EMC Design for Boiler Controller - Assignment Example

EMC Design for Boiler Controller 0. Introduction Any electrical or electronic system has to work satisfactorily within itself and with it’s electromagnetic environment. If there are any unwanted current or voltage present in a circuit or system , it may disturb the performance of other devices by reaching them through conduction , induction or by non – ionizing radiation. This disturbance is termed as electro – magnetic interference (EMI). For such an interference to take place , there has to be a source or transmitter which gives out unwanted voltage or current , a transmission path that leads to interference and a receiver or victim circuit that gets disturbed by the source. Among these three , any one of the part may be unplanned or not properly designed. Any electronic design must control this Electro Magnetic Interference for proper functioning of the electrical or electronic system. This Electro Magnetic Interference control can be achieved by changing or adjusting the signal levels or noise levels in the circuitry. Such a design is said to be Electro Magnetically Compatible (EMC). This Electro Magnetic Compatibility (EMC) is a prime requirement in standard electrical or electronic design procedures. The paper discusses about the concepts related to Electro Magnetic Compatibility like Electro Magnetic Compatibility design principles. The paper also relates these concepts to the design of a Boiler Controller with five sub systems. The EMC mitigation techniques like filtering , shielding , bonding of cables , PCB layout , etc. have to be detailed for this Boiler Controller. Also system hardening recommendations has to be proposed for system in case of any discrepancy in the design. 2.0. Electronic Design Practice compliant to EMC Directive : Dr. Franz Schlagenhaufer and Mathew Wood ( 2008 ) says that ‘Grounding, shielding, filtering and cabling/wiring are important design criteria to achieve EMC on the equipment level. In order to avoid costly overkill solutions and to tailor individual measures in the case of contradicting requirements, ….their application must be based on sound theoretical principles. A sound knowledge of electromagnetic theory is essential to understand and appreciate EMC measures’. The noise frequency range of EMI ranges from 10 KHz to 30 KHz. Different countries have different standards to be followed for EMC. In Europe the standards that need to be complied for Electro Magnetic Compatibility refers to EU - 336/89. There are different directives for EMC in different countries. These standards specify the design standards, testing for EMC , allowable emissive and susceptive levels of radiation . The common considerations for EMC based design of electronic circuits need to account for allowed distance between the sub systems , identify the frequencies that need to be shielded , identify the source of noise , any unwanted signal has to be grounded properly , the high frequency noisy circuits need to be isolated from other circuits, filters to avoid the interferences of radio frequency signals, suppressing or filtering Electro Static Discharges nearby the circuit, providing separate ground sections for analogue and digital circuits when they are in the same system , power lines have to be de-coupled as they may induce spikes, proper grounding for chassis is also essential. These are some general design guideline that need to be followed for Electro Magnetic Compatibility . 3.0. Overview of Product under design : This product has five modules , the first is a Switch Mode Power Supply ( SMPS ) that gives 5 V at 0.5 A and 12 Volts at 5 Amperes from a 240Vac 50 Hz supply. The second section is a true centigrade temperature sensor - LM35 which is connected via a two-metre lead to the third section. The LM 35 needs between +9 and 20 volts to power it and it has a response of 10mV/oC. the next module is a signal conditioning circuit taking the voltage from temperature sensor and changing it to between 1 volt and 5 volts. the fourth section is a microprocessor board with an on-board ADC and an oscillator running at 100 MHz. The final section is a display and interfacing board, with outputs to a five-digit LCD, an alarm circuit and disabling system for a boiler , and the last output is connected to an RS232 socket which has to be connected to the control room , about 20 metres distant from the boiler . A keypad is mounted on to this board to program, set the temperature and assist in calibrating the system . The overall power requirement is less than 30 watts for the system and may therefore require cooling . All the modules except the temperature sensor have to be placed in a chassis. The product needs to function in an industrial environment where there may be interferences due to printer , mobile phones , electric fans , stepper motor based machines , lights , etc. 4.0. Design of EMC compatible Boiler Controller : In general there are two principles that govern system electro magnetic compatibility, they are EMI suppression and EMI containment as specified by microATX ( 1998 ) . It states that ‘ if an electronic system does not generate EMI, then the electronic system is EMC-compliant. Suppression is generally accomplished at the board level. .. also if an EMI source can be completely contained , then the electronic system is EMC- compliant. Containment is generally accomplished at the system level (e.g., use of a shield )’. Practically , using both the methods gives a good design with respect to EMC . The system under design has a LM 35 temperature sensor , that takes 9 – 20 V input and is planned to be placed apart from all other sub systems. All the other sub systems are placed in a casing no bigger than 50x100x200 mm. When two or more sub systems are placed in such a small casing, there are possibilities of EMI. To mitigate these effects the major factors like PCB layout , filtering , shielding of cables , preventing unintentional RF signal , Common Mode chokes , shielding of areas and volumes , bonding , cable routing , reduction of power harmonics , suppression of arcs and sparks , segregation or partitioning , etc . have to be considered. 4.1. Common Mode filtering : The most prominent mitigation for EMI – filtering by Common Mode chokes has to be applied to the first sub system in this boiler controller. It is the SMPS operating at 40 KHz. The boiler system also requires a fan for cooling. Mediano.A. ( 2007 ) says that ‘ These systems must comply EMC regulations both conducted (9kHz to 30MHz) and radiated (30MHz to 1GHz) and both elements (SMPS and fan) are key elements to define the EMC profile’. These EMI can be seen in fig.1. as presented by Mediano.A.( 2007 ) Fig.1. Fan current - comparison of SMPS with linear power supply as presented by Mediano.A. ( 2007). The key element that could cause the problem happens to be the Power Supply cord because it radiates Common Mode EMI in the range 30 - 300 MHz and conducts Common Mode EMI in the range 9 KHz - 30MHz. This could be measured by a Low Impedance Stabilisation Network ( LISN ) . Henry.W.Ott ( 1988) suggests many techniques to minimise these EMI in SMPS , which include filters ( X caps , Y caps , Common Mode chokes, Differential Mode capacitors ) shields in transformers , snubbers , ferrites. Robert West ( 1995 ) proposes a practical Common Mode Induction Filter as shown in fig.2. Fig. 2. Common Mode Induction Filter as proposed by Robert West ( 1995 ) 4.2. PCB Layout : The next consideration is proper PCB layout that is capable of emissions and immunity control. Tony Maddocks and Martin Ganley (2007) of ERA technology say that the ‘ track layout is the most important consideration and the frequency range of importance is 300 – 2000 MHz.’ They also say that a multi layer configuration gives up to ‘30 dB improvement over single sided boards and careful interface design is important for is required to prevent energy transfer to external cables’. The possible EMI radiations relevant to PCB is shown in fig.3. Fig.3. Common & Differential Mode radiation in PCB as quoted by Tony Maddocks.T. and Ganley. M. (2007) of ERA technology. Another factor in the PCB design for EMC considerations is the arrangement of components. Fig.4 shows a preferred arrangement of components in a PCB for EMC, where the high speed components are placed internally and slow speed components have to be placed at the outer portion of the board. Fig. 4. Preferred arrangement of components in PCB , as quoted by Tony Maddocks.T. and Ganley. M. (2007) of ERA technology. 4.3. Shielding of Cables: According to Keith Armstrong and Tim Williams of Radio communications agency , shielding of cables reduces the EMI. The principle is similar to that of screened cables that are used to reduce coupling of the inner cable with the external environment. this screen functions differently at different frequencies and has different effects for electric and magnetic coupling. To reduce EMI, at low frequencies the screened cable has to be grounded only at one end. This gives to better shielding from capacitively coupled interferences. To have magnetic shielding, both ends need to be grounded as this allows flow of induced current in the screen that opposes the induced current in the centre conductor. Using twisted pair cables with the screen grounded at one end also minimizes low frequency magnetic field pick – up. Another factor that has to be taken care of is the length of the cable , if it is a quarter wavelength at the operating frequency, external fields may induce field currents. An open circuit at one end leads to standing wave pattern , also stray capacitances allow flow of screen currents. Thus to have good screening, it needs to be grounded at both the ends to divert these currents . 4.4. Partitioning : A non partitioned system with many sub systems may have common mode currents. To alleviated this the sub systems can be placed far apart within the chasing, but this may lead to high induce ground voltages. The only way to control such effects is to have an overall shield and to use filters for each interface. Here again maintaining the integrity of the shield poses a problem. To avoid all these, Partitioning can be done. This partitioning separates the critical and non critical sections. In this boiler controller the critical section is the Microprocessor circuitry which is highly susceptible to EMC. Elmac services (1997) proposes a partitioning method as shown in fig.5. Here care has to be taken to have less number of filtered interfaces. Fig.5. System partitioning as advised by Elmac services (1997). 