A Clamping-Circuit-Based Voltage Measurement System for High-Frequency Flying Capacitor Multilevel Inverters

Abstract

In an era where high-frequency flying capacitor (FC) multilevel inverters (MLI) are increasingly gaining attention in energy conversion systems that push the boundaries of power density, the need for a compact, fast, and accurate FC voltage monitoring is also increasing. In this paper we designed and developed a new FC measurement system, based on precise sampling of the inverter switching node voltage, through a bidirectional clamping circuit. The deviation of FC voltages from their nominal values are extracted by solving a set of linear equations. With a single sensor per phase and no isolation requirements, as opposed to dozens of sensors in traditional FC monitoring, our approach results in significantly lower cost, complexity, and circuit-size. Detailed device-level simulations in LTspice and system-scale simulations in Matlab, validate the accuracy and speed of the proposed measurement system and the balancing strategy in steady state, abrupt load change and imbalance conditions. Experiments carried out in a 3-phase Gallium-Nitride 5-level inverter prototype, reveal a gain in precision and bandwidth that is more than 30 times that of conventional methods, at a fraction of their cost and footprint. The recorded performance renders the developed sensor an ideal solution for fast MLIs based on wide-bandgap technology.

A Generalized Phase-Shift PWM Extension for Improved Natural and Active Balancing of Flying Capacitor Multilevel Inverters

“…Such a PWM that generates the "right" amount of independent ZSS is also a requirement for active balancing that employs ac-side monitoring. For example, in [32] we show that the FC voltages can be extracted by clamping (i.e., saturating) and sampling the inverter voltage solely during the ZSS. This method increases the resolution and accordingly broadens the measurement bandwidth to more than 200 kHz, allowing its application in high frequency GaN-based inverters.…”

“…Interestingly, this is the same condition for optimal natural balancing, that is to produce a number of independent ZSS equal to the FC count. Although this has been established for N = 5 in [32], it remains an open challenge for an arbitrary number of levels N.…”

“…An additional benefit of this approach is that the produced ZSS and their sequence in time are generated analytically without the need for complex or time-consuming simulations. This feature, along with the straightforward derivation of the solution to the system of equations for the FC voltages allows the direct application of the precise measurement system of [32] for active balancing. It is worth noting that in case this ac-side monitoring system is to implemented, the inverter switching frequency should be bounded by the limits of ( 11), according to [32], where ω is the angular fundamental frequency and TADC is the sample and hold time of the analogue-to-digital converter (ADC) module that captures the inverter switching node.…”

“…Further to the natural balancing simulations we investigated the performance of the ac-side monitoring system with an analytical calculation of the ΔVC signals [32]. To record a sharp FC voltage variation, the inverter was subjected to a step dclink change from vDC = 200 V to vDC = 300 V while the modulation index was fixed at ma = 0.8.…”

“…A viable alternative for accurate FC measurements with minimum component count adopts the concept of ac-side monitoring, which involves sampling the inverter output voltage at specific switching states and extracting the FC voltages from these measurements [32]- [35]. Particularly, we have demonstrated in [32] increased measurement bandwidth and accuracy by combining the MPSPWM of [22] with a bidirectional clamping circuit, making the approach ideal for high-frequency GaN MLIs. Yet, this measurement method requires the production of all ZSS from the PWM, which is feasible for up to 5-level inverters with MPSPWM.…”