Half-bridge LLC resonant DC-DC converter operating principle
The LLC half-bridge resonant DC-DC converter is currently widely used in the fields of electric vehicles and industrial manufacturing. Such converters have advantages such as high efficiency, high stability, and low loss. Therefore, they have become increasingly popular in recent years. The favor of manufacturers and consumer groups. This article will briefly analyze the working principle of the half-bridge LLC converter to help engineers better understand the operating characteristics of this type of converter.
Let's take the most common LLC half-bridge resonant converter as an example. The main circuit of this type of converter is shown in the figure below. From the figure below, we can see that the two main switching transistors Q1 and Q2 form a half-bridge structure and the driving signal is a complementary mode with a 50% duty cycle (usually In order to prevent the upper and lower arms from passing through, there will be a certain dead time in the signal band.) Q1 and Q2 complement each other to generate a square wave voltage Vmid as the input of the resonant tank. In this structure, the resonant capacitor Cr, the resonant inductor Lr, and the transformer magnetizing inductance Lm constitute a resonant network of the LLC, in which the resonant capacitor Cr also functions as a DC blocking capacitor. On the secondary side of the transformer, the diodes D1 and D2 form a full-wave rectifier circuit with a center tap. The output voltage is filtered by the output capacitor Cout for use by the load.
From this structure, we can clearly see that this half-bridge LLC resonant DC-DC converter uses two series inductors and one series capacitor in its structure, where Lr is the resonant inductor and Cr is the resonant capacitor. Lm is the magnetizing inductance. These two series inductances and series capacitors form two resonant frequencies. When the secondary rectifier diode is turned on, the voltage across the secondary side of the transformer clamps the magnetizing inductance Lm, Lm is constant and does not participate in the resonance. The resonant element is the resonant inductor Lr and the resonant capacitor Cr. The resonant frequency is defined as:
When the secondary rectifier diode is in the cut-off state, the transformer in the structure is disconnected from the secondary side. At this time, no energy is transmitted from the primary side to the secondary side, and Lm is no longer clamped by the secondary voltage, and the resonant element is The resonant inductor Lr, the resonant capacitor Cr, and the magnetizing inductor Lm define the resonant frequency as:
In the normal operation process, since the half-bridge LLC resonant DC-DC converter adopts variable frequency control, its energy transmission is adjusted by changing the switching frequency fs. Under normal circumstances, the half-bridge LLC resonant converter has different operating modes. According to the relationship between the switching frequency fs and resonant frequency fr, fm, and the input voltage and load, LLC resonant converter can be divided into fs> Fr, fm