Development of Current Measurement Techniques for High Frequency Power Converters

Development of Current Measurement Techniques for High Frequency Power Converters Mehrdad Biglarbegian, Shahriar Jalal Nibir, Hamidreza Jafarian, and Babak Parkhideh Electrical and Computer Engineering Department Energy Production and Infrastructure Center (EPIC) University of North Carolina at Charlotte Charlotte, USA email: {mbiglarb, bparkhideh}@uncc.edu Abstract—Current sensing plays dominant role in power Almost in all the power electronics applications such as converters, where current information can be used for controlling, power conversion in solar Photovoltaics, wind turbine systems, monitoring and protection. One of the most challenging goals in motor drives, hybrid electric vehicles, current sensing’s role is modern power electronics converters is to increase switching highlighted significantly. Having accurate current sensing frequency for the purpose of miniaturization, and improve the makes controlling, protection and monitoring feasible at performance especially for end-users. Therefore, the need for accurate, lossless, and fast response current sensors is more different levels of power electronics systems [14]-[19]. Modern highlighted. This paper presents a comprehensive review of research in the field of high frequency current sensing is now different current sensing schemes for power electronics more focused on the investigation of alternative and contactless applications. The Challenges for implementation of conventional measurement approaches for higher efficiency. In traditional methods for high frequency (>1MHz) and high current converters Si-based power converters, the switching frequency of the will be addressed. More specifically, technical issue and hardware power circuits were relatively low (<100kHz), hence the rate of difficulties for developing Rogowski-based as well as Hall effect change current is limited. Thanks to the development of new current sensors are discussed and finally, a new technique for generations of WBG transistors, switching frequency of new improving the performance of Anisotropic Magneto-Resistive converters is increasing dramatically, i.e. >1MHz and >30V, (AMR) at 1MHz and 30V with a new technique is shown. which requires more accurate, faster response and higher Keywords—Hall effect,Rogowski-based, Anisotropic Magneto- bandwidth current sensors [20]. Resistive high frequency converter, current sensing, GaN converter In many articles, characteristics and advantages of the implemented current sensing methods are described according I. INTRODUCTION to the unique specification of the application [20]-[23]. To the best of the author’s knowledge, no prior work has been To improve the efficiency and to enhance the power density published to present comprehensive design, analysis and of power electronics converters, increasing the switching prototyping of different current sensing techniques and also frequency opens a new road map for the future of converters. proposing solutions for hardware implementation. The After decades of significant efforts to improve the performance contribution of this work is to review the different current of power Metal Oxide Semiconductor Field Effect Transistors sensing techniques and challenges associated with the operation (MOSFETs), remarkable progress is made in the development of the existing methods for high frequency and high current of state of the art Wide Band Gap (WBG) power transistors, converters. This paper has organized as: In Section II, the which enables circuit miniaturization to enter into a new level characterization and categorization of suitable techniques for of research and industrial process. Although WBG current sensing of power electronics applications will be semiconductors such as Silicon Carbide (SiC) and Gallium addressed through describing challenges in various prototypes. Nitride (GaN) are invented a few decades ago, utilization of These techniques are categorized as resistive-based, filter- these transistors in the power electronics circuits has been based, inductor-based, hall-effect based, and finally magneto- commercially available around 20 years ago. Due to high resistor (MR) based sensors. In section III, the proposed method electron volts, WBG semiconductors provides unique for enhancing the performance of AMR current sensor opportunity for implementation and fabrication of power including all the experimental results under open loop converter converters at much lower space, with higher efficiency and operation will be presented following with section IV, for much more reliability [1]-[9]. However, the process toward conclusion and future work. miniaturization of high frequency power electronics converters suffers from several difficulties such as hardware layout II. CHARACTERIZATION OF POSSIBLE CURRENT SENSING challenges, thermal management, finding alternatives for TECHNIQUES FOR POWER ELECTRONICS CONVERTERS passive components (especially inductor), and finally effective There are various techniques to implement current sensing of control and monitoring scheme for fault diagnostics [10]-[13]. power electronics converters. Utilization of these methods depends on applications and it could be varied based on control The other method is to measure the internal resistance of the strategy, monitoring and over current protection. This paper inductor in the market. This method, inherently has lower cost focuses on possible techniques related to high frequency current because of using the same inductor of the converter and sensing, which can be implemented onto Printed Circuit Board potentially has higher accuracy. However, this method can only (PCB) to reach higher power density and integration. In order measure the current across the inductor path, which basically to evaluate the performance of these techniques the following can be varied from one topology to another. In the other words, factors have been taken into account: simplicity, response time, this technique cannot always capture the current across the accuracy, power consumption, practicality of implementation required section for monitoring. Besides, this method cannot be for high frequency converters, sensitivity with respect to implemented in high power applications and also where the temperature and offset adjustment, and topology dependency. switch current information is required. In general, the most common techniques are categorized as: Measuring electrical resistance of MOSFETs have been resistor-based, filter-based, inductor-based, hall-effect based, used in some articles [23]. This technique assumes that a and Magneto-resistor based. transistor behaves like a resistor when it is the active region. This technique theoretically can be effective because there is no need to add an additional current sensor on the power path that A. Resisor-based current sensing can reduce the cost and the loss of the system. In most of One of the simplest concepts to measure the current is proposed articles, this technique relies on calculating resistor-based current sensing. The objective of this method is () according to (1): to measure the voltage drop across a sense resistor, which can be a simple representation of actual current. The crucial point is / (1) the performance of the measurement resistor had to be precisely = () ( − ) characterized before as the characteristic of the sense resistor significantly affects the voltage drop across that and where, and are length and width of transistor channel, consequently changes the current measurement. These is the capacitance of oxide, is electron mobility and is techniques can be effectively applied for most of low switching frequency and high current or high switching frequency and low the voltage across the switch and shows the threshold current applications. There are multiple methods for voltage of the MOSFET. However, in reality threshold voltage implementing this technique such as an external sense resistor, and the capacitance of oxide can be varied over the time under internal resistance of the filter inductor, measurement of the thermal and electrical aging. Moreover, it has been also shown the internal resistance of transistors in both Si-based and WBG- drain source of transistor resistance during turn-on , and () based technologies will be affected by junction temperature. SENSEFET [4]. In general, since the voltage drop across the Therefore, the accuracy of this method is not guaranteed due to resistor at high current increases proportionally to the current, variable performance of under hard switching, and the ohmic losses are also very high. Even considering the lower () resistance in the main current path cannot really solve the issue, higher temperature. as more conditioning circuit needs to be added. But since the SENSEFET approach has also been implemented in various lower resistance provides lower voltage drop, this removes all applications for MOSFET technologies. In this technique, by the current ripple information, which cannot be practically adding another transistor with scaled down version of /, the retrieved by adding any additional circuits. Therefore, this actual waveform can be captured. The drain and gate of the method in power electronics converters has major issues, measurement transistor would be basically the same as the main especially at high power and high frequency. Figure 1 shows switch. Then, the source of measurement transistor will be the schematic of resistor-based current sensing. connected through Kelvin connection to a resistor, where the voltage drop

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