This paper analyzes and discusses the transmission mechanism and suppression options of conduction, radiated noise and common mode noise generated by various Voltage regulators that seriously affect the basic performance of portable devices.
Electric field shielding technology
Since the electric field exists between two surfaces or entities with different potentials, it is only necessary to shield the device with a grounded shield, and it is relatively easy to limit the electric field noise generated inside the device to the inside of the shield. Such shielding measures have been widely used in monitors, oscilloscopes, switching power supplies, and other devices with large voltage swings. Another common practice is to provide a ground layer on the circuit board. The electric field strength is proportional to the potential difference between the surfaces, and the well is inversely proportional to the distance between them. For example, an electric field may exist between the source and a nearby ground layer. In this way, by using a multi-layer circuit board, a ground layer is provided between the circuit or the line and the high potential, and the electric field can be shielded.
However, when using a ground plane, attention should also be paid to capacitive loads in high-voltage lines. Capacitors store energy in the electric field, so that when a ground layer is placed close to a capacitor, a capacitor is formed between the conductor and ground. The dv / dt signal on the conductor will generate a large conduction current to ground. In this way, in the control of radiated noise At the same time, it reduces the performance of conducted noise.
If electric field scattering occurs, the source is most likely to be in the place with the highest potential in the system. In power supplies and switching regulators, attention should be paid to switching transistors and rectifiers, because they usually have a high potential, and because of the heat sink, they also have a relatively large Surface area. Surface mount devices also have this problem because they often require large area circuit board copper to help dissipate heat. In this case, you should also pay attention to the distributed capacitance between the large-area heat dissipation surface and the ground layer or power layer.
Magnetic field shielding technology
The electric field is relatively easy to control, but the magnetic field is completely different. The use of high permeability materials to close the circuit can play a similar shielding role, but this method is very difficult and expensive to implement. Generally speaking, the best way to control magnetic field scattering is to minimize it at the source. In general, this requires the selection of inductors and transformers with low magnetic radiation. It is also important to pay attention to minimizing the size of the current network during circuit board layout and connection line configuration, especially those circuits that carry large currents. Not only do high-current loops radiate magnetic fields outward, they also increase the inductance of the wires, which can cause voltage spikes on lines carrying high-frequency currents.
Reducing magnetic radiation-using an air gap ferrite core to make the inductor
Circuit designers tend to choose commercial transformers and inductors, but whether they are designing or selecting commercial transformers and inductors, a little knowledge of magnetic materials will help designers to do specific applications Make the most appropriate choice.
According to the relevant experimental data, the increase of the air gap in the ferrite core (or other types of high permeability cores) will force the magnetic flux to pass out of the core, allowing the inductor or transformer to store energy in the surrounding magnetic field of the device. Adding an air gap to this ferrite core will reduce the slope, and at the same time reduce the equivalent permeability and related inductance. The inductance decreases due to the change in slope, while the maximum current increases due to the change in slope, while the saturation magnetic induction B remains unchanged. Therefore, the maximum energy stored in the inductor (1 / 2L12) has increased. This increase can also be confirmed by applying a voltage to the inductor and then observing the time required to reach saturation Bsat. The energy stored in the magnetic core is the integral of (V & TImes; i) dt. Because for a magnetic core with an air gap, the same voltage and time always have a higher current, so the corresponding Energy Storage is also higher.
However, using a core with an air gap increases the magnetic radiation in the space around the inductor. Take the axial magnetic core as an example, because it has a large air gap, it has strong magnetic radiation during operation. It is for this reason that it is not used in many noise-sensitive applications. Axial core-spool-shaped ferrite-is the simplest and cheapest ferrite core with an air gap. Wound the coil on the middle axis to form an inductor. Since the coil is directly wound on the magnetic core, no other processing is required besides the lead-out of the coil, so the cost is very low. In many cases, the wire is led out through a metalized area at the bottom of the core, allowing the inductor to be surface mounted. Other surface mount inductors are fixed on a ceramic or plastic top cover, and the coil is led out through the top cover.
Some manufacturers put a ferrite shield outside the shaft core to reduce radiation. This method is effective, but at the same time it also reduces the air gap, thus reducing the energy storage of the magnetic core. Since ferrite itself does not store much energy, a small air gap is usually reserved between the magnetic core and the shield, which will cause this type of inductor to radiate a part of the magnetic field. However, at some acceptable level of scattering, the axial core is a good compromise between cost and EMI.
