EMC design analysis of isolated DC/DC converters

With the development of power and electronic technology, switching power supply modules have gradually replaced traditional rectifier power supplies, with advantages such as relatively small size, high efficiency, and reliable operation. However, due to the high operating frequency of switching power supplies, high currents can be generated internally. The rate of voltage change (i.e., high dv/dt and di/df) causes strong electromagnetic interference in the switching power supply module, which is conducted through. Coupling modes such as radiation and crosstalk affect the normal operation of its circuits and other electronic systems. Of course, it can also be affected by electromagnetic interference from other electronic devices. Electromagnetic interference can cause transmission signal distortion and affect the normal operation of electronic equipment. High energy electromagnetic interference such as lightning and electrostatic discharge can damage electronic equipment in severe cases. For some electronic devices, electromagnetic radiation can lead to important information leakage, and in serious cases, it can threaten national information security. This is the electromagnetic compatibility issue we are discussing. In addition, the country has begun to enforce 3C certification for some electronic products. Therefore, whether electronic devices can meet electromagnetic compatibility standards will be related to whether the product can be sold on the market. Therefore, the research on electromagnetic compatibility of switching power supplies is very important.

1. Analyze internal noise interference sources

1.1 Noise interference caused by diode directional recovery

Power frequency rectifier diodes are commonly used in switching power supplies. High frequency rectifier diode. Due to the fact that these diodes operate in the ON/OFF state, as shown in the figure, during the transition of the diode from the OFF state to the ON state, a high voltage peak UFP will be generated; There is a reverse recovery time during the transition of the diode from the on state to the blocked state. During the reverse recovery process, due to the presence of diode packaging inductance and wire inductance, the reverse voltage peak URP will generate a transient reverse recovery current peak IRP, which is the source of electromagnetic interference.

1.2 Electromagnetic interference generated by switch tube switches

Under positive and negative loads. In a push-pull bridge converter, the current waveform flowing through the switch tube is similar to a rectangular wave under resistive load, rich in high-frequency components, and can generate strong electromagnetic interference. In flyback converters, the current waveform flowing through the switch tube is similar to a triangular wave under resistive load, with relatively few high harmonic components. When the switch is turned on, due to the short turn-on time, there is wire inductance in the inverter circuit, which can generate large DV/DT and peak voltage. When the switch is turned off, due to the short closing time, large DI/DT and high current spikes will occur. Sudden voltage changes can cause strong electromagnetic interference.

Electromagnetic interference caused by magnetic components such as inductors and transformers

Input filter inductors exist in switching power supplies. Power transformer. Isolation transformer. Magnetic components such as output filter inductors, and parasitic capacitance exist between the primary and secondary isolation transformers. High frequency interference signals are coupled to the secondary through parasitic capacitance; Due to winding technology and other reasons, the original. The secondary coupling is not ideal, and leakage will generate electromagnetic radiation interference. In addition, the coil winding of the power transformer flows through high-frequency pulse currents, forming high-frequency electromagnetic fields around it; The pulsating current in the inductor coil will generate electromagnetic radiation, and voltage peaks will occur when the load is suddenly switched off. At the same time, when it is saturated, it will produce a sudden change in current, resulting in electromagnetic interference.

1.4 Electromagnetic interference caused by control circuits

Periodic high-frequency pulse signals such as high-frequency pulse signals generated by oscillators can generate high-frequency and high harmonics, which can cause electromagnetic interference to surrounding circuits.

1.5 Other electromagnetic interference

There will also be ground loop interference in the circuit. Common impedance coupling interference, and control power noise interference. In addition, unreasonable wiring can cause electromagnetic interference to cross talk or radiate to adjacent wires through distributed mutual inductance between coupling capacitors and wires, thereby affecting the normal operation of other circuits. There is also electromagnetic interference caused by thermal radiation. Thermal radiation exchanges heat in the form of electromagnetic waves, affecting the normal and stable operation of other electronic components or circuits.

2. External electromagnetic interference

For certain electronic equipment, external electromagnetic interference includes harmonic interference, lightning, solar noise, electrostatic discharge, and surrounding high-frequency emission equipment.

Electromagnetic compatibility design of switching power supply

When setting electromagnetic compatibility standards for switching power supplies, it is important to first clarify the electromagnetic compatibility standards that the system needs to meet; Identify key circuits in the system, including strong interference source circuits. High sensitivity circuit; Identify electromagnetic interference sources and sensitive equipment in the working environment of power supply equipment; Then determine the electromagnetic compatibility measures for the power supply equipment. Therefore, the electromagnetic compatibility design of switching power supplies mainly starts from the following three aspects:

1) Reduce electromagnetic interference energy of interference sources;

2) Cut off the transmission path of interference;

3) Improve the anti-interference ability of disturbed equipment.

