Abstract: The mechanism of electromagnetic disturbance of switching power supply is analyzed, and corresponding suppression measures are proposed. The issues that need attention in electromagnetic compatibility design are discussed.
Key words: switching power supply; electromagnetic compatibility; electromagnetic disturbance; coupling channel
1 Introduction
Electromagnetic compatibility refers to the coexistence of various electrical devices in a limited space, time and spectrum without causing degradation in performance. It includes both electromagnetic disturbance (EMD) and electromagnetic sensitivity (EMS). EMD refers to the electrical noise emitted by electrical products, and EMS refers to the ability of electrical products to resist external electromagnetic disturbances. A device with good electromagnetic compatibility should be immune to the surrounding electromagnetic environment and not cause electromagnetic disturbance to the surroundings.
The power switch tube in the switching power supply generates a large voltage and current jump during the on/off process at high frequency, thus generating strong electromagnetic disturbance, but the frequency range of the disturbance (<30MHz) is relatively low. The geometry of most small-power switching power supplies is much smaller than the wavelength corresponding to 30MHz electromagnetic field (about 10m in air medium). The electromagnetic disturbance phenomenon studied by switching power supply systems belongs to the range of stable field. When studying their electromagnetic disturbance problems, the main considerations are It is conducted harassment.
2 electromagnetic disturbance
Discussion Electromagnetic disturbance is generally carried out in three aspects: the characteristics of the disturbance source, the characteristics of the coupling channel of the disturbance and the characteristics of the victim.
2.1 Main source of electromagnetic disturbance in switching power supply
The electromagnetic disturbance sources in the switching power supply mainly include switching devices, diodes and nonlinear passive components; in the switching power supply, improper wiring of the printed circuit board is also a major factor causing electromagnetic disturbance.
2.1.1 Electromagnetic disturbance generated by the switching circuit
For switching power supplies, electromagnetic disturbances generated by switching circuits are one of the main sources of disturbance for switching power supplies. The switching circuit is the core of the switching power supply and is mainly composed of a switching tube and a high frequency transformer. It produces dv/dt which is a pulse with a large amplitude and a wide frequency band and rich harmonics. The main reasons for this type of pulse disturbance are:
1) The switch tube load is the primary coil of the high frequency transformer and is an inductive load. When the switch tube is turned on, the primary coil generates a large inrush current, and a high surge spike voltage appears at both ends of the primary coil; when the switch tube is disconnected, due to the leakage flux of the primary coil, a part of the energy is not From the primary coil to the secondary coil, this portion of the energy stored in the inductor and the capacitor and resistor in the collector circuit form a damped oscillation with a spike that is superimposed on the turn-off voltage to form a turn-off voltage spike. This power supply voltage interruption produces the same magnetizing inrush current transient as when the primary coil is turned on. This noise is conducted to the input and output terminals, causing conducted disturbances, which may cause breakdown of the switching transistor.
2) The high-frequency switching current loop formed by the primary coil of the pulse transformer, the switching tube and the filter capacitor may generate large space radiation and form radiated disturbance. If the capacitance of the capacitor is insufficient or the high frequency characteristics are not good, the high frequency impedance on the capacitor causes the high frequency current to be conducted in differential mode to the AC power source to form a conducted disturbance.
2.1.2 Electromagnetic disturbance generated by diode rectifier circuit
The |di/dt| of the reverse recovery current generated by the rectifier diode in the main circuit is much smaller than the |di/dt| of the reverse recovery current of the freewheeling diode. As a source of electromagnetic disturbance, the rectifying diode reverse recovery current forms a large disturbance intensity and a frequency bandwidth. The voltage jump generated by the rectifier diode is much smaller than the voltage jump generated when the power switch in the power supply is turned on and off. Therefore, it is also possible to study the rectifier circuit as part of the electromagnetic disturbance coupling channel without considering the effects of |dv/dt| and |di/dt| produced by the rectifier diode.
2.1.3 Relationship between dv/dt and load size
The dv/dt generated when the power switch is turned on and off is the main source of disturbance for the switching power supply. Theoretical analysis and experiments show that the load increases, the value of |dv/dt| generated by shutdown increases, and the load change has little effect on the opening |dv/dt|. Due to the difference in |dv/dt| generated when turned on and off, the externally generated disturbance pulse is also different.