Other cores with different shapes can also increase the air gap (or not) according to the application requirements. For example, the pot-shaped core, EI core and EE core all have a central post or shaft, and an air gap can be opened on it. Opening an air gap in the center of the magnetic core and completely surrounding it with a coil helps to reduce the magnetic radiation of the air gap to the external space. This type of inductance is usually more expensive because the coil must be wound independently of the magnetic core, which is assembled around the coil. In order to facilitate design and assembly, you can purchase a magnetic core with an air gap on the bottom bracket.
Perhaps the best performing magnetic core in terms of reducing magnetic radiation is a magnetic ring with a distributed air gap. This magnetic core is formed by mixing a filler material and a high-permeability metal powder and pressing it. The metal powder particles are separated by a non-magnetic filler to form a small air gap. Although they are distributed throughout the magnetic core, their function is similar to a total "air gap". The coil surrounds the magnetic system, so that the magnetic field forms a ring along the magnetic ring in the middle of the coil. When the wire surrounds the entire circumference of the magnetic ring, it completely surrounds the magnetic field and shields it.
The energy loss of the distributed air gap type magnetic ring is sometimes higher than that of the ferrite core with an air gap. This is because eddy currents are easily formed in the metal particles constituting the core, which causes the core to heat up and reduces the power supply efficiency. Because the coil must pass through the center of the magnetic ring, the winding is more difficult, so this type of inductance is also more expensive. The coil winding can be done by the machine, but this type of machine is more expensive and more expensive than the traditional type of winding machine. The operation is slower.
Some ferrite magnetic rings have a discontinuous air gap. The impact radiation generated by this core is higher than that of the above-mentioned distributed air gap magnetic cores, but typical air gap magnetic rings have a relatively low energy loss because they close the magnetic field. The ability of ginger is better than other types of ferrite cores with discontinuous air gap. Surrounding the air gap with a coil can reduce the magnetic radiation, and the ring-shaped magnetic core is more conducive to confine the magnetic field inside the core.
Transformer design should consider to avoid leakage inductance
Transformers have many limitations common to inductors because they are wound with the same magnetic core. In addition, the transformer has some unique features. The characteristics of an actual transformer are close to that of an ideal transformer-the voltage is coupled from the primary to the secondary at a voltage ratio proportional to the winding turns ratio.
In the transformer equivalent circuit, the distributed capacitance between the windings is equivalent to the capacitors CWA and CWB. The main problem brought by these factors is the common mode scattering problem in the isolated power supply. The winding capacitances CP and CS are very small and can usually be ignored at the operating frequency of the switching power supply and regulator. The role of the magnetizing inductance LM is very important, because excessively high magnetizing current will cause transformer saturation. Like the inductor, the magnetic radiation of the transformer will increase in saturation. Saturation also causes higher core energy loss, higher temperature rise (possibly causing thermal runaway), and reduced coupling between windings.
Leakage inductance is caused by the magnetic field of only one winding of the winding and the other winding of the winding, although this parameter is intentionally designed to be compared in some coupled inductors and transformers (like the common mode choke discussed above) Large, but for switching power supply, leakage inductance LLp and LLs are often the most troublesome parasitic components, and the magnetic flux of the two windings at the same time turns the two windings into one. All transformer windings surround the magnetic core, so any leakage inductance exists outside the magnetic core, and in the air, it will generate magnetic radiation to the outside world.
Another problem caused by leakage inductance is that when the current changes rapidly, a large voltage is generated, which is manifested in most switching power supply transformers. Such a large voltage will damage the switching transistor or rectifier due to overvoltage. Absorption buffers (usually a resistor and capacitor in series) are often used to dissipate the energy of such voltage spikes, so that the voltage is controlled. On the other hand, some switching devices are designed to withstand a certain repetitive avalanche breakdown, dissipate a certain amount of power, and eliminate the need for external buffers.
The measurement of transformer leakage inductance is very simple, just short circuit the secondary coil and then measure the primary inductance. This measurement also includes the secondary leakage inductance coupled through the transformer. In most cases, this leakage inductance must also be considered because it also increases the voltage spike on the primary side. The corresponding peak energy can be calculated according to the formula E = 1 / 2L12. In this way, the power consumption caused by leakage inductance is the energy of each peak multiplied by the switching frequency: P: = 1 / 2L12f.
The specific requirements for transformers are related to different power supply topologies. Some topologies directly couple energy through transformers-such as half-bridge, full-bridge, push-pull or forward converters-requiring very high magnetizing inductance to prevent saturation. In these circuits, the primary and secondary coils of the transformer simultaneously transmit current, coupling energy directly through the transformer. Since only little energy is stored in the magnetic core, the transformer can be made smaller. This kind of transformer is usually made of a magnetic core with no air gap ferrite or other high permeability materials.
