Abstract
Keywords - current evaluation, hybrid vehicle, optical fiber sensor, compact design, inverter
A current evaluation methodology has been developed that can evaluate transient current behavior of semiconductor chips in densely packaged hybrid vehicle (HV) inverters. This paper describes this measurement and analysis technology for current in HV inverters using an optical current sensor as an example of the latest evaluation technology in the power electronics field.
Minimizing the size of HV components is of critical importance for popularizing HVs, and is one of the main challenges in HV development. In order to meet this challenge, technology to accurately evaluate the transient electrical characteristics of HV components is needed.
However, conventional current sensors cannot measure the current of parallel power devices in the HV inverter power module accurately without altering the structure of the inverter because these sensors are too large to probe inside the inverter. Initially, it was attempted install a 0.1 m shunt resistor into the inverter power module, but measurement was impossible due to the effect of current resonance on the shunt resistor. Next, it was attempted to wrap a Rogowski coil, i.e., a toroidal coil with an air core, around the conductor, but the measurement results were not sufficient to evaluate a current balance of several amperes due to the effects of error created by external magnetic fields.
For this reason, the possibility of adopting a type of optical current sensor utilized by the electric power industry was examined as a response to these issues. This sensor utilizes the Faraday effect of optical fibers to measure current with high accuracy unaffected by magnetic fields. Furthermore, it has a thin and flexible structure, which allows it to be wound around the semiconductor wire bonding in the inverter power module. However, this sensor is only capable of measuring frequency bands between five kHz and fifty kHz, which is too low to measure current in HV inverters, which reach a frequency of several MHz. These technical issues were solved by combining improvements to the amplifier and optimization of gain and frequency characteristics with a measurement method that removes noise by averaging the measured current from inverter DC operation. As a result, it is now possible to measure current for parallel chips in an HV inverter, and the accuracy of current simulation technology used in design has been greatly improved.
Both developed technologies are very valuable for optimizing design of compact HV inverters.