2.2 Coupling channel of electromagnetic noise of switching power supply
There are two ways to describe the coupling path for switching power supplies and system conducted disturbances:
1) dividing the coupling channel into a common mode channel and a differential mode channel;
2) A system function is used to describe the characteristics of the coupling channel between the disturbance and the victim.
This article uses the first method to discuss.
2.2.1 Common mode and differential mode disturbance channels
When the switching power supply is powered by the grid, it converts the electrical energy drawn from the grid into a different characteristic electrical energy supply load. At the same time, the switching power supply is a noise source. The coupling channel is used to harass the power grid, the switching power supply itself and other devices. Usually, common mode and differential mode disturbance are used for analysis.
"Common mode harassment" means that the size and direction of harassment are consistent, and it exists between any relatively large ground of the power source or between the center line and the earth. Common mode disturbance is also known as longitudinal mode disturbance, asymmetric disturbance or ground disturbance. It is the disturbance between the carrier fluid and the earth.
"Differential mode disturbance" refers to the same size and opposite direction, which exists between the power phase line and the neutral line and between the phase line and the phase line. Differential mode disturbance is also called normal mode disturbance, transverse mode disturbance or symmetric disturbance. It is the disturbance between the carriers.
Common mode disturbances indicate that disturbances are coupled into the circuit by radiation or crosstalk, while differential mode disturbances indicate that the disturbance originates from the same power circuit. Usually these two kinds of harassment exist at the same time. Due to the imbalance of the line impedance, the two kinds of harassment will also transform each other in the transmission, which is very complicated. Common mode disturbance is mainly caused by |dv/dt|, and |di/dt| also produces certain common mode disturbances. However, in low-voltage and high-current switching power supplies, common mode disturbance is mainly caused by |dv/dt| or by |di/dt|, which needs further study.
In the case where the frequency is not very high, the disturbance source, the coupling channel and the victim of the switching power supply essentially constitute a multi-input and multi-output electrical network, and decomposing it into common mode and differential mode disturbance to study is complicated. A method of processing the network, which is more appropriate in some situations. However, the division of the coupling channel into the common mode and the differential mode channel has certain limitations. Although the common mode component and the differential mode component can be measured, the common mode component and the differential mode component are generated by which components, and it is not easy to determine. . Therefore, some people use the system function method to describe the coupling channel of switching power supply disturbance, that is, to study the relationship between the system function of the coupling channel and each component, and establish the circuit model of the coupling channel. The results of many system analyses, such as sensitivity analysis, modal analysis, etc., can be used to study the debugging and prediction of EMD for switching power supplies. However, there is still a lot of work to be done to analyze the coupling channel of the disturbance using the system function method.
2.2.2 Stray parameters affect the characteristics of the coupled channel
In the conduction disturbance frequency band (less than 30MHz), the coupling channel of most switching power supply disturbances can be described by circuit network. However, any of the actual components in the switching power supply, such as resistors, capacitors, inductors, and even switches and diodes, contain spurious parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. The equivalent circuit of the switching power supply, including the stray parameters of each component and the coupling between components, will be much more complicated. At high frequencies, the spurious parameters have a great influence on the characteristics of the coupled channel, and the presence of distributed capacitance becomes a channel for electromagnetic disturbance. In addition, when the power of the switch tube is large, the collector generally needs to be provided with a heat sink. The distributed capacitance between the heat sink and the switch tube cannot be ignored at high frequencies, and it can form a space-oriented radiation disturbance and power line conduction. Common mode harassment.
3 suppression of electromagnetic disturbance
The suppression measures for the EMD of the switching power supply are mainly
1) reduce the intensity of disturbance of the disturbance source;
2) Cut off the route of harassment.
In order to achieve this goal, it is mainly from selecting the appropriate switching power supply circuit topology; using the correct grounding, shielding, filtering measures; designing reasonable component layout and printed circuit board wiring and other aspects.
3.1 Reduce the disturbance of the switching power supply itself
Reducing the disturbance of the switching power supply itself is the fundamental way to suppress the disturbance of the switching power supply, and is an effective method for making the electromagnetic interference of the switching power supply lower than the specified limit value.
