High-Bandwidth Isolated Voltage Measurements With Very High Common Mode Rejection Ratio for WBG Power Converters

Abstract

Galvanically isolated voltage measurements are becoming increasingly important for the characterization of converter systems with fast switching Wide-Bandgap (WBG) semiconductors. A very high Common Mode Rejection Ratio (CMRR) > 80 dB for frequencies up to several tens of MHz is required to accurately measure, e.g., the high-side gate-source or drainsource voltage in a half-bridge, or voltages on floating potentials as, e.g., found in multi-level converters. Common to all listed measurement scenarios is the fast changing reference potential, which acts as Common Mode (CM) disturbance. This article derives the minimum necessary CMRR at different frequencies to constrain the time-domain measurement error below a certain limit. Thereby, only the switched voltage and the voltage transition rate (dv/dt) of the CM disturbance are to be considered and not the actual converter switching frequency f sw . Afterwards, a galvanically isolated measurement system with a CMRR > 100 dB up to 100 MHz and an analog measurement bandwidth of 130 MHz is presented. Critical design aspects to achieve this performance are investigated. Compared to commercially available isolated voltage probes, the presented measurement system does not require any additional equipment like an oscilloscope to perform and visualize measurements, since the data is already digitized/sampled and thus can be transmitted directly to a host device (e.g., computer or monitoring system) with corresponding Graphical User Interface (GUI) software. Experimental verification in frequency-and timedomain confirms that the performance is on par with the best commercially available isolated voltage probes.

Mitigating EMI Noise in Propagation Paths: Review of Parasitic and Coupling Effects in Power Electronic Packages, Filters, and Systems

“…In the gate drive circuit, the parasitic capacitances caused by the DC-DC power isolation stage such as the semiconductor-based isolation barrier and the monolithic transformer provide extra CM paths which may cause gate signal interference and worsen the WBG device's switching behavior, affecting the power electronics system's EMI performance from the noise source [34]. Including the coaxial shunt resistor in the circuit for current measurement increases the length of the power loop with more parasitic inductance, while the parasitic capacitances induced by other sensing techniques and voltage probes affect the precision of the measurement as well as introduce extra CM paths and generate more CM current as the probes are connected to the grounded oscilloscope [35], [36]. In WBG systems, the short cables act as transmission lines due to the high switching speed, which results in reflected wave phenomenon (RWP) and causes overvoltage on a connected motor, stressing the motor winding insulation and aggravating EMI issues [37]- [39].…”

Parasitic Capacitive Couplings in Medium Voltage Power Electronic Systems: An Overview

“…The CM capacitance (commonly denoted as input capacitance) is most often given as a rated value in the datasheet, which is typically modelled in parallel with a large resistor. When the voltage probe is used to measure at a high dv/dt node, the input capacitance of the probe can introduce high-frequency capacitive CM currents, leading to high requirements in terms CM rejection ratio to avoid the capacitive coupling affecting the desired measurement [240]. In [241], a number of commercially available MV probes have been tested and the CM capacitance of the probe has been found to have a significant impact on the accuracy and reliability of the measurements.…”

Remote Low-Cost Differential Isolated Probe for Voltage Measurements

“…In the classification of analogue isolated high-voltage probes, two types can be distinguished according to the location of the isolation: those using a non-intrusive sensor/measuring coupling system [1,2] and those using an intrusive one [3,4], that connect a circuit to the system and then use a coupling method, either optical, capacitive, inductive, or through the power supply, to achieve the isolation. In [1], the authors present a nonintrusive probe for the non-contact measurement of voltages of 10 kV and a bandwidth of 4.5 MHz, based on the electric field coupling principle using the dual-pin-type probe method.…”

“…On the other hand, the high-bandwidth isolated voltage probe presented in [4] is intended for the measurement of source gate voltages of floating transistors in bridge rectifier and inverter configurations. Its input voltage range is ±75 V and it has a bandwidth of 130 MHz.…”