Wide Bandwidth Low Complexity Isolated Current Sensor to be Employed in a 10kW/500kHz Three-Phase Unity Power Factor PWM Rectifier System Gerold LAIMER Johann W. KOLAR +41-1-632-7447 laimer@lem.ee.ethz.ch +41-1-632-2834 kolar@lem.ee.ethz.ch Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory ETH-Zentrum/ETL I16 CH-8092 Zurich SWITZERLAND / Europe Abstract. A current sensor employing a Hall-based field sensing ASIC in combination with a current transformer is proposed. The sensor is characterized by a measuring range of ±50A, an upper bandwidth limit of 20MHz and high dv/dt-immunity up to 10kV/µs. The sensor functional principle and dimensioning are discussed in detail. Parameters determining the sensor bandwidth are clarified, the theoretical considerations are verified by experiments. Finally, measures for further increasing the sensor bandwidth are proposed. 1 INTRODUCTION As shown in [1] by application of SiC diodes in combination with latest power MOSFET technology (CoolMOS) the switching frequency of high power three-phase PWM rectifier systems could be increased to 500kHz at high efficiency resulting in a power density of up to 10kW/dm 3 . However, the high switching frequency and/or the high switching speed do pose special requirements on the bandwidth and dv/dt-immunity of the phase current sensing used for input current control. Commercially available current sensors which are comprising an AC current transformer and a Hall-based DC and/or low-frequency current transducer in closed- loop (compensating) [2] or open-loop [3] operation show a bandwidth of typ. <200kHz, and do exhibit high frequency output signal oscillations (cf. Fig.9 in [4]) and a relatively large dv/dt-sensitivity (cf. Fig.1). Therefore, the sensors cannot be employed in very high switching frequency applications. In this paper therefore an open-loop current sensor with ±50A measuring range is proposed, which is characterized by a high upper bandwidth limit of 20MHz, high dv/dt-immunity, high linearity and low offset drift and does show a low realization effort. In Section 2 the basic functional principle of the sensor is described and measures for minimizing the dv/dt-sensitivity are discussed. In Section 3 the procedure of dimensioning the sensor is treated in detail. The influence of the number of turns of the secondary and of the current transformer stray inductance on the sensor upper bandwidth limit is shown in Section 4. There, furthermore, a sensor upper bandwidth limit of 20MHz is verified experimentally. As the comparison of the sensor output signal to the output of a 50MHz current probe (Tektronix A6302) shows (Section 5) the sensor is ideally suited for measuring the input current of high switching frequency PWM rectifier systems. v 1 2 v L D T 1 2 (a) 0.5ms/Div 2A/Div LEM LA55-P (b) 2A/Div TEK A6302 2A/Div TEK A6302 100V/Div 2A/Div 0.2ms/Div LEM LA55-P (c) Fig.1: Testing of a closed-loop/compensating Hall-based current transducer (LEM LA55-P) in a 500kHz DC/DC boost converter (a) simulating transducer operating conditions as occurring for input phase current measurement in three-phase PWM rectifier systems; (b): LEM LA55-P output signal (upper trace) and actual current time behavior acquired by a 50MHz Tektronix current probe (A6302) for employing the sensor in the connection to the supplying voltage source v 1 (position 1 in (a)); (c): LEM LA55-P and A6302 output signal for current measurement at position 2 in (a); furthermore shown: power transistor drain-to-source voltage. Currents in (b) and (c) are shown with equal scales. Finally, in Section 6 modifications of the current sensor construction for further increasing the upper bandwidth limit are proposed.