Contents I. Introduction
The miniaturization of power electronics has increasingly become a popular research topic at the forefront of advancing power electronic technology, offering significant improvement in achievable converter power density and reliability in modern-day electric power systems. One method of miniaturizing power electronic converters is to replace passive energy storage components with actively controlled power electronic converter solutions that perform the same overall energy storage function. This is realized by operating the power converter replacement with transformed line voltage and current, achieved by operating an equivalent circuit network with a pulsewidth-modulated (PWM) switching frequency scheme exceeding the line frequency by several orders of magnitude at minimum. Owing to both the transformation in low-frequency line voltage and current processed by the energy storage device and high-frequency (HF) PWM switching operation, the energy storage capacitance or inductance may then be considerably reduced. There, however, exists an inherent tradeoff in increased system losses present in both the power electronic switches and equivalent-series resistance (ESR) of the converter capacitive and inductive elements. Therefore, it is crucial to explore and develop alternative energy storage options that effectively address the limitations posed by aluminum electrolytic capacitors (AECs). Gu et al. [1] have proposed a way to minimize the capacitance requirements by using an intermediate dc link where they boost the dc voltage to reduce the capacitance. However, this method of capacitance reduction increases the loss to a great extent especially in the low-power systems. The emergence of the active power decoupler (APD) concept approximately a decade ago presented a novel solution to address the aforementioned challenges [2], [3], [4]. The APD is a power electronic converter topology which aims to replace bulky dc link passive capacitor banks typically comprising electrolytic capacitors (ECs), a capacitor technology that is not preferred due to its short life span and low reliability, with an active power electronic circuit solution [5]. These dc link capacitor banks are commonly employed as energy storage devices in single-phase dc to ac power inverters to sufficiently compensate for inherent mismatch between the constant dc input power and fluctuating ac output power of the converter during single-phase dc-ac conversion. This power compensation enables the ability to maintain double-line frequency voltage ripple arising from such power mismatch at the input dc link within an acceptable limit for satisfactory converter lifespan, reliability, and operation [6], [7], [8], [9], [10], [11] albeit with an increased energy storage system cost. Without an active power electronic solution to alter the voltage and current stress imposed on the capacitor, it is infeasible to directly replace ECs with more desirable capacitor technologies such as ceramic and film, which boast significant improvements in the reliability concerns of ECs. This is because directly replacing ECs with such capacitor technologies without an interfacing converter results in the capacitor bank volume increasing significantly to meet the required energy storage capacitance. Existing works have extensively analyzed several APD topologies such as buck, boost, half-bridge, and stacked capacitor in Fig. 1 which is practically integrable in an inverter system with reasonable complexity.
Commonly studied dc link APD topologies. (a) Buck, (b) boost, (c) half bridge, and (d) stacked capacitor.
