Control Design for Photovoltaic Power Optimizers Using Bootstrap Circuit

The non-inverting buck-boost converter has emerged as one of the most promising topologies for distributed maximum power point tracking applications. Pressure to reduce cost and volume of photovoltaic module integrated converters often compels bootstrap capacitor type gate drivers. The bootstrap technique is a simple and economical solution to create a floating power supply for high-side gate drivers. However, it comes with a major challenge, as the low-side switch must be periodically turned on, to recharge the high-side bootstrap capacitor. In the buck and the boost mode, one of the switching legs of the converter must be held with the high-side switch on, which eventually discharges the high-side capacitor. When the non-inverting buck-boost input voltage is regulated by means of a single voltage loop, recharging the bootstrap capacitor causes an undesired ripple in the PV module voltage, degrading the regulation quality. In order to mitigate this problem, this paper proposes a cascaded control technique, entailing regulation of the PV module voltage and inductor current. The cascaded controller dramatically improves the rejection of the disturbance induced by the periodic recharge of the bootstrap capacitor, thanks to the fast action of the current loop, regulating the average value of the inductor current. Converter transfer functions and controller design are derived and validated. Experimental results clearly show that the cascaded control scheme outperforms the traditional voltage control scheme, resulting in a better regulation of the PV module voltage. This main advantage comes at the cost of an additional measurement of the average inductor current.

I. Introduction

As global installed PV capacity continues to grow at an exponential rate [1], power electronic concepts and converters used in PV plants are evolving. In distributed maximum power point (DMPPT) applications, a maximum power point tracking (MPPT) converter is embedded with every PV module [2], [3]. While increasing the energy yield, by mitigating the negative effects of partial shading and module mismatching [4], such architecture provides opportunities for enhanced data gathering, diagnostic and safety. At the start of 2017, in [5], it was estimated that more than 70% of U.S. residential installations would use module integrated converters by the end of that same year. On one hand, dc-ac module integrated converters (ac-MICs or microinverters) can be used to interface each PV module directly to the ac grid, however due to the large voltage step-up ratio required, they demand high-voltage rated components, together with double stage conversion or galvanic isolation, challenging low costs and high efficiencies [6]. On the other hand, dc-dc module integrated converters (dc-MICs or power optimizers), require a much lower voltage conversion ratio, since a PV inverter is still used for interfacing to the ac grid. dc-MICs are implemented with low voltage rated components and they do not need galvanic isolation, hence representing an economically attractive and highly efficient solution [7]. It is estimated that power optimizers increase the PV system annual performance up to 6%, recovering on average 36% of the power lost from partial shading [8]. A system deploying dc-MICs is shown in Fig. 1. In this configuration, the dc-MICs are preferred to have both buck and boost capability [6]. The non-inverting buck-boost is a well suitable dc-MIC topology [4], [9], due to its versatility, basic structure [10], [11] and high efficiency [4].