What issues should be considered in controlling EMI/EMC in PCB board design?
The most common EMI/EMC precautions used by electronic R&D engineers are shielding, filtering, grounding, and wiring. However, with the integration of electronic systems, cost, quality, and functionality are considered, and the speed of product launch is required. Engineers must conduct EMI/EMC predictive analysis and design at the initial stage of design, avoid problems in the later stages of development, adopt passive control methods that save repairs, and receive twice the result with half the effort. This article describes the issues that should be considered when controlling EMI/EMC at the beginning of product design.
1 PCB board design
1.1 PCB layer number and function distribution
When designing a circuit board, the first thing to consider is the number of layers of the PCB and the distribution of signals, power, and ground. The number of layers is determined by functional specifications, noise suppression, signal type, trace distribution, impedance matching, active component density, number of networks, and so on. Pressing RF radiation on the PCB level is better than working on the case or metal coating on the plastic case.
Table 1 shows the general distribution of the number of layers of the board and the signal, power, and ground. These allocation methods are not static and can be modified as required by the functional requirements and the number of winding layers required (RountingLayers). The important point to be grasped is that each winding layer must be adjacent to a complete plane.
PCBs are generally even layers, and two-layer boards are often used for frequencies below 10 kHz. If more than 3 complete planes are provided, ie one power supply and two grounds, the highest speed clock is routed to the adjacent groundplane and not adjacent to the powerplane for optimum EMI effects. This is the basic concept of suppressing EMI suppression on the PCB.
1.2 Power and Grounding
The most important consideration in high-speed PCB design is to supply power to each part of the circuit to minimize noise. It is like developing a non-interfering power supply. A good grounding impedance should be zero, so a good reference voltage can be provided to all circuits without EMI. In fact, in a real power network, because there is a non-zero value transfer delay current, there should be some limited impedance, such as resistance, inductance, or capacitance, which are dispersed throughout the board.
Another problem in high-speed PCB boards is the alternating electromagnetic field generated by the AC signal, which circulates in a closed loop surrounded by wires that can make the circuit's crosstalk and radiation more severe. The performance of the distributed power supply is determined by the different potentials of the circuits on the board. The purpose of the design is to minimize the impedance of the power supply network. There are usually two ways to solve this problem, using the power bus and the power plane.
The general designer tends to use the power bus because it has a reasonable cost, so it is preferred, but the power bus shares the entire layout layer with the signal line, and there are many power supplies to all devices. Usually the bus is long and narrow, so its impedance is relatively large, which is why the current is limited to the path defined by the bus. The EMI generated by the device is associated with the device on the power bus.
As for the power plane because it is full of the entire layout layer, and the impedance of the power plane is a small part of the power bus. On the power plane, the noise current is scattered because the current path is not limited. The path impedance along the line is also lower, so the power plane is quieter than the power bus. Another function of the power plane is that there is a return path for all signal supply in the system that can be used to limit many high speed noise problems.
At low speeds, current flows to the path of the lowest resistance. At high speeds, the inductance of the return current path is much lower than the effective resistance. The high-speed return current is the path of the lowest inductance, but this path is not the lowest. The path of the resistor. This lowest inductance return path is placed directly below a signal conductor with a minimum total loop between the return current paths. The power plane provides a return path for all signals in the system, and the current can be returned via VCC or ground.
The shorter the route, the shorter the transmission delay. If the length of the wire exceeds 1/6 of the length of the rising edge of the electron, the signal delay is greater than the effective part of the transmission time, so the signal route must be regarded as a transmission line, an inappropriate The terminal transmission line is prone to reflections, which in turn destroy the signal. Too short a terminal line will produce a negative reflection to slow down the conversion time and slow the data flow; while a too long terminal line will produce a positive reflection which can be interpreted as a multi-tasking signal due to this frequency Underneath, with high impedance and transmission rate, it can be effectively coupled with adjacent path lines.
