EMC Aspects of PMW Controlled Loads in Vehicles
Abstract: The number of electrically driven loads in a modern vehicle is constantly increasing. Many loads that former were mechanically driven will in the future be driven by electricity. This implies that a number of electronic systems have to be packed together in the limited space in a vehicle. When different electronic systems are placed close to each other, there is always a risk for electromagnetic interference between the different systems causing malfunction or even failure. It is important to ensure that this does not happen, and this concept is called electromagnetic compatibility, EMC. EMC implies that different electrical systems should be able to work in close proximity without affecting each other. From the EMC point of view, integration of electric traction drives in present vehicles represents a considerable challenge. In order to save energy, many electrical loads can be controlled on demand. A common and energy efficient way to do this is to use a method called pulse width modulation, PWM, where the load voltage is pulsed in order to create the desired average output voltage. When this method is employed, the voltage pulses are present on the conductors between the power electronic converter and the load. Since the space in a vehicle is limited, it is often not possible to place the power electronic converter close to the load. Consequently, long conductors are often required between the power electronic converter and the load. The steep edges of the voltage pulses and the fundamental of the square wave, called the switching frequency, together with the long conductors cause electromagnetic interference problems. These disturbances could interfere with, for example, the radio in the vehicle. In this thesis, different electromagnetic compatibility aspects of a pulse width modulated system are investigated. Some solutions are proposed in order to mitigate the disturbances. The solutions involve increasing the rise and fall times of the voltage pulses and employing a randomly varying switching frequency. Also the effects from different conductor layouts, such as using the vehicle body sheet metal as a current return path or having the lead-in and return conductor close to each other, are investigated. In order to evaluate the results from the different setups, the voltage across the load and the radiated magnetic field are measured. The experimental results in this thesis show that a conductor should be used for current return and that this conductor should be placed as close to the lead-in conductor as possible in order to suppress electromagnetic noise. It is also shown that a randomly varying switching frequency will give a more broadband noise in the switching frequency range. Increasing the resistance of the gate resistor mitigates the disturbance in the higher frequency areas at the expense of increased switching losses.
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