4.5. Other mitigation techniques : External RF signals that are easily picked up by I/O wires can be avoided by using special EMC filters or low equivalent series resistance capacitors. Also HF transients and Electro Static Discharges (ESD) need to be reduced by using RF filters and over voltage protectors. Conducted interferences may be reduced by suppression capacitors. RF coupling between cables can be mitigated by using metal free fiber optic cables , routing the cables such that the Common Mode emissions return path is ideal , classify the cable based on the signal carried and then route cables carrying similar signal near to each other. Recommendation for system Hardening : Brewer Ron ( 2008 ) states that ‘If the system is composed of unintentionally radiating RF devices that are being installed into a benign RF environment, EMC hardening simply may consist of a good ground and some careful cable routing and box shielding. Its different if the system is composed of multiple RF receivers and transmitters installed in a small, hostile RF environment along with similar types of equipment.’ System hardening may need frequency assignment , proper placement of components , correct orientation of components , scheduling the operations according to priority , also using circuit breakers, transient absorbing devices, trip switches , and fuses would help to improve system hardening against EMC. To further improve EMC of the boiler system , a new system could be used. This stand by system has to be designed to have very less external emissions. Also they should not be susceptible to radiations from outside. This could be achieved by using shields and screens. This screening may involve screened room or metallic racks or a highly conductive chassis for the boiler controller circuitry kit ( chasing). This reduces the EMI radiations to and from the boiler circuitry and the other control room computer or temperature sensor that is attached with the boiler. The field inside the chasing (sphere) is maintained at zero according to Faraday’s law. Here, the EMI induced E – field cause electrical charges of different polarity through the screen and these charges tend to nullify the EMI field inside the chasing. Additional EMI suppressors or transient filters can be used to harden the system. These may be in the form of capacitors after the rectifier in SMPS , to suppress the harmonics further . The British Standards ( BS 2135 ) states that the maximum value of EMI suppressor Capacitor to be 0.0032-0.064uF. Another hardening remedy for the LM 35 temperature sensor is to use a bypass capacitor from VIN to ground along with a series RC damping circuit with R value being 75Ω in series with a capacitor of value 0.2 or 1 µF between output and ground. The mains power for the stand by system can be made to pass through a power – line filter with a shielded enclosure , which provides adequate attenuation of EMI signals. Conclusion : This article discusses about the design of a system for EMC. The basic EMC directives and the design rules have been discussed. Later the EMC design specifications for the given Boiler Controller has been discussed with respect to various mitigation techniques like cable shielding , system partitioning, PCB layout and other mitigating methods. Finally system hardening options has also been explained for the Boiler Controller. References: Armstrong.K. and Williams.T., EMC awareness, radio communications agency , http://www.emcuk.co.uk/awareness/Pages/MitigationTechniques/Design/ShieldingCables.htm Dr. Schlagenhaufer .F. and Mathew Wood , 2008 , EMC DESIGN PRINCIPLES, IEEE EMC Symposium. 908 ; 910. 3M Electronic Adhesives & Specialties. Elmac services , 1997 ,Designing for EMC, EMC for the small business , 1st May 1997 at the IEE, Savoy Place, London. Henry W. Ott . , 1988 , Noise Reduction Techniques in Electronic Systems”, John Wiley & Sons, 2nd Edition . Maddocks . T. and Ganley .M. ,2007 , PCB Design Principles for EMC , ERA Technology, EMC and Safety Engineering Department Mediano.A. , 2007 , Troubleshooting EMC in designs with switching power supplies and fans , Group of Power Electronics and Microelectronic (GEPM), I3A, University of Zaragoza, This work was supported in part by the Spanish MEC under Project TEC2007-64188, by DGA (Regional Government of Aragón) and University of Zaragoza (Spain) under project UZ2006-TEC-01 . microATX , 1998 , EMC Design Suggestions , Version 1.0 Ron .B. , 2008 , Systems EMC design and analysis. Evaluation Engineering , EMC Test , part 1. http://www.entrepreneur.com/business journals. West .R ., 1995 , Common Mode Inductors for EMI Filters Require Careful Attention to Core Material Selection, from PCIM magazine, reprinted from July 1995 issue. Read More