1) Reduce the disturbance generated during the process of power tube on and off
The above analysis shows that the main disturbance of the switching power supply is the dv/dt from the power switch tube. Therefore, reducing the dv/dt of the power switch tube on and off is an important aspect to reduce the disturbance of the switching power supply. It is generally believed that soft switching technology can reduce the dv/dt of the switching tube on and off. However, some current research results show that the soft switch does not significantly reduce the disturbance of the switching power supply as expected. No experimental results show that the soft-switching converter is significantly superior to the hard-switching converter in terms of EMC performance.
The PWM flyback converter, quasi-resonant zero-current variable-frequency switching forward converter, multi-resonant zero-voltage variable-frequency switching flyback converter, multi-tuning zero-voltage variable-frequency switching forward converter, voltage clamp have been systematically studied. The EMD characteristics of a multi-resonant zero-voltage fixed-frequency switching flyback converter and a half-bridge zero-voltage variable-frequency series resonant converter are discussed. The effects of snubber circuit, clamp circuit, frequency conversion and fixed-frequency control on the level of disturbance are discussed. The experimental results show that the zero-voltage fixed-frequency switching converter with voltage clamping has the lowest EMD level.
Therefore, the use of soft switching power supply technology, combined with reasonable component layout and reasonable printed circuit board wiring, the EMD level of the switching power supply has a certain improvement.
2) Switching frequency modulation technology
The frequency-invariant modulation is changed to random modulation, frequency modulation, and the like. The disturbance generated by the modulated pulse with constant frequency is mainly the harmonic disturbance of the modulation frequency in the low frequency band, and the disturbance of the low frequency band is mainly concentrated on the harmonic points. The switching frequency modulation method proposed by F.Lin [3], the basic idea is to distribute the energy concentrated on the switching frequency fc and its harmonics 2fc, 3fc... to the frequency bands around them by modulating the switching frequency fc. Above, thereby reducing the EMD amplitude at each frequency point to reach a limit lower than the EMD standard. Although the method of switching FM PWM cannot reduce the total disturbance energy, it distributes the energy to the baseband of the frequency point so that each frequency point does not exceed the limit specified by EMD.
3.2 Grounding
“Grounding†has signal grounding inside the device and the device is connected to the earth. The two concepts are different and the purpose is different. The classic definition of "ground" is "equipotential point or plane as a circuit or system reference."
3.2.1 Signal grounding of the device
The signal ground of the device may be a ground reference point for the signal at a point or piece of metal in the device, which provides a common reference potential for all signals in the device.
Floating and hybrid grounding are introduced here, in addition to single-point grounding and multi-point grounding.
1) Floating
The purpose of using floating ground is to isolate the circuit or equipment from a common grounding system, or a common conductor that may cause circulation. Floating can also make circuit coordination between different potentials easier. Methods for achieving floating of circuits or equipment are transformer isolation and optical isolation. The biggest advantage of floating is that it has good anti-harassment performance.
The disadvantage of floating space is that since the device is not connected to the common ground, it is easy to cause static electricity accumulation between the two. When the charge is accumulated to a certain extent, the potential difference between the device ground and the common ground may cause severe electrostatic discharge, and become A source of violent harassment.
A compromise is to connect a large bleeder resistor between the floating ground and the common ground to release the accumulated charge. Pay attention to the impedance of the control release resistor. Too low resistance will affect the eligibility of the device leakage current.
2) Hybrid grounding
Hybrid grounding makes the grounding system exhibit different characteristics at low and high frequencies, which is necessary in wideband sensitive circuits. The capacitor has a high impedance to low frequency and DC, thus avoiding the formation of ground loops between the two modules. When the DC ground and the RF ground are separated, the DC ground of each subsystem is connected to the RF ground through a capacitor of 10 to 100 nF. The two grounds should be connected at a low impedance, and the connection point should be selected at the highest flip speed ( Di/dt) The point at which the signal is present.
3.2.2 Equipment 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 purpose of the equipment to connect to the earth is
1) Ensure the safety of the equipment operators.
2) Discharge the accumulated charge on the chassis, avoiding the accumulation of electric charge and increasing the potential of the chassis, resulting in unstable circuit operation.
3) Avoid changing the potential of the device to the earth under the action of the external electromagnetic environment, resulting in unstable operation of the device.