Taking isolated DC/DC converters as an example, the electromagnetic compatibility design of switching power supplies is discussed.

3.1 Electromagnetic Compatibility Design of DC/DC Converter Input Circuit

As shown in Figure 2, FV1 is a transient voltage suppression diode RV1 is a voltage sensitive resistor with strong transient surge absorption ability, which can effectively protect rear components or circuits from damage caused by surge voltages. Z1 is a DC EMI filter, and the ground wire must be short. It is best to install it directly on a metal housing and ensure its input. Shielding and isolation between output lines can effectively cut off conducted interference along input lines and radiated interference along space. L1 and C1 form a low-pass filter circuit. When the inductance saddle of L1 is large, D1 and R1 must also be added to form a freewheeling circuit to absorb the electric field energy released when L1 is disconnected. Otherwise, the voltage peak generated by L1 forms electromagnetic interference. The magnetic core used for inductor L1 is preferably a closed magnetic core. Leakage of magnetic fields from the air gap open loop magnetic core can form electromagnetic interference. C1 has good capacity and reduces the ripple electromagnetic field around the input line.

3.2 Electromagnetic compatibility design of high-frequency inverter circuit

As shown in Figure 3, a half bridge inverter circuit composed of C2.C3.V2.V3 is a switching device such as LGBT or M0SFET. When V2. V3 is switched on and off, due to the short switching time, the lead wire is inductive. Due to the presence of transformer leakage inductance, the circuit will generate a high di/dt.dv/dt, thereby forming electromagnetic interference. Therefore, an absorption circuit composed of R4.C4 is added to both ends of the transformer primary side or to both ends of V2.V3. In design, G4C5.C6. Generally, low inductance capacitors are used, and the capacitor capacity depends on the conductor inductance. The current value and allowable overshoot voltage value in the same circuit are obtained from LI2/2=C △ V2/2 to obtain C (L is the circuit inductance, I is the circuit current, and △ V is the overshoot voltage value).

In order to reduce △ V, it is necessary to reduce the inductance of the circuit leads. Therefore, a device called multilayer low inductance composite busbar is often used in design. The patented busbar device applied for by the Group can reduce the circuit inductance to a sufficiently small lonh level, thereby reducing electromagnetic interference in high-frequency inverter circuits.

The rapid switching action under high current or voltage is the basis for generating electromagnetic noise. Therefore, try to choose a circuit topology with low electromagnetic noise. For example, under the same conditions, the electromagnetic noise generated by a dual transistor forward topology is less than that generated by a single transistor forward topology, and the electromagnetic noise generated by a full bridge circuit is less than that generated by a half bridge circuit. In addition, using ZCS or ZVS soft switching conversion technology can effectively reduce electromagnetic interference in high-frequency inverter circuits.

Figure 4 shows the current on the switch tube after adding a buffer circuit. Compared to the waveform without a buffer circuit, the rate of change of current and voltage after the buffer circuit is greatly reduced.

Due to the fact that a transformer is a heating element, poor heat dissipation conditions will inevitably cause the temperature of the transformer to rise, resulting in thermal radiation. Therefore, the transformer must have good heat dissipation conditions.

The high-frequency transformer is usually packaged in an aluminum shell box and injected with electronic silicone. The aluminum box can also be installed on the aluminum radiator to form a better electromagnetic shield for the transformer and ensure good heat dissipation effect. Reduce magnetic radiation.

3.4 Electromagnetic compatibility design of output rectifier circuit

Figure 6 shows a half wave rectifier circuit, with D6 as the rectifier diode and D7 as the freewheeling diode. Due to D6. D7 operating in a high-frequency switching state, the electromagnetic interference sources of the output rectifier circuit are mainly D6 and D7. R5.G12 and R6.C13 are connected to D6.D7, respectively.

Reducing the number of rectifier diodes can reduce the energy of electromagnetic interference. Therefore, under the same conditions, half wave rectification produces less electromagnetic interference than full wave rectification and full bridge rectification.

In order to reduce the electromagnetic interference of diodes, it is necessary to select diodes with soft recovery characteristics. Low reverse recovery current. The reverse recovery time is short. Theoretically, Schottky barrier diodes (SBDs) are the conductors of most carriers, without minority carrier storage and recombination effects, so there will be no reverse voltage peak interference. However, in fact, for Schottky diodes with higher reverse operating voltages, the reverse recovery current will increase as the thickness of the electronic barrier increases, and electromagnetic noise will also occur. Therefore, at a low output voltage, Schottky diodes will generate less electromagnetic interference than other diodes.

3.5EMC design of output DC filter circuit

The output DC filter circuit is mainly used to cut off electromagnetic conducted interference propagating along the wire to the output load terminal, reducing electromagnetic interference caused by electromagnetic radiation around the wire.

