Application of Good Electromagnetic Compatibility Design Rules for the System.
With the emerging technologies and increased complex and complicated electronic systems the subject of Electromagnetic Compatibility (EMC) commands much of the attention of designers. The environment for which these complex electronic systems are designed and implemented is getting tougher to deal with because of the presence of high frequency radio transmitters and the power circuits systems are using switching power supplies etc. Every year the number of electronic appliances increases greatly.
While designing the various parts of a complex electronic system, the designer has to adopt techniques and good practices that will increase the chances of the final system becoming compliant to the EMC directive. According to the EMC Directive the designer has to make sure that the function of any of the system component will not be disturbed due to the emission or radiation from any other component. This directive also makes sure that in their operating environment, all of the products tolerate the existing disturbance.
Here the big responsibility is on the designer of the finished product/system to adopt the techniques so that the final system is EMC compatible. A designer has to measure the environmental effects on the design of the system and also take into account the effects of the design on the environment. The former is called as immunity test and later is known as emission test.
1.2 Application of Good EMC Design Rules for the System
In order to design a computer room in the bank, it is necessary to adopt techniques and practices that are in compliance with the EMC directives. A designer has to consider the EMC production volume and the estimated system cost for each of the sub-systems as well as for whole system. In case of a high volume and low cost system, the overall cost of the finished system can be reduced by spending more resources and time on the design, but for a low volume and high cost system, the design time can be reduced by using costly system solutions and components.
All of the sub-systems will have some of the internal as well as the external effects that will have an impact on and can create problems for the overall computer room. There is a great need to keep an eye on these effects because it can deteriorate overall system performance. To ensure good computer room performance the following are good EMC design rules for the system that must be taken into consideration.
1. Identify the Noise Sources
Equipment like printers, UPS etc will only be used when needed, whereas the servers, thermal management system etc will be working all the time. As all of the devices will be switched on for 24 hours a day, even if they are not in use, they will be producing internal and external noises. It is necessary to identify all noise sources. This should be done as close as possible to the source and farthest away from the sensitive parts of the circuit. As all equipment is being used in close vicinity and in absence of the enforcement of EMC directives, there is a high possibility that emissions from these pieces of equipment will cause a rise in temperature. Therefore, the thermal management system will be activated quite often. This continuous change of heat and thermal radiation can result in a degraded performance of the system. So the designer should handle the noise that is transmitted and received on the system inputs and on the power cables.
2. The Path to Ground
In order to avoid noise-related problems, the best way is to prevent noise generation. In the given system it is not possible because most of the noise cannot be avoided because it is generated as a side effect of the intended performance of other system components. AC or DC currents, high or low power current, signals or noise currents, all types of currents always try to find the easiest path to ground. Every sub-system will generate noise that can affect the performance of other sub-systems and can result in interruption of the overall system. Designers try to control the path to ground for all signals. They ensure that this path is far away from the circuits and signals that may be disturbed. By adopting this technique, transmitted noise will find a path to ground before leaving the system and received noise will find a path to ground before reaching the sensitive system components and parts.
3. System Zones
In designing a system that is in accordance with the EMC directive, it is good practice to split the overall system into different sub-systems or zones and handle them separately, because handling the system as a whole is a very complicated task. This is the case in this system. The whole system is divided into four sub-systems so that the designer can focus on the operation of each individual sub-system and the interaction between these systems. Now the designer has to consider the noise that is being emitted from the sub-system and the noise tolerance level of the sub-system. In this problem the system is divided into four sub-systems that are set apart from each other in such a way that the noisy circuits are separated from the sensitive ones. Another way would be to divide them into zones and put a sub-system inside each one.
4. RF Immunity
The environment in which this system is employed is a complex office environment where there are several pieces of electrical equipment like PCs, printers, telephones including mobile phones, PDAs, electric fans, charging circuitry, fluorescent lighting, etc. The pieces of electrical equipment which are outside of the system can emit noise. This noise can be picked up and conducted into the main system due to long I/O and power cables because they normally act as good antennae. The amount of the RF energy that is allowed into the system must be kept as low as possible to avoid this coupling.
5. ESD and Transients
We have seen that the complete system is made up of equipment that is switched on all the time and the second important point is that the system is operating in an office environment where there are many employees working. By taking into consideration these two important factors, it is very important that all of the system components must be properly enclosed in the equipment, only leaving the I/O pins outside. This equipment enclosure is important because system components always produce Electrostatic Discharge (ESD) so the user should be protected from touching the sensitive system components. Incorrect values of signals can be generated due to this ESD discharge that can induce currents in nearby paths.
6. Power Supply, Power Routing and Decoupling Capacitors
As the computer room is in a bank headquarter and it has to be protected from the risk of any power supply failure, it is important that the power supply to this electronic system must be adequate. It is necessary that the system must remain stable and noise emission from the device is minimized. This can be achieved so that the correct and sufficient decoupling of power lines is ensured.
Shielding is important because in some cases it is not possible to get the noise level of a system low enough, but in some cases shielding is done to make a system easier to use. The shielding depends on the type of system and it can cover the whole system or only some of the system components. When the system is divided into different sub-systems, it is easy to determine which zone needs to be shielded. Therefore, in this system each sub-system can be properly shielded to reduce the noise level of the system low enough to ensure the best performance of the overall system.