Signals at the line load can be combined to form a ring to reduce the speed of the system. It can also cause erroneous timing and even disrupt system functionality. Therefore, the terminal resistance of the terminal line should be reduced, or limited to no reflection, so that the resistance value matches the characteristic impedance of the transmission line to effectively suppress the reflection.
Incorporating a load-bearing resistor between parallel terminals reduces the load impedance, but it has the disadvantage that it has a higher current at the positive voltage output, which can be passed through the power supply and the grounding resistor. The use of the two resistors is a so-called Thevenin effect; although this method is very good, because the resistance is between the power supply and the ground, a large power supply current is required.
Another technique is to incorporate resistors and capacitors that allow AC shorts and DCs to open. This circuit can be referenced and considered an AC termination. The design of the load termination technology can limit the first reflection.
Another option is to increase Zs to be equal to Zo and connect Zs to the power supply. When Zs is added, the power supply will generate a new impedance Zo. We can also consider using the terminator on both the power supply and the load side to make half of the received signal and reduce the huge reflection. In digital circuits, this technique is only used for lines connected to receiving components.
Handheld devices place special emphasis on EMI design on printed circuit boards, and in most cases the surface mount components of the board produce emissions that are larger than the copper foil lines on the board. The same current flowing through the copper foil wire must flow through the IC as well, ensuring that the area between the copper foil wire and its nearest reference datum is less than the current loop area from the chip pin to the board and back to the device's power and ground pins. , the chip can emit more energy than the copper foil wire. In addition, if the two copper foil wires are equal in length and carry the same signal, the radiation of the copper foil wire physically higher than the nearest solid reference plane will be compared. Big. Simply put, the higher the distance from the datum, the higher the radiation.
2 Adoption of shielding measures
For design engineers, the use of shielding materials is a way to effectively reduce EMI. A wide variety of housing shielding materials are now available, ranging from metal cans, sheet metal and foil strips to spray coatings and coatings on conductive fabrics or tapes (such as conductive paint and zinc wire coating). Whether it is metal or a plastic coated with a conductive layer, once the designer has determined that it is the outer casing material, it is time to start selecting the liner.
2.1 Metal shielding efficiency
Only materials with high magnetic permeability such as metal and iron can achieve higher shielding efficiency at very low frequencies. The magnetic permeability of these materials decreases as the frequency increases. In addition, if the initial magnetic field is strong, the magnetic permeability is lowered, and the mechanical method of making the shield into a prescribed shape also reduces the magnetic permeability. In summary, the choice of high permeability materials for shielding is very complex, and it is often necessary to seek solutions from EMI shielding material suppliers and related consulting agencies.
Under high frequency electric field, a thin layer of metal can be used as the outer casing or lining material to achieve good shielding effect, provided that the shielding must be continuous and the sensitive part is completely covered without gaps or gaps. However, it is impossible to manufacture a seamless and notched shield in practice. Since the shield is divided into a plurality of parts, there is a gap to be joined, and it is usually necessary to punch holes in the shield. Install the cable with the card or assembly components. The difficulty in designing a shield is that voids are inevitably created during the manufacturing process and are required during operation of the equipment. Manufacturing, panel connections, vents, external monitoring windows, and panel mounting components all require holes in the shield to greatly reduce shielding performance. Although trenches and gaps are unavoidable, it is advantageous to carefully consider the length of the trench associated with the wavelength of the operating frequency of the circuit in the shield design.
2.2 shielding design key
Equipment generally needs to be shielded because there are some slots and gaps in the structure itself. The required shielding can be determined by some basic principles, but there is still a difference between theory and reality. For example, when calculating the size and spacing of pads at a certain frequency, the strength of the signal must also be considered, as is the case when multiple processors are used in a device. Surface treatment and gasket design are to maintain long-term shielding for EMC performance. The key factor.
