Electrical products are born with EMI (Electro-Magnetic Interference). Multifunction of electronic products meet people's different expectations, which also leads to the generation of a series of EMI issues that directly threaten people's health and environmental security. Effect of EMI on automotive is so protruding that it influences the performance of external radio devices and creates interference towards electronic devices inside vehicles to some extent. Therefore, it's of much necessity for us to understand EMI and to focus on some tips for automotive circuit design as preventive measures.
EMI issue usually occurs in electronic products. It can be seen everywhere in electronic products, between electronic devices, components and systems. EMI results from so many reasons among which there's only a core, that is, electrostatic discharge (ESD) interference. Elements leading electronic circuits to be interfered by static electricity come in two aspects: unreasonably-designed distribution parameters in electronic circuit and environmental elements that reduce stability of circuits, finally causing interference.
EMI withstand by automotive mainly derives from electrical devices in cars. Its transmission modes contain conduction and radiation while interference sources include circuit network interference, electrostatic discharge interference and electromagnetic radiation interference etc.
• Circuit network interference primarily derives from electrical devices inside vehicles. Those devices create interference pulse signals that are then transmitted in a mode of electromagnetic waves around network, stopping ambient radio receiving equipment from normal running.
• Electromagnetic radiation interference refers to the interference resulting from a sudden change (pulse) that is caused when circuit becomes powered on or off. It especially occurs to inductive and capacitive carrier when a suddenly high voltage is generated at the transient moment of power on and off, leading radiation frequency to be high as well. This type of interference not only affects on-board radiation receiving device, but damages automotive electrical devices in different degrees as well.
All kinds of electronic control units applied by vehicles don't normally run unless a stable power supply is provided. As soon as a voltage fluctuation occurs in power supply system, electronic modules won't be able to work at a normal state.
Tips in Automotive Circuit Design
To stop vehicles from suffering from abnormality due to EMI, some preventive measures should be made in order to prohibit or reduce EMI so that automotives and automotive electronic devices are capable of smoothly performing.
Tip 1: Electromagnetic conduction interference should be reduced.
Detailed measures in automotive circuit design can be summarized as the following:
• Effective area of each circuit loop should be minimized.
• When it comes to EMI resulting from leakage inductance on transformer, magnetic shielding has to be implemented on transformer. Furthermore, effective area of each loop should be minimized.
• For circuits suffering from serious EMI, double lines can be applied to transmit signals to reduce EMI.
Tip 2: Automotive circuit network interference should be prohibited.
• Leads connecting automotive electrical devices should be shortened and should NOT be parallel or close to each other.
• Rational leads routing plays an essential role in prohibiting circuit network interference.
• Power filtering capacitance should be enlarged with RC decoupling circuit added to reduce circuit coupling.
Tip 3: Automotive electromagnetic radiation interference should be prohibited.
• R-C-D network protection should be implemented.
• Effective transient prohibition measures should be made at interference source, which not only reduces electromagnetic radiation of capacitive loading, but protects and controls contact points of capacitive loading.
• Metal shield should be applied.
Any interference on field can be prohibited in shielding methods.
• Within the receiving frequency of mobile communications, aluminum is usually picked for electromagnetic shielding. Due to high-frequency skin effect, electromagnetic shield needn't to be too thick.
• Contactless engine or distributorless ignition system can be applied to eliminate interference source and reduce electromagnetic radiation.
Tip 4: Shielding should be undertaken.
Shielding plays an effective part in stopping electromagnetic interference. Usually materials with high conductivity are selected as a shield. It's been testified that excellent performance of high-frequency field shielding depends on high-conductivity materials such as steel or aluminum while that of low-frequency field shielding depends on materials with high magnetoconductivity such as magnet steel, iron and beryllium alloy. Appliances tending to generate spark or electric arc should be shielded with a metal shield so that negative effect can be eliminated of electromagnetic compatibility (EMC).
Tip 5: Filtering is also an efficient method to control EMI.
Filter is regarded as the most commonly-used method to defeat EMI and it is capable of resisting EMI that directly enters through circuit path. It is summarized that increasing signal amplitude is a crucial method of anti-EMI. When it comes to weak sensor signals, circuit can be amplified with amplitude increased, which will drastically reduce EMI.
EMI on automotive circuit has lots of bad effect on automotive electronic components. It not only shrinks automotive lifespan but also destroys people's driving security. As a result, numerous preventive measures can be made to minimize the influence of EMI on vehicle electronic components with automotive running security and reliability ensured.