It can be seen that in addition to the consideration of personnel safety and equipment safety, the connection of equipment to the earth is also an important means to suppress the occurrence of harassment.
3.3 Shielding
An effective method for suppressing the disturbance radiation generated by the switching power supply is shielding, that is, shielding the electric field with a material having a good electrical conductivity, and shielding the magnetic field with a material having a high magnetic permeability. In order to prevent the magnetic field leakage of the pulse transformer, a closed loop can be used to form a magnetic shield, and in addition, the entire switching power supply is shielded by an electric field. Shielding should consider heat dissipation and ventilation problems. The venting holes on the shielding casing are preferably circular and porous. The number of holes can be increased under the condition of ventilation, and the size of each hole should be as small as possible. The joints should be welded to ensure the continuity of the electromagnetic. If the screws are used, pay attention to the short screw spacing. Filtering measures should be taken at the introduction of the shielded enclosure and at the lead-out line. Otherwise, these would become disturbance antennas and seriously reduce the shielding effect of the shielded enclosure. If the electric field is shielded, the shielding case must be grounded. Otherwise, the shielding effect will not be achieved. If the magnetic field is shielded, the shielding case does not need to be grounded. The outer casing of the non-embedded external switching power supply must be shielded by electric field. Otherwise, it is difficult to pass the radiation disturbance test.
3.4 Filtering
The power filter is installed between the power line and the electronic device to suppress conducted disturbances from the power line and to reduce conducted disturbances introduced from the power grid. It plays an important role in improving the reliability of equipment.
The electromagnetic disturbance generated by the switching power supply is mainly conducted by conduction disturbance, and the conducted disturbance is divided into two types: differential mode disturbance and common mode interference. Common mode disturbances typically produce larger radiated EMDs than differential mode disturbances. The most effective way to suppress conducted EMD is to use passive filtering techniques.
As a dual-port network EMD filter, its suppression of disturbances depends not only on the topology of the filter itself, but also on the input and output impedance values ​​of the EMD filter to a large extent. Due to the variability of EMD filter impedance and load impedance and their possible direct connection to the grid, the input and output impedance of the power supply EMD filter are not only mismatched but often well known. This has resulted in the design method and theory of the mature communication filter that cannot be fully applied to the EMD filter design. This is the main problem facing the design of power wave filters.
3.5 Component layout and printed circuit board wiring
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, that is, the radiated disturbance EâˆI·A·f2. The premise of using this relationship is that the channel size is much smaller than the wavelength of the frequency.
The above relationship shows that reducing the passage area is the key to reducing radiation disturbance, which means that the components of the switching power supply are closely arranged with each other. In the primary circuit, the input capacitor, the transistor, and the transformer are required to be close to each other, and the wiring is compact; in the secondary circuit, the diode, the transformer, and the output capacitor are required to be close to each other.
On the printed board, the positive load flow wires are respectively placed on both sides of the printed board, and the two current-carrying conductors are managed to keep the two current-carrying conductors parallel to each other because the external magnetic field generated by the parallel-carrying positive load current conductors tends to Offset each other.
The electromagnetic coupling between the wires is performed by the electric field and the magnetic field. Therefore, attention should be paid to the suppression of the coupling between the electric field and the magnetic field during wiring. The method of suppressing the electric field is
1) Maximize the distance between the lines to minimize the coupling of the capacitors;
2) Using electrostatic shielding, the shielding layer should be grounded;
3) Reduce the input impedance of sensitive lines.
The method of suppressing the magnetic field is
1) reduce the loop area of ​​the disturbance source and the sensitive circuit;
2) Increase the distance between the lines so that the mutual inductance between the coupled disturbance source and the sensitive circuit is as small as possible;
3) It is best to route the disturbance source at right angles to the sensitive circuit to greatly reduce the coupling between the lines.
4 Conclusion
The purpose of electromagnetic compatibility design of switching power supply is to make the product work normally under certain electromagnetic environment. That is to say, the power supply product should meet the immunity limit value specified by the standard. When there is certain electromagnetic disturbance, there is no performance degradation or At the same time, the power supply product meets the electromagnetic limit value requirements of the standard, and does not constitute a pollution source for the electromagnetic environment, but achieves electromagnetic compatibility.
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