Design of an intelligent switch for fully integrated power nets

Abstract

This thesis focuses on the structural design of a proposed intelligent switch system and demonstrates how the electrical power system of a vehicle can be simplified with advanced power management features by using the proposed intelligent switch system. The design concept of the proposed scheme is basically a DC power bus with a number of power switches controlled remotely by means of DC powerline communication. With the proposed system, users are able to manage and install any appliance into a vehicle without installing new wires. However, even though the AC powerline communication technology is mature and has been widely used in many applications, there is no DC powerline communication system in the market due to the technology limitations. The major difficulties of realizing DC powerline communication are due the low impedance and low frequency cut-off characteristics of the DC powerline, which introduces serious signal attenuation and distortion to the carrier signal. These problems lead to poor signal to noise ratio. In order to mitigate these effects, a high efficiency carrier transceiver which consists of a carrier transmitter and a carrier receiver is required for the intelligent switch system. The proposed transmitter is basically an H-bridge switching circuit with a capacitor and a 1:100 transformer connected in series. The H-bridge is designed to operate in switching mode to minimize power losses while the switching frequency of the switches is set slightly lower than the resonance frequency of the series capacitor and transformer. The capacitor and the transformer behave like a LC circuit and will resonance at the switching frequency and transmit a sine-wave like carrier to the powerline at high power efficiency. Data symbols '0' and '1' are modulated into continuous frequency changes. Since it is almost impossible to have an ambient noise varying at the same pattern as the modulated symbols and existing at the same time, the problem of noise jamming is minimized. Also since the transformer boosted up the current acquired from the power bus one hundred times, the signal to noise ratio can be maintained at an acceptable level. The design of the receiver is based on the simple ratio detector which has been widely used in commercial radio. The ratio detector converts frequency change into voltage variation, which will then be digitalized by an analog to digital converter (A/D converter) and decoded by a microcontroller. Since the transceiver circuitry is very simple and no special component is needed, the objective of developing a simple and low cost in-vehicle intelligent switch system is achieved.