LLC technology has gained widespread popularity and is expected to remain a leading choice in power electronics. The half-bridge LLC resonant circuit can be configured in different ways depending on the connection of the resonant capacitor. Typically, there are two common configurations, as shown in Figure 1. The main difference lies in how the resonant cavity is connected. The left diagram uses a single resonant capacitor (Cr), which results in higher input current ripple and RMS values, but offers simpler wiring and lower cost. On the right, a split resonant capacitor (C1 and C2) is used, which reduces both the input current ripple and RMS value. In this configuration, only half of the RMS current flows through each capacitor, and the total capacitance required is half that of the single capacitor version.
The basic principle of the LLC half-bridge resonant circuit involves two distinct operating regions: zero voltage switching (ZVS) and zero current switching (ZCS). These modes are determined by the resonant frequencies of the circuit. There are two key resonance points: one between Lr and Cr, and another involving Lm, Cr, and the load. As the load increases, the resonant frequency also rises. The formulas for calculating these two resonance points are as follows:
[Image: Resonant frequency equations]
To achieve maximum efficiency, the operating frequency should be set close to fr1, which is the resonant frequency of the Lr-Cr series resonant circuit. When the input voltage decreases, reducing the operating frequency can increase the gain. By carefully selecting the resonant parameters, the LLC converter can operate in ZVS mode regardless of changes in load or input voltage.
In general, the switching behavior of the LLC half-bridge resonant circuit is similar to that of a standard half-bridge circuit. However, due to the addition of the resonant tank, the upper and lower MOSFETs operate differently, allowing them to turn on at zero voltage. The working waveform is illustrated below.
[Image: Ideal LLC half-bridge waveform]
This waveform shows the operation of the circuit over six distinct stages. Here’s a detailed breakdown of each phase:
**T0 ~ T1**: Q1 is off, and Q2 is on. At this point, the current in the resonant inductor is negative, flowing through Q2. During this phase, the transformer leakage inductance does not participate in the resonance, and energy is supplied by Cr and Lr. This phase ends when Q2 turns off.
**T1 ~ T2**: Both Q1 and Q2 are off. This is the dead time of the half-bridge. The resonant inductor current remains negative, discharging Q1's output capacitance (Coss) and charging Q2's. Once Q2’s output voltage matches the input voltage, Q1 will turn on with zero voltage. The transformer secondary side is isolated from the primary during this time.
**T2 ~ T3**: Q1 turns on, and Q2 turns off. The resonant inductor current continues to flow back to the input via Q1’s body diode. D1 is conducting, providing energy to the output. This phase ends when the resonant inductor current reaches zero.
**T3 ~ T4**: The resonant inductor current becomes positive, and the same process repeats as in T2-T3. Lr and Cr continue to resonate, while Lm charges. This phase ends when Q1 turns off.
**T4 ~ T5**: Both Q1 and Q2 are off. The resonant inductor current charges Q1’s Coss and discharges Q2’s until it reaches zero. This creates a condition for Q2 to turn on with zero voltage. The transformer secondary is again isolated.
**T5 ~ T6**: Q1 is off, and Q2 turns on. Since Q2’s Coss was discharged during T4-T5, it turns on at zero voltage. The resonant inductor freewheels through Q2, and D2 supplies the output. This phase ends when the resonant inductor current returns to zero, restarting the cycle.
From the above analysis, it is clear that the circuit operates primarily at a high resonant frequency composed of Lr and Cr, except during the dead time. The transformer’s leakage inductance is clamped by the output voltage, appearing as part of the Lr-Cr resonant tank. It does not fully participate in the resonance. Due to this passive load, the LLC converter may not operate at very high frequencies under light loads. However, this design ensures zero-voltage switching under all load conditions, making it highly efficient and reliable.
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