Active power decoupling (APD) is an emerging technique for second-harmonic ripple filtering in single-phase power conversion systems. It reduces the dc bus capacitance requirement while adding extra active components to the converter. Though capacitance reduction is favourable for improving converter power density and reliability, added active components often make the circuit complex apart from increasing the power losses. Therefore, APD with minimum active component count and reduced loss have gained significant research interest. This article discusses the synthesis of minimum switch count APD topologies that use low-power rated devices. Low-power APD circuits help to reduce the filter loss, making their efficiency comparable to that of the passive filters. The topology derivations are elaborated, and the significant attributes of the derived filters are compared with the state-of-the-art active and hybrid filters to comprehend their relative merits and limitations. The operations of the synthesized unique topologies are experimentally validated using laboratory hardware.

Second-harmonic filter is a critical constituent of single-phase power conversion system that helps to reduce the dc bus capacitance requirement. As the converter performance largely depends on the filter efficacy, an improved filter design target, characterised by superior ripple attenuation, compact size, low power loss, and less failure rate, continued to trigger significant research interest. Given the design and performance trade-offs between conventional passive filters and cutting-edge active filters, hybrid filters have emerged as an attractive solution capable of combining the desired features. Though it is easy for new designs to adapt an active filter with tunable characteristics, more often than not, it is impracticable for an existing system due to constraints on accessibility and space. However, tuning, retrofitting or replacement of the existing passive filters can be necessary in the interest of superior filtering and converter performance. Therefore, an easily integrable active circuit, named solid state tuning restorer (SSTR), is proposed here to enhance the performance of an existing LC filter. The second-harmonic tuned passive LC filter, along with SSTR, forms a hybrid filter with adjustable characteristics capable of maintaining the tuned state over a range of operating frequencies and LC parameters. The volt-ampere (VA) rating of SSTR, being only 2% of the main converter, facilitates easy integration into an existing dc bus LC filter. Even in case of SSTR failure, the configuration offers a graceful degradation in the second-harmonic filter characteristics without disrupting the main converter. Thus, it improves the performance of an existing second-harmonic LC filter while not compromising its reliability. The operation, design constraints, and control methodology of the proposed SSTR are discussed, and the performance is validated through experiments on a hardware prototype.

Flexible characteristics, tunable parameters, and improved power density are the desired attributes that have led to widespread research on active capacitors and inductors (ACI) in power conversion systems. However, the two-terminal realisation of ACI is challenging due to different trade-offs between power density, device stress, and reliability of the added components. Moreover, the requirement of handling second-harmonic ripple makes bulk electrolytic capacitors nearly indispensable in two-terminal ACIs. Thus, the scope of power density improvement becomes constrained. Hence, active capacitors requiring minimum or no bulk dc-link capacitance are seen as an attractive technology in recent literatures. However, they can mimic either inductive or capacitive behaviour with a single converter unit, restricting their adaptability across different applications. This limitation is addressed in this work with the proposed unified active capacitor and inductor (UACI), realised without any bulk dc capacitor. It emulates variable capacitance and inductance while offering a smooth transition from one characteristic to another. High effective impedance due to a small susceptance at transition duty prevents undesirable current overshoot during the transition. A laboratory-scale prototype of the proposed ACI is fabricated, and the hardware is verified up to +/-450 VAR.

Active second-harmonic (2ω) filters have become an integral technology for single-phase power converters in high power density designs. The series-stacked buffer (SSB) has emerged as an attractive topology among the existing solutions due to its low power, high efficiency, and compact design. However, one of the challenges in adopting SSB lies in its control, where conventionally hysteresis current control has been adopted. This results in a wide variation in the switching frequency, making the digital control implementation and filter design complex. On the other hand, fixed-frequency pulse-width modulation (PWM) control necessitates developing a model for the systematic controller design to ensure stability and desired filtering performance. The assumption of SSB modelled as a second-harmonic current source, independent of the dc bus circuit components, fails to capture the dc bus loading effect on the SSB. In this work, a dynamic model of SSB is eveloped where the main dc bus and its passive elements are taken into account. The derived model gives a systematic procedure for the controller design while ensuring the desired 2ω filtering. The stability limits and parameter sensitivities of the model are studied analytically and in simulations. The analytical model is validated, and the proposed controller performance and stability limit are verified experimentally on a hardware prototype.