Since the seam will cause the shield to decrease in conductivity, the shielding efficiency will also decrease. It should be noted that the attenuation below the cutoff frequency depends only on the length to diameter ratio of the slit. For example, a length to diameter ratio of 3 can achieve 100 dB of attenuation. When perforation is required, the waveguide characteristics of the apertures above the thick shield can be utilized; another way to achieve a higher length to diameter ratio is to attach a small metal shield, such as a suitably sized gasket. The above principle and its extension in the case of multiple seams form the basis of the design of the porous shield.
Porous thin shield: There are many examples of porous, such as vents on thin metal sheets. When designing the spacing of the holes, the design must be carefully considered. Seams and joints: electric welding, brazing or soldering are between the sheets. In the usual way of permanent fixing, the metal surface of the joint must be cleaned so that the joint can be completely filled with conductive metal. Fixing with screws or rivets is not recommended because the low resistance contact between the fasteners is not easily maintained for a long time.
The purpose of the conductive pad is to reduce the slots, holes or gaps in the seam or joint so that RF radiation does not emanate. The EMI gasket is a conductive medium used to fill the gaps in the shield and provide a continuous low impedance contact. Typically an EMI gasket provides a flexible connection between two conductors to pass current through one conductor to another.
The selection of sealed EMI gaskets must take into account variables such as: shielding efficiency in a specific frequency range, mounting method and sealing strength, current compatibility with the enclosure, corrosion resistance to the external environment, operating temperature range, cost, most commercial linings The pads have sufficient shielding properties to allow the device to meet EMC standards, and the key is to properly design the gasket within the shield.
2.3 Selection of shielding materials
A wide variety of shielding and gasketing products are currently available, including a copper joint, a metal mesh, a metal mesh and oriented wire embedded in the rubber, a conductive rubber, and a polyurethane foam liner with a metal coating. Most shielding material manufacturers offer SE estimates that can be achieved with a variety of liners, but keep in mind that SE is a relative value and depends on pores, liner size, gasket compression ratio, and material composition. Pads are available in a variety of shapes for a variety of specific applications, including wear, slip, and hinged applications. Many liners currently have adhesive or have a fixture on the liner, such as a squeeze insert, a pin insertion or a barb device.
Among the various types of liners, coated foam liners are one of the most versatile products on the market. These liners are available in a variety of shapes, thicknesses greater than 0.5 mm, and reduced thickness to meet UL combustion and environmental sealing standards. There is another new type of gasket, the environmental/EMI hybrid liner, which eliminates the need for a separate sealing material, reducing the cost and complexity of the shield. The outer coating of these liners is UV-stabilized, moisture-proof, wind-proof, and anti-cleaning solvent. The inner coating is metallized and has high conductivity. Another recent innovation is the installation of a plastic clip on the EMI liner, which is lighter in weight, shorter in assembly time, and less expensive than conventional pressed metal liners, making it more attractive.
3 signal path considerations
Generating EMI requires the cooperation of many variables, as EMI is the result of a passive component other than the normal state. Some of the behavioral characteristics of these passive components at high frequencies are commonly referred to as "hidden circuits." Hardware engineers generally assume that these components have a single frequency response. As a result, it selects components based on the functional characteristics of the time domain, regardless of the actual performance in the frequency domain. Many times, when the designer bends or breaks the rules, many EMI situations arise.
Once you understand these hidden behaviors, it is easy to design a product that meets the requirements. At the same time, we must also consider the hidden behavior brought by the switching speed of the active components, which hides the components of inductance, capacitance and resistance.
When the signal is transmitted to the line in the board, the crosstalk between the wires is obvious. In the high-speed design, only the ground line is not recommended by all signals. To confirm that each signal has its own return path to reduce Crosstalk source. The return signal current is based on the inductance of the individual path and flows between all ground paths. In low-inductance lines, there is more return current, and these low-inductance lines are placed close to the signal line, but less on the external path. Compared with the traditional signal transmission line, the conventional signal transmission line has only one signal line and the ground line for the current return. As for the differential signal transmission mode, two signal lines and one current return ground line are required. The signal-derived signal returns a current problem that not only provides a low-impedance path for a single signal, but also limits the impedance.
