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 across the resistor is representative of main current.

Gate 1 W/L=n W/L=1 Main Switch Sense Switch Vsense IL Vin Vout Q1 Q2 Gate Gate 2 Vsense

Fig. 2. General schematic of SENSEFET. This technique should be implemented in an integrated package for higher performances. Fig. 1: Schematic of external sense resistor for capturing the inductor and output current. The voltage drop across the resistor theoretically This technique could be effective at low frequency and chip can represent the actual current waveform; however, this method in design, it is not isolated and it has some technical challenges practice has a lot of issues for high frequency and high current such as difficulty of calibration, impedance matching, and applications. bandwidth limitation at discrete design. The latter comes from the fact, the measurement transistor typically should have much This method initially has been introduced by [3], but due to lower /, which requires both transistors have the same some challenges like temperature dependency of resistance in fabrication technology. At the end, all the features of resistor- inductor path, inherent tolerance of passive components, and based sensing approaches have been compared in Table-I. requirement for the active calibration were not fully developed. In [22], proposed a new technique for accurate filtering, TABLE-I: RESISTOR-BASED SENSING COMPARISONS actively calibrate the measurement, and consequently compensate the dynamic changes of passive components. Resistor-based External Inductor RDS(on) SENSEFET However, this approach like resistive-based current sensing Resistor Resistance sensing method, potentially has big challenges at high current and high High Current Poor Poor Good Good frequency due to lack of isolation, variability of passive Measurement components over the time, complexity for implementing in Accuracy High High Low Moderate discrete design.

Bandwidth High Low High Moderate C. Inductor-based current sensing DC current No Yes No No Saturation The other approach is induction-based methodology which Yes No Yes Yes DC Offset introduced in various types to induce the voltage or current in Variation the secondary circuit as the measurement. In wide area, these Temperature Medium Medium High Medium approaches can be also implemented with various technique, Sensitivity but they can be categorized as three main groups: AC current Hysteresis No No No No transformers, DC current transformers, and Rogowski-based Saturation current sensing techniques. The AC current transformer is not a new concept. This Contactless No No No No Operation method basically induced the current in the secondary winding and potentially has high performance for current measurement Power High Low Low Low and can be applied in many power electronics applications. Consumption Although previously these transformers have been built in High No No Yes No bulky, nowadays some companies like Coilcraft released high Frequency performance current transformer sensor (CST7030 and Possibility CST4835) in small packages. They are able to measure the Integration Yes No Yes Yes current at 20A, 48V and 1MHz. However, due to the Possibility importance of capturing the DC current of converters, they are Complexity Low Low Medium High not very attractive for power electronics modules. In DC current transformer, they typically have at least two cores with a rectifier, then the voltage drop across a resistor at B. Filter-based current sensing the measurement section can provide current information. Due to the utilization of coil inductors, they can suffer from the The other approach for current measurement is a filter- saturation at high switching frequency, and their typical based technique, where the new RC filter in parallel has been applications in the market are not going beyond 100kHz. added to the RL network of the output filter. By knowing the Rogowski-based current sensing, due to its inherent total impedance of the inductor, equivalent series resistor of simplicity, and no bandwidth limitation is among the most inductor, additional capacitance and resistor will be added popular techniques in high frequency power converters. The accordingly such that the voltage drop across the filter stage can Rogowski-based current sensor typically consists of an air-core represent the current waveform. inductor with a low number of turns, where it can be induced

