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Improve PCB EMC Capability PCB Design and Wiring Method

Improve PCB EMC Capability PCB Design and Wiring Method

This paper gives the factors that need to be considered in the PCB layout of the motor control power circuit, especially how to improve the electromagnetic compatibility of the circuit. This paper gives the consideration from the selection of the circuit board and the laying of the ground wire. Finally, the application of these methods is demonstrated through practical cases.

Author: Zhuo Qing

Introduction: This paper gives the factors that need to be considered in the PCB layout of the motor control power circuit, especially how to improve the electromagnetic compatibility of the circuit. This paper gives the consideration from the selection of the circuit board and the laying of the ground wire. Finally, the application of these methods is demonstrated through practical cases.

foreword

In the annual National College Student Smart Car Competition[1] , there will be many students who encounter static interference caused by the friction of the model car wheels on the PVC track.For example, periodontitis caused by beriberi in Bowen[2] The recorded students used anti-static tape to wrap the wheels to reduce the impact of static electricity on the electromagnetic detection circuit.

Question 1: Zhuo Da, the inductance value of our tricycle jumped inexplicably, and the jump became quite powerful. This is the waveform of the inductance value without the magnet wire to push off. Then wrap a layer of anti-static tape on the tire, and the jumping phenomenon is gone. Zhuo Da help us analyze it.

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1 Binding anti-static tape on the tire

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 2 The interference of electrostatic discharge on the output signal of the electromagnetic detection circuit

Electromagnetic Compatibility Design and Test Standards in Motor Control Applications[3] Introduction to EMC design guides for motor control applications[4] The first half of this chapter on EMC precaution standards and test methods in motor control circuits. Due to the space, the second half of the document, that is, the details of the specific PCB wiring considerations, is omitted. The second half of the document is excerpted below.

PCB Design and Routing

In order for motor control circuits to meet Electromagnetic Compatibility (EMC) standards, EMC requirements should be part of the product definition, with the goal of focusing on EMI radiation and reducing the circuit’s exposure to EMI during circuit design, device selection, and PCB layout. Sensitivity.

The following figure shows the general circuit structure of a highly integrated motor control design circuit. Here, you can see that it contains various functional modules. Which of these functions may generate electromagnetic radiation or be susceptible to electromagnetic interference needs to be considered, as well as possible coupling paths between them.

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.1 Three-phase induction motor inverter circuit

1.1 EMC Overview

The following figure shows a simple component that generates EMI:

・ Sources of electromagnetic interference;
・ Electromagnetic coupling path
・ Electromagnetic interference affects the device or the receiving device;

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.1.1 Electromagnetic Interference Model

EMI sources include microcontrollers, charge discharge devices, transmitters, power transient devices such as electromagnetic relays, power switches, and lightning. In microcontroller systems, clock circuits often generate broadband noise.

Although all Electronic circuits may receive EMI signals, the most sensitive circuit signals include: reset, interrupt, fault detection, protection, and control signal lines. Analog amplifiers, control circuits, and power regulator circuits are also susceptible to noise interference.

The coupling path between the interferer and the receiver circuit consists of:

Conduction: The coupling path between the interference source and the receiving circuit is a direct contact, such as a lead, cable or path connection;
・Capacitance: There are various electric fields between two close conductors or leads. When the distance is smaller than the wavelength of the electromagnetic wave, the voltage will change between the gaps;
・Inductance or magnetic field: There is a magnetic field between two parallel conductors or leads, and when the distance is less than the wavelength of the electromagnetic wave, it will cause a voltage change on the receiving conductor;
・Electromagnetic radiation: When the distance between the interference source and the receiving circuit is far, larger than the wavelength of the electromagnetic wave, the transmission and reception are equivalent to a radio antenna, the electromagnetic interference is sent from the interference source, and the radiated electromagnetic wave propagates in the air.

Switching power supplies in motor control circuits are often a major source of EMI. The square wave pulse in the circuit forms rapidly changing large current and voltage, with high di/dt, du/dt waveform has strong nonlinearity, and there are high-order harmonics. Since there are so many frequency components, usually noise signals, they are more likely to interfere with other circuits in the motor control circuit through conduction or radio wave radiation, causing them to malfunction.

Improve PCB EMC Capability PCB Design and Wiring Method

Designers often use damping circuits or soft-switching techniques to greatly reduce EMI in switching power supplies.