The theory of differential signals is quite simple. When two signals are transmitted, the signal to be transmitted is added after the second signal and is equal to the negative value of the first signal, and the first signal is positive. The return current from the second signal is negative. Comparing the two signals at the receiving end to determine the polarity of the logic, in the process of comparison, a reference voltage that is not local is required, and the shift of the ground voltage is between the transmitting end and the receiving end, so that each line is valid. Equal to the effect of the two different lines. If the ground voltage shift is between the transmitter and the receiver, differential reception can be disabled. When a differential signal is transmitted through the connector, the connection of adjacent pins should be maintained. In this method, the return signal current path will be covered and cancelled, and the lines can be tightly combined and moved onto the printed circuit board. Crosstalk comes from different metal routes, and they have interference and EMI, which are generated between any two unbalanced transmission signals. We call this imbalance a common mode interference.
If the differential signal is connected by a twisted pair cable, the transmission performance can be improved. The differential signal includes all the actual return currents of the closed loop formed between the positive and negative signals, and one signal line in the twisted pair. And the closest twist to the return path, when there is a signal transmission delay along the twisted pair, then a magnetic field will come from the different polarity pairs, the magnetic fields from the two lines have different polarities, for the same distance The polarity of the magnetic field is determined by the nearest line. When these lines are rolled around each other, the polarity of the magnetic field will be reversed. The result is that the crosstalk between adjacent twisted pairs is zero and can be ensured. The wires of the strands have the same direction.
Another advantage of using twisted pairs is that the transmission of the electromagnetic field can be reduced for different transmissions, and most of the return current for individual signal streams is at the ground line, which is one of the methods for canceling the image of the radiation field.
In the differential mode, low-voltage differential mode signaling (LVDS) is a faster and more stable signal when it is not dependent on the supply voltage, so it has considerable advantages.
Low-amplitude differential signals can also improve signal integrity at high speeds. Due to the increased demand for data transmission in the communications world, higher frequencies and larger bit widths can cause reflection and crosstalk problems in the transmission line. As the system load increases, the impedance characteristics of the system change and cause impedance mismatch, causing the transmission line to send reflected signals, which can cause bit errors or prolong system stabilization time, making time allocation more difficult when speed increases. Techniques for transmitting signals in differential modes such as LVDS can solve this problem by accepting common mode noise from differential lines. In addition, lower amplitude differential techniques can reduce reflection because low voltage amplitude can limit the energy supplied to the transmission line.
4 power supply needs to take measures
Power supply design options help reduce EMI, especially for filters, choke coils, and controller frequency modulation components, all of which are ways to reduce harmful emissions from portable devices.
An important design issue for power supplies to reduce EMI is the frequency modulation of the switch. Frequency modulation minimizes EMI by dispersing energy over a wider frequency range. Directly related to the amount of EMI reduction is the modulation level and modulation speed. Frequency modulation can use economical inductors instead of AC input choke coils to meet EMI limits and specifications.
As for the filter, single or multi-section can be selected. The single-section filter is small and cheap, but circuit parasitics and component parasitics may occur. In addition, the choke is also an important consideration in the power supply field. The power supply includes a bridge. A rectified input filter that absorbs supply frequency currents that are fairly narrow and have relatively high peaks. The most basic form of a poor mode choke can transmit a series of inductors that simultaneously transmit/block high frequency conducted emissions. Typically, the differential mode choke coil is wound on a solenoid made of iron powder or ferromagnetic material. The common mode choke coil is a simple inductor designed for common mode EMI filters. The choke coil is composed of two windings of the same winding to eliminate the electromagnetic field caused by the differential mode current, and the annular choke coil is one of the best choke coils for attenuating the radiation. The ring is a ring shaped iron core with a coil that passes through the center of the ring. The magnetic field surrounds the central motion of the iron core, limiting the magnetic field to the interior of the iron core.
When an EMI problem occurs, the engineer should explore the problem in a logical analysis. The pattern describing EMI must have three elements: the source of energy, the recipient of energy interference, and the coupling path between the source and the receiver.