The LLC half-bridge resonant circuit is a widely used topology in power electronics, especially in high-efficiency DC-DC converters. It typically features two configurations based on the coupling mode of the resonant capacitor, as illustrated in Figure 1 below. The primary difference lies in how the resonant cavity is connected. In the left configuration, a single resonant capacitor (Cr) is used. This design results in higher input current ripple and RMS values but offers a simpler wiring layout and lower cost. On the right side, the circuit uses a split resonant capacitor (C1 and C2), which reduces both the input current ripple and the RMS current. In this case, only half of the RMS current flows through each capacitor, and the total capacitance required is half that of the single capacitor solution.
The basic principle of the LLC half-bridge resonant circuit revolves around its ability to operate in zero-voltage switching (ZVS) and zero-current switching (ZCS) regions. This converter has two distinct resonant frequencies: one determined by the series resonance of Lr and Cr, and the other influenced by the magnetizing inductance (Lm), the resonant capacitor (Cr), and the load conditions. As the load increases, the second resonant frequency also rises. The formulas for calculating these two resonant points are shown in Figure 2 below.
To achieve maximum efficiency, the operating frequency should be set close to fr1, which corresponds to the resonant frequency of the Lr-Cr series resonant circuit. When the input voltage decreases, lowering the operating frequency can increase the gain. By carefully selecting the resonant parameters, the LLC converter can maintain ZVS operation regardless of variations in load or input voltage.
In general, the switching behavior of the LLC half-bridge circuit is similar to that of a standard half-bridge, but the addition of the resonant cavity leads to differences in the operation of the upper and lower MOSFETs. This allows for ZVS turn-on, improving efficiency and reducing switching losses. The working waveform of an ideal LLC half-bridge resonant circuit is shown in Figure 3 below.
The waveform is divided into six distinct stages, each representing a different operating condition of the circuit. These stages illustrate the dynamic interaction between the resonant inductor (Lr), the magnetizing inductance (Lm), the resonant capacitor (Cr), and the MOSFETs. Understanding these stages is crucial for analyzing the performance and behavior of the LLC resonant converter under various operating conditions.
During T0–T1, Q1 is turned off, and Q2 is turned on. The resonant inductor current is negative, flowing through Q2. At this stage, the transformer leakage inductance does not participate in the resonance, and the energy comes from the Lr-Cr resonant tank. This phase ends when Q2 turns off.
In T1–T2, both Q1 and Q2 are off, creating a dead time. The resonant inductor current continues to be negative, discharging Q1’s output capacitance (Coss) and charging Q2’s Coss until the voltage across Q2’s output capacitor matches the input voltage, enabling ZVS for Q1 in the next cycle. During this time, Lm, Lr, and Cr resonate together.
From T2–T3, Q1 turns on while Q2 remains off. The resonant inductor current is still negative, flowing back to the input through Q1’s body diode. D1 conducts, providing power to the output. This phase ends when the resonant inductor current reaches zero.
In T3–T4, the resonant inductor current transitions from negative to positive, and the same switching pattern repeats. Lr and Cr continue to resonate, while Lm is charged by the current. D1 remains active, delivering power to the output.
During T4–T5, both MOSFETs are off again, creating another dead time. The resonant inductor current charges Q1’s Coss and discharges Q2’s Coss, preparing for ZVS turn-on of Q2. The secondary side of the transformer is isolated from the primary, and Lm participates in the resonance.
Finally, in T5–T6, Q2 turns on at zero voltage, as its Coss has been discharged. The energy is freewheeled through Q2, and D2 provides power to the output. At this point, Lm no longer participates in the resonance, and the cycle repeats.
Overall, the LLC half-bridge resonant circuit operates efficiently by leveraging the resonant properties of Lr and Cr. The transformer leakage inductance acts as a passive load, allowing the circuit to maintain ZVS operation across a wide range of loads and input voltages. This makes the LLC topology particularly suitable for applications requiring high efficiency and low electromagnetic interference (EMI).
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