse spike current and spike voltage generated in the tube can be buffered. get over. The buffer absorption circuit can reduce the amplitude of the spike voltage and reduce the rate of change of the voltage waveform, which is very beneficial for the safety of the semiconductor device. At the same time, the buffer absorption circuit also reduces the spectral composition of the RF radiation, which is beneficial to reduce the energy of the RF radiation. Clamp circuits are primarily used to prevent the risk of breakdown of semiconductor devices and capacitors. 5倍。 The breakdown voltage of the primary winding is selected to be 1.5 times the induced voltage of the primary winding. When the voltage on the TVS exceeds a certain amplitude, the device turns on quickly, draining the surge energy and limiting the amplitude of the surge voltage to a certain amplitude. The saturable core coil or the microcrystalline magnetic bead may be connected in series on the positive electrode lead of the switch tube drain and the output diode, and the material is generally cobalt. When the normal current is passed, the core is saturated, and the inductance is very small. Once the current is flowing in the opposite direction, it will generate a very large back EMF, which effectively suppresses the reverse surge current of the diode.

2.3 Shielding measures

An effective way to suppress radiated noise is to shield. The electric field can be shielded with a material with good electrical conductivity, and the magnetic field can be shielded with a material with high magnetic permeability. In order to prevent the magnetic field leakage of the transformer and make the primary and secondary coupling of the transformer good, the closed magnetic ring can be used to form the magnetic shielding. For example, the leakage flux of the can core is significantly smaller than that of the E type. For the connection line of the switching power supply, the power line should use a shielded wire to prevent external interference from being coupled into the circuit. Or use EMC components such as magnetic beads and magnetic rings to filter out high-frequency interference from power supplies and signal lines. However, be aware that the signal frequency cannot be disturbed by the EMC component, ie the signal frequency is within the passband of the filter. The outer casing of the switching power supply also needs to have good shielding characteristics, and the joints must meet the shielding requirements specified by EMC. Through the above measures, the switching power supply is protected from external electromagnetic environment and does not interfere with external electronic equipment.

2.4 Winding of the transformer

The leakage inductance must be minimized when designing high frequency transformers. Because the larger the leakage inductance, the higher the peak voltage amplitude generated, the greater the loss of the drain clamp circuit, which inevitably leads to a decrease in power supply efficiency. To reduce the leakage inductance of the transformer, measures such as reducing the number of turns of the primary winding, increasing the width of the winding, and reducing the insulation between the windings are generally employed.

The main parasitic parameters of the transformer are leakage inductance, inter-winding capacitance, and cross-coupling capacitance. The cross-coupling capacitance between the transformer windings provides a path for common mode noise to flow through the entire system.

Faraday shields are used in the winding process of the transformer to reduce the cross-coupling capacitance. The Faraday shield is simply wrapped between copper and aluminum foil between the primary winding and the secondary winding to form a surface shield isolation region and grounded, wherein the primary winding and the secondary winding are interleaved to reduce the crossover. Coupling capacitor. In the installation procedure, the heat sink is generally required to be grounded. The parasitic capacitance between the drain and the heat sink of the switch provides a path for common mode noise. A copper foil or aluminum foil can be added between the drain and the heat sink and grounded. Reduce this parasitic capacitance.

2.5 Application of Grounding Technology

The switching power supply needs to pay attention to the connection of the ground wire. The ground wire bears the heavy responsibility of the reference level, especially the reference ground of the control circuit, such as the ground level of the current detecting resistor and the ground level of the voltage dividing resistor without the isolated output.

(1) Signal grounding of the device. The signal ground of the device, possibly a point or piece of metal in the device as the ground reference point for the signal, provides a common reference potential for all signals in the device. Such as floating ground and hybrid grounding, in addition to single point grounding and multi-point grounding.

(2) The device is connected to the earth. In engineering practice, in addition to seriously considering the signal grounding inside the equipment, the signal ground of the equipment, the casing and the earth are usually connected together, and the earth is used as the grounding reference point of the equipment.

The ground level attenuation of the control signal should be as small as possible. Therefore, the control part is grounded at one point, and then the common connection point is connected to the power ground. This grounding method separates the noise source from the sensitive circuit. In addition, the ground wire should be as wide as possible, and the blank area can be filled with copper to reduce the ground level error and EMI.

Surface mount components are used as much as possible in the device to make the assembly density higher, the volume is smaller, the weight is lighter, the reliability is higher, the high frequency characteristics are good, and electromagnetic and radio frequency interference are reduced.

2.6 PCB component layout and routing

The string between wires, wires and cables in the PCB is one of the most difficult problems in printed circuit board circuits [7]. The radiated disturbance of the switching power supply is proportional to the product of the current in the current path, the loop area of ​​the path, and the square of the current frequency. Therefore, the layout design of the PCB will directly affect the electromagnetic compatibility of the whole machine. When designing a switching power supply printed circuit board, you must start with an optimized layout and routing.

(1) The layout of the printed circuit board usually conforms to the following principles: 1. The wires used for the input and output terminals should be avoided as much as possible. It is best to add the ground wire between the wires to avoid feedback coupling. 2. The printed circuit board wire should be wide wire as much as possible, especially the power cable and the ground wire. 3. The curved portion of the printed conductor generally adopts a circular arc shape; 4. The dedicated zero-volt line The trace width of the power cable (1 mm, the power cable and ground wire are as close as possible, etc.).

(2) The layout of components must generally conform to the following principles: 1. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.

2. Center around the core components of each functional circuit and arrange it around it. Components should be evenly, neatly and compactly placed on the PCB to minimize and shorten leads and connections between components.

3. For circuits that operate at high frequencies, the distribution parameters between components should be considered. The general circuit should make the components equally arranged as much as possible.

4. The components located at the edge of the board are generally not less than 2 mm from the edge of the board.

3 Conclusion

The switching power supply is getting smaller and smaller, and the power density is getting larger and larger. The EMI/EMC problem has become a key factor in the stability of the switching power supply, and it has received more and more attention. There are many electromagnetic compatibility control strategies and control technologies for switching power supplies, such as suppression of interference transmission channels, separation of space, separation of time, frequency management, and electrical isolation. In the design of the switching power supply, only the comprehensive use of various electromagnetic interference suppression technologies can effectively improve the electromagnetic compatibility of the switching power supply, and truly meet the needs of various occasions.