As shown in Figure 7, the LC filter circuit composed of L2.C7.C18 can reduce the size of output current and voltage ripple, thereby reducing electromagnetic interference transmitted through radiation. The filter capacitors C17.C18 should be connected in parallel as much as possible to reduce the equivalent series resistance, thereby reducing the ripple voltage. The output inductance L2 should be as large as possible to reduce the magnitude of the output ripple current. In addition, it is best to use a closed loop magnetic core without gaps for inductor L2, and it is best not to use a saturated inductor. In design, please remember that there are changes in current and voltage on electric wires, and electromagnetic fields will propagate along space, forming electromagnetic radiation.

C19 is used to filter common mode interference on wires. Try to choose low induction capacitors with short wiring. C20.C21.C22.C23 used to filter differential mode interference on the output line should be selected as reliable.

Z3 is a first-class EMI filter, which can be single stage or multi-stage depending on the situation, but Z3 needs to be directly installed on a metal chassis with filter input. The output line is preferably shielded and isolated.

3.6 Electromagnetic compatibility design of contactors, relays, and fans

Relays, contactors, fans, etc. After power loss, the coil will generate large voltage peaks, resulting in electromagnetic interference. Therefore, a diode or RC absorption circuit is connected in parallel at both ends of the DC coil, and a voltage sensitive resistor is connected in parallel at both ends of the AC coil to absorb the voltage peaks generated after the coil loses power. If the contactor coil power supply is the same as the input power supply of the auxiliary power supply, it is best to pass an EMI filter. Relay contacts can also generate electromagnetic interference, so RC absorption circuits are added at both ends of the contacts.

3.7 Electromagnetic compatibility design of switching power supply box structure

1) The selection of materials in the structural design of the switching power supply box and the principle of shielding materials are high interference electromagnetic field frequency and good shielding effect; When the frequency of interference electromagnetic field is low, the shielding effect is good; In some cases, if high frequency and low frequency electromagnetic fields are required to have good shielding effects, metal materials with high conductivity and high magnetic permeability are usually used.

2) Hole. Clearance. Adopting electromagnetic shielding method can achieve good electromagnetic compatibility effect without redesigning the circuit. The ideal electromagnetic shielding is a seamless one. No holes. Non permeable conductive continuum, low impedance metal seal, but switching power supplies require input. The output line is perforated. "If no measures are taken, the overlapping gaps between cooling ventilation holes and other structural components of the box will lead to electromagnetic leakage and reduce the shielding efficiency of the box.". Even completely lost. Therefore, in the design of switching power supply boxes, it is best to weld the overlap between metal plates and use electromagnetic pads or other shielding materials; The aperture on the box is less than 1/2 of the shielding electromagnetic wave length, otherwise the shielding effect will be greatly reduced; For ventilation holes, when shielding requirements are not high, perforated metal plates or wire mesh can be used, which not only requires high shielding efficiency, but also requires good ventilation effect to improve shielding efficiency. If the shielding efficiency of the box still cannot meet the requirements, shielding paint can be sprayed on the box. In addition to shielding the entire switching power supply box, it can also partially shield components, components, and other interference sources or sensitive equipment in the power supply equipment.

3) When designing the box structure, a low impedance current leakage path should be designed. The box must have reliable grounding measures to ensure the current carrying capacity of the grounding wire. At the same time, sensitive circuits or components should be kept away from these leakage circuits or electric field shielding measures. For the surface treatment of structural components, generally required surface treatment is silver, zinc, nickel, chromium, tin, etc. Specifically, consideration should be given to conductivity, electrochemical reaction, cost, and electromagnetic compatibility.

3.8 Electromagnetic compatibility design in component layout and wiring

For the layout of internal components of switching power supply equipment, electromagnetic compatibility requirements must be considered. Interference sources within the device can affect the normal operation of other components or components through radiation and crosstalk. Research has shown that the energy of interference sources can significantly attenuate over a certain distance. Therefore, a reasonable layout is conducive to reducing the impact of electromagnetic interference.

The EML input/output filter is preferably installed at the inlet and outlet of the metal chassis to ensure shield isolation of the input and output lines.

Sensitive circuits or components that are away from heat sources.

Switching power supply products should generally follow the following wiring principles.

1) The input line and output line of the main circuit are separated.

2) The EMI filter input line is separated from the output line.

3) The main circuit line is separated from the control signal line.

4) High voltage pulse signal lines should preferably be separated.

5) Separate wiring avoids parallel wiring, and can be vertically crossed. The spacing between harnesses exceeds 20mm.

6) Cables should not be connected to metal enclosures and radiators to ensure a certain distance.

7) Twisted pair cables are used in EMC designs.