by main current of the conductor. Then the picked up current Sensing Active Calibration & Compensation information can be transformed to the voltage through an Circuit integrator circuit. They might be equipped with a reset signal to Gate 1 indicate the start and end point of integration [7], [11]. The captured voltage can be a good representative of actual current V in Vout waveform. In literatures, this technique has been implemented successfully in many different ways, including high switching Gate 2 frequency WBG-based converters. There is also a commercially available product (Si850X) that can measure the current at 20A and 1MHz switching frequency. However, in practice this technique has some sensitivities, which technically Fig. 3. General schematic of Filter-based current sensing make the layout of high switching frequency and high current very difficult to design. sensor area, implementing on wider area, or also bringing a new Conductor V shielding on control section of the sensor. One of the most sense common recommendation is to keep the pick-up current coil far Air Core from any magnetic component, which gives more immunity for Inductor R2 pick-up coil. However, these solutions also sacrifice the power density and brings more complexity for the measurement. In practice measuring the inductor current and transistors R are very critical for high frequency converters. The information 1 can be utilized for implementing the advanced controlling Fig. 4. General schematic of Rogowski current sensor methods, or diagnostics of power switches. In Rogowski-based method, inducing the current in the inductor creates a mutual In order to demonstrate the impact of performance inductance in power loop, which potentially is a hazard for high placement and their sensitivities, full-bridge DC/AC converter frequency converters. This basically comes from the fact that has been prototyped. By neglecting the functionality of the any unwanted magnetic fields near the pick-up coil should be sensor, the performance of the overall converter has been avoided to reduce the chance of susceptibility of this method to verified at 500kHz, 15A and 30V output under unipolar Pulse noise. Therefore, this method needs accurate knowledge, Width Modulation (PWM) switching. As it shown in figure 5, careful understanding and proper design of Electromagnetic 6, the performance of the sensor at top and bottom switches are Interfaces (EMI) on converter power stage. Figure-7 shows the very different. performance improvement of Rowgowski-based current sensor

under high frequency power converter with new layout considerations.

Fig. 5. Performance of Rogowski-based current sensor under 500kHz converter at the full bridge converter. Yellow: drain-source voltage, Green: Inductor current ripple captured by high bandwidth amplifier Fig. 7. Improvement of the performance of Rogowski-based current current gun. Pink: Top switch current captured by sensor SiliconLab- sensor with new layout consideration at 100kHz and half bridge Si8503. configuration. Yellow: Load voltage, Green: Top switch current captured by sensor SiliconLab-Si8503. Pink: PWM gate signals D. Hall effect-based current sensing One of the most popular methods for measuring both AC and DC current is using Hall-effect sensors, which works according to Lorentz force. Due to generation of magnetic fields in passing of current through a conductor, a voltage can be induced across a Hall element and represents current waveform as shown in figure 8. Hall-effect sensors due to their low power consumption, easier implementation on high switching frequency converters, immunity respect to noise and compact designs are very attractive for both academia and industry. In general, there are two types of Hall-effect sensor under open-loop and closed loop operation. Both have great capability to capture high current (~30A), and relatively high Fig. 6. Performance of Rogowski-based current sensor under the same converter at the full bridge converter. Yellow: Drain-source voltage, bandwidth (~1MHz). But their sensitivity to temperature drift Green: Inductor current ripple captured by high bandwidth amplifier and offsets will be changed accordingly. Therefore, this needs current gun. Pink: Bottom switch current captured by sensor to be considered as careful design of Hall cell geometry. These SiliconLab-Si8503. sensors both at academia and industry have been studied at There are multiple ways to improve the performance of the various current, and their characteristics have been sensor such as decoupling power and control stage around the demonstrated vastly [12], [21]. significantly improved due to lower layout constraints. However, by comparing the experimental results of figure-9, 10 which depicts the improvement steps for high frequency converters with Hall-effect sensors show careful design constraints also need to be considered. This would include intelligent decoupling and isolation of power stage and control PCB stage to reduce reflection of hall-effect sensor by radiated noise and EMIs. It is worthwhile to mention that DC current measurement could be potentially a big issue due to saturation Copper Trace at high current level.

E. Magneto resistor-based current sensing Fig. 8. General operation of contact-based hall effect sensor while current passing through the hall element. Black arrow: current vector, Anisotropic Magneto-Resistors are based on metal alloys as Red arrow: voltage vector, Blue: Magnetic field vector opposed to MRs which are based on low bandgap semiconductors such as InSb/InAs. The most widely used AMR At the moment, there are only two high bandwidth hall-effect devices, which developed and integrated into a chip is current sensors available in the market, which are operating at composed of four Permalloy (Ni0.81F0.19) AMRs in a full 1MHz (Allegro-ACS730 and AKM-CQ3303). The performance of AKM current sensor has been characterized under 500kHz sensitivity Wheatstone bridge configuration [24]-[26]. MR DC-AC converter at 500kHz, 15A and 30V output under based current sensors work on the principal of detecting the unipolar PWM switching. magnetic fields generated by current travelling through a trace on the Printed Circuit Board (PCB). Generally, the MR based current sensor is placed on top or underneath a trace carrying the current without any conductive contact with the current trace. The low frequency current through the PCB trace generates a uniformly distributed magnetic field, which passes through the sensor along the default axis and thus the sensor responds by sensing the magnetic field [27].

