With the improvement of the integration of IC devices, the miniaturization of devices and the increasing speed of devices, the EMI problem in electronic products is becoming more and more serious. From the point of view of EMC/EMI design of system equipment, dealing with EMC/EMI problems in PCB design stage of equipment is the most effective and lowest cost means to make system equipment meet EMC standards. This paper introduces EMI control technology in digital circuit PCB design. ”
Generation and suppression principle of EMI
EMI is caused by electromagnetic interference source transferring energy to sensitive system through coupling path. It includes three basic forms: conduction through wires or common ground wires, space radiation or near-field coupling. The harm of EMI is to reduce the quality of transmission signal, cause interference or even damage to the circuit or equipment, and make the equipment fail to meet the technical index requirements specified by EMC standards.
To suppress EMI, the EMI design of digital circuits should be carried out according to the following principles:
According to the relevant EMC/EMI technical specifications, the indicators are decomposed into single-board circuits, which are controlled by levels.
EMI is controlled from three aspects: interference source, energy coupling way and sensitive system, so that the circuit has a flat frequency response and ensures the normal and stable operation of the circuit.
Start with the front-end design of equipment, pay attention to EMC/EMI design, and reduce the design cost.
EMI control technology of digital PCB
When dealing with various forms of EMI, specific problems must be analyzed. In the PCB design of digital circuit, EMI control can be carried out from the following aspects.
Device selection
When designing EMI, the speed of device selection should be considered first. In any circuit, if a device with a rise time of 5ns is replaced by a device with a rise time of 2.5ns, the EMI will increase by about 4 times. The radiation intensity of EMI is proportional to the square of frequency. The highest EMI frequency (fknee), also known as EMI emission bandwidth, is a function of signal rise time rather than signal frequency: fknee=0.35/Tr (where Tr is the signal rise time of the device)
The frequency range of this radiated EMI is 30MHz to several GHz. In this frequency band, the wavelength is very short, and even a very short wiring on the circuit board may become a transmitting antenna. When EMI is high, the circuit will easily lose its normal function. Therefore, in device selection, on the premise of ensuring circuit performance requirements, we should try our best to use low-speed chips and adopt appropriate driving/receiving circuits. In addition, because the lead pins of devices have parasitic inductance and parasitic capacitance, the influence of device packaging form on signals can not be ignored in high-speed design, because it is also an important factor to generate EMI radiation. Generally, the parasitic parameters of SMD devices are smaller than those of plug-in devices, and those of BGA packages are smaller than those of QFP packages.
Selection of connectors and definition of signal terminals
Connector is the key link of high-speed signal transmission, and it is also the weak link that easily generates EMI. In the terminal design of the connector, more grounding pins can be arranged to reduce the distance between the signal and the ground, reduce the effective signal loop area that generates radiation in the connector, and provide a low impedance backflow path. When necessary, consider isolating some key signals with pins.
Laminated design
On the premise of cost permitting, increasing the number of ground layers and making the signal layer close to the ground plane layer can reduce EMI radiation. For high-speed PCB, the power plane and the ground plane are closely coupled, which can reduce the power impedance, thus reducing EMI.
overall arrangement
According to the direction of signal current, reasonable layout can reduce the interference between signals. Reasonable layout is the key to control EMI. The basic principles of layout are:
Analog signals are easily interfered by digital signals, so analog circuits should be separated from digital circuits;
Clock is the main interference and radiation source, so keep away from sensitive circuits and make the clock routing the shortest.
Circuits with high current and high power consumption should be arranged in the central area of the board as far as possible, and the influence of heat dissipation and radiation should be considered at the same time;
Try to arrange the connector on one side of the board and keep away from the high-frequency circuit;
The input/output circuit is close to the corresponding connector, and the decoupling capacitor is close to the corresponding power supply pin;
Considering the feasibility of layout for power division, multi-power devices should be laid across the boundary of power division area to effectively reduce the impact of planar division on EMI.
The return plane (path) is undivided.
wiring
Impedance control: High-speed signal lines will exhibit the characteristics of transmission lines, so impedance control is needed to avoid signal reflection, overshoot and ringing, and reduce EMI radiation.
Classify the signals according to the EMI radiation intensity and sensitivity of different signals (analog signals, clock signals, I/O signals, buses, power supplies, etc.), so as to separate the interference source from the sensitive system as much as possible and reduce the coupling.
Strictly control the routing length, number of vias, cross-division, termination, wiring layer, reflow path, etc. of clock signals (especially high-speed clock signals).