1.3 System Analysis
One of the major rules to meet the criteria of the EMC directive is to divide a large system into different sub-systems. In this complex office environment, the electronic system is divided into four sub-systems and each is handled separately. Each system required different techniques and practices in designing to come up with a finished system that is in compliance with the EMC directive. The system has to follow the above mentioned and explained design techniques, but here the system analysis is done by focusing each of the systems separately.
1.3.1 Sub-System 1
In this case, all of the devices are very sensitive to fluctuations in temperature. It is very important to handle the noise that is transmitted and received on the system inputs and on the power cables, because it can alter the temperature of the equipment, so identification of noise sources is very important. The second important thing is good cable grounding so that the noise cannot couple with the sensitive parts of the systems. Careful placement of ground connections goes a long way towards reducing the noise voltages that are developed across ground impedances. A proper RF immunity is also highly important to avoid noise coupling, ESD discharge and transients with the sensitive system parts so that this sudden temperature rise can be avoided.
1.3.2 Sub-System 2
In this sub-system it is very important that the ITC equipment does not interfere with each other and with other sub-systems. A major part of EMC design is concerned with the interfaces between the unit and its cables. The cables may intentionally carry high frequency signals, such as data or video; a more potent interference source is common mode noise coupled onto the cable at the interface, and flowing in all its conductors or in its screen. In this sub-system there is a need to control the interaction of the ITC equipment’s internal circuit directly with the external environment so that the three pieces of equipment do not disturb each other’s operations. All of the internal structure of the ITC equipment, especially PCBs and wiring and conductive metal parts like heat sinks and chasses members, must be designed in such a way to minimize their effectiveness as accidental antennae. All of the conductive parts should be bonded together to stop HF potentials arising between them. The amount of RF energy that is allowed into the system must be kept as low as possible to avoid this coupling so that it has RF immunity. The ESD discharge as well as LCD discharge is well maintained.
1.3.3 Sub-System 3
This sub-system is responsible for sufficient power supply. To maintain this power supply and to minimize the emission of noise from the UPS, correct and sufficient decoupling of power lines is very important. As this system is switched on all the time, there is some ESD discharge, and a proper shielding is necessary to avoid any problem. Ventilation holes or louvres must also be constructed from several small holes or slots and they will ensure that the temperature inside the UPS will not rise higher than its tolerance level.
1.3.4 Sub-System 4
Again this sub-system also ensures the stable behaviour of the whole system because it monitors overall system performance. As this is the software monitoring system which is operated from a separate PC it consists of very noisy or very susceptible circuits. It is necessary that good PCB layout practice and proper interface control must be adopted to meet the requirements. Shielding is again important to maintain the internal temperature, and good cable grounding and RF immunity is important so that the noise can not couple with other system components.
1.4 Recommendation from Problem Solving Approach
If the above recommended system design process fails to meet the EMC standards then an EMC problem solving process will be adopted. This process involves troubleshooting as an integral part, but still a hit and trial method will be used to overcome the problem. The initial step is to gather and then evaluate existing system information. It is important to note all of the symptoms and the environment in which the system is designed and implemented. The second step is the preliminary diagnosis which makes it possible to make some tentative evaluations. After diagnosing, it is important to make and carry out the plan.
As this is a large and sensitive system, there is a possibility that the majority of high frequency problems that are related to emission, immunity or self-compatibility may arise. If this problem occurs then at high frequencies a ground return plane should be used because wires and traces have high impedance. As the whole system is divided into four zones or sub-systems, there is a possibility of poor ground cabling so that the emissions from one sub-system may disturb other sub-systems. In order to avoid this problem the current should be diverted back to the shielded enclosure or the current blocked with a high impedance ferrite choke.
Another problem of emission can arise due to improper shielding integrity in the case of PCs and UPS, so proper shielding tests should be carried out to overcome poor shielding integrity by analysing or measuring the crystal oscillator harmonics. The switching devices can emit radiations like ESD and LCD emissions from work stations, monitors etc. Another problem is the cross talk that can occur due to the self-compatibility problem. This can happen in cases where a noisy source current comes into close proximity with very sensitive devices like the thermal management system. It may vary the temperature of the device and auto system shut down can occur frequently that may disturb overall system performance. These problems like emissions, power disturbance and cross talk can be solved by controlling the path of current. Problems related to RF immunity and power disturbance always involve a cable so cabling must be done in a more careful way.
1.5 Evidence of Research
- Designing for EMC, Tim Williams, Elmac Services, Jan, 2010.
- AVR040: EMC Design Considerations, Atmel Corporation.
- Electromagnetic compatibility Part 2 Design principles, C. Christopoulos.
- Top Ten EMC Problems & EMC Troubleshooting Techniques, Kenneth Wyatt, DVD, Colorado Springs, Rev. 5, June 19, 2007.
- Systems EMC troubleshooting, Kimmel, William D, Gerke, Daryl D Electromagnetic News Report, Jul/Aug, 1998.