Copper Trace

PCB Fig. 9. Performance of Hall-effect current sensor under 500kHz unipolar switching converter at full bridge converter. Yellow: Drain- source voltage on slow top switch, Green: Drain-source voltage on slow bottom switch, Pink: Inductor current ripple captured by AKM- CQ3303.

Copper Trace

PCB

Fig. 11. General operation of the contactless AMR sensor: magnetic field generations at low frequency (top) and high frequency (bottom) are shown.

Fig. 10. Improvement of performance of Hall-effect based current For high frequency current, especially above 1 MHz, the sensor under 500kHz unipolar switching converter at full bridge configuration. Pink: Drain-source voltage on top switch, Blue: Drain- generated magnetic field is concentrated mostly on the edges of source voltage on bottom switch, Green: Inductor current ripple the trace due to skin effect that results in a non-uniform captured by sensor AKM-CQ3303. magnetic field distribution around the PCB trace. Consequently, the detected magnetic field by the sensor is very As it shown in figure-10, the performance of hall-effect weak at higher frequencies. For high frequency applications, sensor compared to Rogowski-based current sensor has been due to skin effect, the magnetic field distribution is non-uniform and affects the sensitivity of the AMR based current sensors. Therefore, placing the sensor on the opposite side of the PCB with respect to the current trace is not effective for accurate sensing above 1MHz. For better detection bandwidth and sensitivity, the magnetic field passing through the default axis of the sensor needs to be concentrated to make the field more uniform. Therefore, alternative and innovative magnetic concentration techniques need to be implemented to make the field normalized and more uniform. One of the techniques that can be utilized effectively to enhance the sensing bandwidth of the MR based sensors is the ‘folded trace’ method, which addresses the challenges related to high frequency current detection by modifying the layout of the current trace. The proposed folded trace method also provides shielding for the sensor from external EMI generated in the high frequency power (b) converter circuits [27]. At the end, the comparison of common Fig. 15. Experimental results: Operation of synchronous GaN buck possible current sensing techniques for high frequency is converter at 3A, 30V and 1 MHz with folded trace technique. Significant improvement of the current sensing in proposed method is summarized in Table-II. observed. Green: Gate signals, Blue: Actual current captured amplifier current gun Yellow: AMR sensor measurement.

TABLE-II: CURRENT SENSING TECHNIQUES COMPARISONS Sensing Transformer Rogowski Hall AMR Technique High Current Good Good Good Very Measurement Good Accuracy High High High High Bandwidth Low High High Very High DC current No No Yes No Saturation DC Offset No No Yes No Variation

Fig. 13. Folded trace technique to normalize and intensify the fields Temperature Low Low High Medium near the sensor Sensitivity Hysteresis Yes No Yes No Saturation Contactless No No No Yes Operation Power Moderate Low Low Low Consumption High Low High High Very High Frequency Possibility Integration No Yes Yes Yes Possibility Complexity Low High High High

Fig. 14. Experimental results: Operation of synchronous GaN buck converter at 3A, 30V and 1 MHz with normal bare trace. Green: Gate III. CONCLUSION signals, Blue: Actual current captured amplifier current gun Yellow: AMR sensor measurement. This work presented to show the significant effect of current measurement at high frequency and high current power electronics converters. This paper focused on the possible solutions for implementing sensing techniques on power converters and explicitly addresses their characteristics and features. More specifically, this paper focused on prototyping [14] H. Jafarian, I. Mazhari, B. Parkhideh, S. Trivedi, D. Somayajula, R. Cox, DC to AC converters using Rogowski-based, Hall-effect based, and S. Bhowmik, “Design and implementation of distributed control architecture of an AC-stacked PV inverter,” pp. 1130–1135, 2015. and AMR current sensing. The difficulties and challenges for [15] F. B. Costa; A. Monti; F. V. Lopes; K. M. Silva; P. 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