Other power supply topologies require transformer cores to store a certain amount of energy. The transformer in the flyback circuit stores energy through the primary coil in the first half of the switching cycle. In the second half of the switching cycle, energy is released and fed to the output through the secondary coil. As in the case of inductors, high permeability cores without air gaps are not suitable for transformer energy storage. In contrast, the magnetic core must have a discontinuous or distributed air gap. This will make the size of the component larger than when there is no air gap, but it eliminates the need for additional energy storage inductors, thus saving costs and space.
About crosstalk and suppression
The so-called crosstalk means that there is no direct connection between the source of noise (interference) and the object being interfered with, but when their respective wires or leads are close to each other, there will be stray capacitance and parasitic inductance.
When two or more conductors are close together, there will be capacitive coupling between them, and a large voltage change in one conductor will couple current to other conductors. If the conductor is low-impedance, the coupling current produces only a small voltage. The capacitance is inversely proportional to the distance between the conductors and proportional to the area of ​​the conductors. In this way, reducing the area of ​​adjacent conductors and increasing the distance between them will help reduce the conduction noise.
Another way to reduce coupling between conductors is to add a ground or shielding layer. A ground wire between conductors (in many cases, a power bus or other types of low-impedance nodes) can bypass the capacitively coupled interference signals to ground, thereby preventing mutual interference between conductors. But it should be done with caution. If the line carrying the fast dV / dt signal is placed close to a ground layer, and the ground layer is connected to the ground through a high-impedance interconnection, then the above-mentioned rapidly changing signal will couple into the ground layer. Furthermore, the ground plane will couple to the sensitive line, so that instead of improving it, it will make the noise problem worse. If the ground plane does not need to carry large currents, it tends to use thin wires to connect it to ground. However, thin wires have a relatively large inductance, which causes the ground layer to exhibit high resistance to rapidly changing voltage signals.
When wiring, make sure that the ground plane does not couple noise to sensitive parts of the circuit. For example, input and output bypass capacitors often transmit current through the ground plane. High-frequency currents can have a non-negligible effect on sensitive circuits. To avoid this problem, separate layers are often used on the circuit board, which are used for power and signal grounding, respectively. Connect different levels at a single point, then the noise on the high-power ground plane will not be noticed to other levels. This approach is similar to a star ground where all components are grounded at a single point (all lines converge to the ground point in a "star" shape). The effect of star grounding is equivalent to the use of independent power and signal grounding, but it cannot be implemented in a relatively complex large circuit containing many grounding components.
If a node is known to be sensitive to noise, all lines and wires connected to the node should be kept away from those nodes with large voltage changes. If this is not possible, a good ground or shield needs to be added. Good capacitance bypass can also reduce the sensitivity of these nodes to crosstalk. Usually, a small capacitor connected between the node and ground, or between the node and the power bus, can form an appropriate bypass.
When choosing a bypass capacitor, make sure that it has a sufficiently low impedance in the frequency range that may cause problems. ESR and ESL may make the impedance of the capacitor higher than expected at high frequencies. Therefore, ceramic capacitors with low ESR and ESL are commonly used for high-frequency bypass. Ceramic dielectrics also have a greater impact on performance. A higher-capacity dielectric (such as Y5V) will cause the capacitance to change significantly with changes in voltage and temperature. At the highest rated voltage, the capacity of capacitors made of this ceramic will be as much as 15% lower than the capacity without bias. A better dielectric has a slightly lower capacitance, but the suppression of crosstalk has a lower correlation with bias and temperature, and in many cases can provide a more stable and better bypass.
The placement of bypass capacitors is also very particular. In order to suppress high-frequency noise, it is best to make the signal line that needs to be bypassed directly routed through the bypass capacitor. In Figure 8a, the line in series with the capacitor will increase the ESR and ESL, increase the high-frequency impedance, and greatly reduce the effect of the capacitor as a high-frequency bypass. A better way of wiring (see Figure 8b) is to make the line pass directly through the capacitor. In this way, the discrete ESR and ESL of the line will assist the capacitor to produce a better filtering effect.
Some nodes cannot use bypass measures because doing so will change their frequency characteristics. An example is a resistor divider used for feedback. In most switching power supplies, a resistive feedback voltage divider divides the output voltage to a level acceptable to the error amplifier. The large-capacity bypass capacitor added to this feedback node and the resistance on the node form a pole. Because the voltage divider is part of the control loop, this pole becomes a hope for the loop characteristics. If the pole frequency does not exceed a tenth octave of the corner frequency, the phase or gain effects it produces will adversely affect the loop stability.
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