(1) Twisted pair and coaxial cables can effectively suppress electromagnetic interference commonly used in pulse signal transmission lines. It is best to use twisted pair shielded wires for control auxiliary power lines and sensor signal lines. Due to the small circuit area between twisted pairs, the amount of current induced on each two adjacent circuits of the twisted pair is equal. In the opposite direction, the generated magnetic fields cancel out each other, thereby reducing mold difference interference caused by radiation. However, the number of turns of the twisted pair is preferably even, and the more wavelengths per unit, the better the effect of decoupling. When using, it should be noted that both ends of twisted pair and coaxial cables cannot be grounded at the same time, and only one end can be grounded. Grounding both ends of the shielding layer can shield electric and magnetic fields, while single end grounding can only shield electric fields. When using coaxial cables, it should also be noted that their shielding layer must completely cover the signal line to ground, that is, the connector and cable shielding layer must be 360. Overlap to effectively shield the electromagnetic field. As shown in Figure 8, the exposed portion of the signal line can still form compatible coupling with the outside world, reducing shielding efficiency.

(2) Ribbon cables are suitable for short distance signal transmission. In order to reduce the electromagnetic radiation of differential mode signals, it is necessary to reduce the loop area formed by signal lines and signal return lines. Therefore, when designing a ribbon cable layout, it is best to separate the signal line from the ground wire. As shown in Figure 9, S is the signal line, and G is the signal line.

3.9 Selection of components

There are three modes of heat transfer, namely conduction, convection, and radiation. Thermal radiation propagates into space in the form of electromagnetic waves, and heat conduction can also be transmitted to other components around it, affecting the normal operation of other components or circuits. Therefore, considering the thermal design of components, a large amount of margin should be left as much as possible to reduce the temperature rise and surface temperature of components. In addition to the special requirements for temperature rise of components, generally speaking, switching power supplies require that the internal component temperature be less than 90 ℃, and the internal ambient temperature should not exceed 65 ℃, to reduce the temperature by 4. Thermal radiation interference.

For digital integrated circuits, from the perspective of electromagnetic compatibility, CMOS devices with high noise capacity should be selected instead of TTL devices with low noise capacity.

Try to use low-speed, narrowband components and circuits.

Select surface mount components (SMD) with small distributed inductance, ceramic dielectric capacitors with good high-frequency characteristics and low equivalent series inductance, high-frequency non-inductive capacitors, three-terminal capacitors, through-center capacitors, etc.

3.10 Control circuit and PCB electromagnetic compatibility design

Signal ground refers to the low impedance path where signal current returns to the signal source. In design, ground loop interference and common impedance coupling interference are often caused by improper grounding methods. Therefore, the grounding method should be reasonably selected. Grounding methods include single point grounding, multi point grounding, and hybrid grounding.

1) Ground loop interference typically occurs on equipment connected through long cables. The reason is that the ground potential of the two devices is different due to the presence of ground loop current. Photocouplers or isolation transformers are commonly used for ground isolation to eliminate ground loop interference. Due to the large parasitic capacitance between the windings of the isolation transformer, even the isolation transformer with shielding measures is usually only used for signal isolation below 1MHz. When the frequency exceeds LMHz, the photocoupler is usually used for isolation.

2) When the ground current of two circuits passes through a common impedance, common impedance coupling occurs. Because the ground wire is a signal return wire, the working state of a circuit inevitably affects the ground wire voltage. When two circuits share a section of ground wire, the voltage of the ground wire will also be affected by the operating state of the two circuits.

It can be seen that both ground loop interference and common impedance coupling are caused by ground wire impedance. Therefore, it is necessary to consider minimizing ground wire impedance and induced impedance in the design.

3) Reduce noise in the control power supply. The sudden change of current on the power supply line can generate noise voltage. Adding decoupling capacitors near the chip can effectively reduce noise. If it is a high-frequency current load, multiple high-frequency capacitors with the same capacity and non-inductive capacitors can be connected in parallel to achieve better results. Please note that the larger the capacitance, the better. The pulse current frequency is mainly selected based on its resonant frequency.

4) Reasonable wiring of printed boards will effectively reduce the radiation of printed boards and improve their ability to resist radiation interference. Please note the following points.

(1) Arrange the ground wire network, that is, arrange the most parallel ground wires on both sides of the dual panel.

(2) For some key signals (such as pulse signals and externally sensitive level signals), it is necessary to minimize the lead length and signal return area. If it is a double-sided board, the ground wire and signal wire can run parallel on both sides of the printed board.

(3) If multilayer circuit boards have both digital and analog components, they must be arranged on the same layer to reduce coupling interference between them.

(4) Common impedance coupling often occurs in actual circuits, so the correct grounding method should be selected based on the actual situation.

4. Conclusion

This paper analyzes the electromagnetic interference sources and generation mechanism of isolated DC/DC converters in detail, and introduces the electromagnetic compatibility design methods of their main circuit and control circuit in detail, which has certain reference value for the electromagnetic compatibility design of other electronic products.