Photovoltaic Inverter Inductive Components and Their Technical Trends

World-wide energy shortages and rising oil prices have become one of the important constraints on the sustainable development of the global economy. In the wave of vigorous development of renewable energy, solar power generation technology is receiving more and more attention due to its superior environmental protection, large capacity, and scale. In photovoltaic power generation systems, in order to maximize the efficiency of injecting power from solar cells into the power grid, the conversion efficiency of PV inverters is one of the most important technical issues in the industry in recent years.

With the breakthrough of technologies such as power semiconductor devices and large-scale digital control, photovoltaic power inductors, which are energy storage filter elements, are becoming more and more important bottlenecks affecting system conversion efficiency. In order to improve system efficiency and reduce inductance loss, copper and expensive high-performance magnetic materials have to be consumed in large quantities, becoming one of the most expensive components in the entire inverter.

Aiming at the above problems, this article will analyze the performance requirements of the inductive components used together and the advantages and disadvantages of the widely used inductive components according to the basic circuit architecture and working principle of various PV inverters , combined with the latest existing magnetic materials As a result, methods and approaches to fundamentally solve this problem are proposed by using inductor design methods such as hybrid magnetic circuit and magnetic integration technology.

First, the classification of photovoltaic inverters

Grid-connected PV inverters have different circuit architectures and working mechanisms due to their different power levels. According to the power level of photovoltaic inverters, they can be roughly divided into four categories: micro-inverters, residential photovoltaic inverters, commercial medium power inverters, and centralized power station inverters.

Micro inverters and their core magnetic components

Due to the influence of factors such as the installation position, dark cloud conditions, and shadow coverage of surrounding leaves, the power generated by each module will vary in different degrees. If they are all connected in series and parallel, an adverse effect will appear like the combination of new and old batteries. When 2-3% of the general battery area is covered by shadows, the total power generation often drops by as much as 20%, which seriously affects the power generation efficiency of the entire system. For this reason, the micro-inverter specializes in independent grid-connected power generation of a single battery module, which can avoid this problem to the greatest extent. This solution has been widely favored once it came out. But a household power often requires a dozen or more such independent inverter units, so whether the inverter can achieve high efficiency and low cost has become an important constraint affecting the widespread application of the system. 

The micro inverters shown in Figs. 1 and 2 are two typical grid-connected power generation topologies. In Fig.1, two staggered critical working mode boost flyback transformers are used first. The duty cycle is based on the sine wave half-wave law, and the power of the single-stage circuit is sinusoidal, isolated, boosted and MPPT (Maximum Power Point). Tracker) control filtering, and then full-bridge half-wave power frequency commutation filtering, which effectively realizes the direct grid-connected power generation of low-voltage DC. This is one of the most promising micro inverters at present. Fig. 2 is a conventional method of full-bridge isolation boosting, filtering, and then full-bridge inverter filtering and grid connection. The obvious disadvantages of this method are that it requires more magnetic components, and there are too many high-frequency switching devices, which lacks advantages in terms of cost and efficiency. In order to simplify the circuit, a hard-switch driver is used without exception to isolate the leakage of the main transformer The inductance is very small. Generally, a flat transformer structure with a multilayer circuit board has to be used, which makes its parasitic capacitance large, the cost is high, and the EMI is more difficult to handle.

For the mainstream CRM Interleave topology, there are two core magnetic components, a flyback power transformer and an AC filter inductor ACL. Flyback transformer FBT (Flyback Transformer), because it works in the critical mode of hundreds of kHz, so the design of this type of transformer must follow the following principles:

1) Ferrite core with high Bs, high frequency and low Pcv loss;

2) In order to reduce the loss of the transformer, it is necessary to use a large and effective cross-section and low magnetic path length to control the magnetic loss. Commonly used magnetic cores include thin or customized products with optimized shapes such as PQ and RM;

3) Maximum control of transformer leakage inductance, using well-coupled winding structure;

4) The internal resistance of the winding is as small as possible. At the same time, the copper wire eddy current loss caused by skin effect and air gap magnetic leakage must be paid attention to.

For the ACL of the micro-inverter post-filtering, because its ripple current is relatively small, high flux or high-frequency characteristic NPF ring-shaped iron-silicon materials are generally used.