Surprisingly, since today’s power transistors typically have higher switching frequencies than the application requires, certain circuit parts may inadvertently amplify noise and harmonic components, complicating EMI problems. These high-frequency interfering signals have the potential to reach the frequency band where radio waves are emitted, so they are sometimes referred to as radio frequency interference (RFI).

Inverter and drive circuits have the ability to generate electromagnetic interference. Circuit designers need to pay special attention to the turn-on and turn-off characteristics of power transistor devices to minimize the generation of electromagnetic interference signals in these circuits. If using discrete IGBTs, or MOSFET devices, designers can flexibly use gate series resistors to control the switching characteristics of the power transistors, making a compromise between power loss and EMI.

If an IPM (Intelligent Power Module) is used, the drive circuit is integrated inside, the parameters of which have been optimized between power loss and electromagnetic interference.

In the design of the motor control circuit, there are also control and sensor functions. They are usually susceptible to electromagnetic interference, and their failure can be avoided by bypassing, filtering, and buffering.

Once EMI sources and interfering circuits are identified, circuit topology is optimized within circuit performance and cost constraints.

Once the initial circuit design and schematics are finalized, efforts need to focus on the heart of electromagnetic compatibility and control: PCB routing. At this stage, segmentation strategies can be considered to consider how the placement and routing of devices with different 3D structures affects the EMI performance of the final product. The trouble of many electromagnetic compatibility problems is usually found and solved during the circuit division and wiring process.

Main steps to address EMC requirements Phase:

1. Circuit definition stage: define the electromagnetic compatibility standards that the design needs to comply with;

2. Circuit design stage: In the process of schematic realization, engineers need to: * Determine the circuits and devices that may form the source of electromagnetic interference in the circuit; * Determine the circuits and devices in the circuit that are susceptible to electromagnetic interference; Possible communication and radio transmission paths to and from the receiving interfering circuit.

3. Design a suitable circuit segmentation strategy for efficient circuit connection and planning.

1.2 Circuit segmentation strategy
  
The key factors of PCB layout and layout that are important for EMI impact include:

1. PCB: Determine the type of PCB, including size and number of layers, usually determined by cost;
2. Ground wire: Determine the circuit ground wire structure, which directly affects the selection of PCB types;
3. Signal: Determine the type of control, power and ground signal, which is determined by the required motor control function;
4. Coupling path: Determine the best means of signal exchange between functional modules, and determine whether to use surface mount or through-hole lead packages for large devices.
5. Device orientation and placement: Considering the longevity of large devices, or devices that need to be installed with heat sinks, they often have requirements for placement and require special treatment.
6. Shielding: For other methods of electromagnetic interference that ultimately fail to meet your electromagnetic compatibility requirements and limitations, consider how to add a shield to the PCB.

1.3 Circuit division
  
After careful planning, the actual division of the circuit needs to be done step-by-step (following logic). The circuit segmentation model in the figure below is the result of taking into account all major EMI issues, and in general it shows:

・ How the circuit function is divided into different modules;
・ How the different modules are laid out;
・ And how the modules are divided by the bottom line;

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.3.1 A scheme of circuit division

In addition, in order to efficiently carry out PCB planning and routing, graphical tools are used to carry out.

In each circuit functional block, the source of electromagnetic interference can be found through the schematic diagram. Since the bottom line layout is very important to meet the electromagnetic interference compatibility, the different modules are clearly separated by the bottom line. Of course this is just an ideal model of the module and ground layout, and you need to get as close to this layout as possible when designing.

So far, we have adopted a top-to-bottom design strategy to meet the electromagnetic compatibility requirements. The advantage of this is that the source of electromagnetic interference that affects the whole world can be determined, and the circuit partition strategy is adopted to lay a good foundation for reducing the electromagnetic interference layout.

1.4 Routing and EMI: PCB Selection and Routing Specifications
  
Below, we begin to discuss the bottom-up approach to electromagnetic compatibility requirements, including intelligent wiring, the layout of electromagnetic transmitters and their interconnection and influence.

1.4.1 PCB

Since the circuit is finally designed and implemented through PCB, we need to consider the selection scheme of PCB, which can better solve the problem of electromagnetic interference.

Conductors with the same electromagnetic wavelength will be sensitive to electromagnetic interference and will also become the emission source of electromagnetic interference signals. Therefore, it is necessary to select the PCB substrate material with the lowest dielectric coefficient when designing.