Signal loop, that is, the loop from signal outflow to signal inflow, is the key to EMI control in PCB design, and it must be controlled during wiring. To know the flow direction of each key signal, the key signals should be routed close to the return path to ensure the minimum loop area.
For low-frequency signals, make the current flow through the path with the smallest resistance; For high-frequency signals, make the high-frequency current flow through the path with the smallest inductance, not the path with the smallest resistance (see Figure 1). For differential mode radiation, EMI radiation intensity (E) is proportional to the current, the area of current loop and the square of frequency. (where I is the current, A is the loop area, F is the frequency, R is the distance to the center of the loop, and K is a constant. )
Therefore, when the minimum inductance return path is just below the signal conductor, the current loop area can be reduced, thus reducing the EMI radiation energy.
The key signal must not cross the divided area.
High-speed differential signal routing adopts tight coupling mode as much as possible.
Make sure the stripline, microstrip line and its reference plane meet the requirements.
The lead of decoupling capacitor should be short and wide.
All signal traces should be as far away from the board edge as possible.
For multipoint connection network, choose appropriate topology to reduce signal reflection and EMI radiation.
Split processing of power plane
Division of power supply layer
When there are one or more sub-power supplies on a main power supply plane, the continuity of each power supply area and sufficient copper foil width should be ensured. The dividing line does not have to be too wide, generally it is 20 ~ 50mil line width, so as to reduce the gap radiation.
Division of ground wire layer
The ground plane layer should be kept intact to avoid division. If it must be divided, it is necessary to distinguish the digital ground, analog ground and noise ground, and connect with the external ground through a common grounding point at the exit.
In order to reduce the edge radiation of the power supply, the power supply/ground plane should follow the 20H design principle, that is, the ground plane size is 20H larger than the power supply plane size (see Figure 2), so that the edge field radiation intensity can be reduced by 70%.
Other control means of EMI
Power supply system design
Design a low impedance power supply system to ensure that the impedance of the power distribution system in the frequency range below fknee is lower than the target impedance.
Use filters to control conducted interference.
Power supply decoupling. In EMI design, providing a reasonable decoupling capacitor can make the chip work reliably, reduce the high-frequency noise in the power supply, and reduce EMI. Due to the influence of wire inductance and other parasitic parameters, the response speed of power supply and its power supply wires is slow, which will make the instantaneous current required by drivers in high-speed circuits insufficient. Reasonable design of bypass or decoupling capacitors and distributed capacitors in the power supply layer can quickly supply current to devices by using the energy storage function of capacitors before the power supply responds. Proper capacitive decoupling can provide a low impedance power supply path, which is the key to reduce common-mode EMI.
ground connection
Grounding design is the key to reduce EMI of the whole board.
Make sure to adopt single-point grounding, multi-point grounding or mixed grounding.
The digital ground, analog ground and noise ground should be separated, and a suitable public grounding point should be determined.
If there is no ground plane in the double panel design, it is very important to design the ground grid reasonably, and it should be ensured that the ground width > the power line width > the signal line width. It is also possible to lay the floor in a large area, but it should be noted that the continuity of a large area on the same floor is better.
For multilayer board design, the ground plane layer should be ensured to reduce the common ground impedance.
Series damping resistor
On the premise that the circuit timing requirements allow, the basic technology to suppress interference sources is to connect small resistance in series at the output of key signals, usually using 22 ~ 33 Ω resistance. These small resistors connected in series at the output terminals can slow down the rising/falling time and smooth the overshoot and undershoot signals, thus reducing the high-frequency harmonic amplitude of the output waveform and effectively suppressing EMI.
block
Key devices can use EMI shielding materials or shielding nets.
The shielding of key signals can be designed as striplines or isolated by ground wires on both sides of key signals.
spread spectrum
Spread spectrum (spread spectrum) is a new and effective method to reduce EMI. Spread spectrum is to modulate the signal and extend the signal energy to a relatively wide frequency range. In fact, this method is a controlled modulation of the clock signal, which will not significantly increase the jitter of the clock signal. The practical application proves that the spread spectrum technology is effective and can reduce the radiation by 7 to 20dB.
EMI analysis and testing
Simulation analysis
After PCB wiring is completed, EMI simulation software and expert system can be used for simulation analysis to simulate EMC/EMI environment, so as to evaluate whether the products meet the requirements of relevant EMC standards.
Scanning test
The electromagnetic radiation scanner is used to scan the assembled and powered-on machine plate, and the electromagnetic field distribution diagram in PCB is obtained. According to the test results, the PCB design is improved.