Understanding Hybrid Inverter Principles 1

Understanding Hybrid Inverter Principles

A complete power generation system consists of an energy storage system paired with a hybrid inverter. To fully grasp how hybrid inverters work, it’s crucial to understand their operational principles. This article series will provide an in-depth look into hybrid inverters, starting with the front-end components.

Front-end of Hybrid Inverter: Lithium Batteries

Let's begin with the voltage specifications of lithium batteries:

• 51.2V Lithium Battery

Example: UZENERGY 5.12KWh energy storage product，
Specification: 51.2V / 100Ah        

• 28.8V Lithium Battery

Example: UZENERGY new product 9.04KWh，
Specification: 28.8V / 314Ah        

Voltage Conversion

Since household appliances use AC (220V or 380V), but lithium batteries provide DC, we need to perform two voltage conversion steps:

1. Step 1: Boosting Voltage
2. Step 2: Inversion

LLC Topology for DC-DC Conversion

To achieve the voltage boosting, we use an LLC topology DC-DC converter. Below is a simplified diagram of the LLC topology and its main characteristics:

Simplified LLC Topology Diagram

Topological Structure

The LLC resonant converter, as one of the important topologies of resonant switching technology, boasts characteristics such as high efficiency, good voltage regulation, and excellent performance across a wide load variation range.

The circuit topology of the LLC resonant converter mainly consists of three parts: the switching circuit (square wave generator), the resonant circuit, and the rectification circuit. Taking the full-bridge LLC converter as an example, the switching circuit is a full-bridge inverter circuit composed of switching devices Q5 to Q8. The resonant circuit includes the resonant inductor Lr, resonant capacitor Cr, and magnetizing inductor Lm, and is connected to the primary side of the transformer. The secondary side of the transformer is a full-wave rectification circuit composed of diodes D1 to D4, which, after being connected to the output capacitor Cin, is then connected to the load.

Key Characteristics of LLC Topology

1. Soft Switching （ZVS）
2. Transformer Turns Ratio
3. H-Bridge Rectifier

Through these processes, we successfully boost the low DC voltage from lithium batteries to DC 400V, setting the stage for the next step of converting it to AC. In the following articles, we will continue to explore the components and detailed principles of inverters, providing a comprehensive understanding of hybrid inverters.

Zero Voltage Switching (ZVS)

1. The most significant feature of the LLC resonant converter is its ability to achieve Zero Voltage Switching (ZVS) across the entire load range.
2. Using ZVS can reduce switching losses.
3. The LLC resonant converter achieves ZVS by utilizing dead time and operating in an inductive mode.

Attached is the schematic diagram.