FR4 is usually used in low frequency circuit design, and its dielectric constant reaches 4 due to the use of epoxy resin as the insulating layer.

The base thickness of the PCB is also important, as it determines the degree of coupling between the different lay-up layers. The ratio of the width of the wire to the thickness of the circuit board determines the degree of coupling between the two layers of wires, which is also important for controlling EMI.

The number of layers of available routing on a PCB is an important factor affecting the electromagnetic compatibility characteristics in a particular design. It is important because it limits how the bottom line is laid and also determines the overall EMI behavior. By laying the ground layer, it is easier to ground the device, and the shielding effect of the ground wire is the key to controlling electromagnetic interference.

The figure below shows the structure of a single-sided board. All power lines, control lines and ground lines need to be completed on one side of the PCB. This complicates wiring and control electromagnetic compatibility issues. Mutual interference may occur between circuits on the same layer. Device grounding also becomes difficult.

When using a single-sided PCB, the surrounding area of ​​the circuit board needs to be used for laying ground wires as much as possible. For the part without leads in the circuit, it is also necessary to lay copper and connect to the ground wire. The copper that is not connected to the ground wire needs to be removed. Using a single panel lacks a flexible means of designing for reliable electromagnetic compatibility.

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.2 Copper laying in a single panel

When using a double-sided board to design the circuit, one side can be used for the ground wire alone to reduce the complexity of the wiring. The cost is only a little higher than that of a single panel. However, there is still interference between the power supply and the control module, so it is critical to separate the source of electromagnetic interference in the circuit from other circuits.

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.3 Double panel structure

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.4 Having through-holes to solder devices with pins

Four-layer boards are often expensive, but can use independent power planes to achieve good self-shielding effects. Components can also be placed on both sides of the board.

In the following two four-layer board layouts, the left method sets the power supply layer inside the circuit board, which limits heat dissipation and also affects the bottom signal layer. Signal lines on the upper and lower layers are also sensitive to external sources of electromagnetic interference.

The measurement on the right is to place the ground wire layer on the outermost side, which has a strong effect of resisting external interference sources, but there will be great self-interference between internal circuits.

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.5 Two four-layer board layouts

1.4.2 Ground wire
  
A good grounding strategy will address sources of interference and sensitive circuits. First of all, it is necessary to consider the prevention of the reference ground wire of all signals in the PCB, which is usually a physical point on the PCB. Sometimes the PCB is placed in the rack or metal shell, and this point on the circuit board is also connected to the rack or metal shell. .

Next, it is necessary to ensure that the path from the circuit ground to this point is as short as possible, which usually needs to be weighed in consideration of the available area:

・ Since there is no dedicated bottom layer in a single panel, it is difficult to consider the ground wire structure;
・ For two-layer boards, one layer can be set as an independent ground layer, and the remaining layer is used for power lines and signal lines;
・A circuit board with more than two layers has more flexibility in the placement of the ground wire, and the ability to improve electromagnetic interference is also very large.

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.6 Multilayer circuit board PCB layout

(1) Reduce the ground wire impedance

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.7 Reduce the length of the ground loop
Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.8 Improved circuit layout

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.9 Recommended circuit layout

(2) Ground wire structure

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.10 Grounding through vias

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.11 Improved ground loop

(3) Circuit connection

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.12 Ground wire assembly method

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.4.13 Layout for a three-phase power system

1.5 PCB layout skills

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.5.1 The copper circuit needs to be connected to the ground wire

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.5.2 Layout of power and ground wires

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.5.3 Decoupling Capacitor Placement Recommendations

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.5.4 Suggested 45° angle of lead

1.6 Case Examples

1.6.1 Case 1

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.1 Case 1: Circuit Board

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.2 Improved PCB

1.6.2 Case 2

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.3 Case 2: PCB

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.4 Case 2: Improved circuit

1.6.3 Case 3

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.5 Case 3: PCB

1.6.4 Case 4

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.6 Case 4: PCB

1.6.5 Case 5

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.7 Case 5: PCB

1.6.6 Case 6

Improve PCB EMC Capability PCB Design and Wiring Method
Figure 1.6.8 Case 6: PCB

Wiring Summary

This paper gives the factors that need to be considered in the PCB layout of the motor control power circuit, especially how to improve the electromagnetic compatibility of the circuit. This paper gives the consideration from the selection of the circuit board and the laying of the ground wire. Finally, the application of these methods is demonstrated through practical cases.

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