Unlike conventional pulse width modulation (PWM) power converters, resonant converters regulate the output voltage by frequency modulation. Therefore, the design method of the resonant converter is also different from the design method of the PWM converter. Among the various types of resonant converters, the LLC series resonant converter (LLC-SRC) of Figure 1 is particularly attractive because of its enhanced output regulation, smaller cycle current, and lower circuit cost.
The series resonance feature allows the switching network in the DC (DC)/DC LLC-SRC (shown in Figure 2) to have a wide range of zero voltage switching (ZVS); therefore, the LLC-SRC can easily be implemented in front-end power applications. 94% efficiency and can operate at high switching frequencies.
Similar to the design process of the PWM converter, when designing the LLC-SRC, the first step is to select the operating frequency required at full load. The rest of the steps are different because there is no duty cycle factor in the resonant converter. The duty cycle remains the same in the LLC-SRC, which is 50%, which is ideal. Figure 3 shows the flow chart for the design of the LLC-SRC (from the TI Power Design Workshop topic "Designing an LLC Resonant Half-Bridge Power Converter").
The gain curves in the Mg/Qe and Mg/fn plots are derived from the LLC resonant tank shown in Figure 1, which is also the linearization circuit for the LLC resonant half-bridge converter.
Figure 3 provides a simple circuit parameter selection process for an LLC resonant half-bridge converter. By examining the fn_min and fn_max positions on the gain curve, you can design a high efficiency LLC resonant half-bridge converter with ZVS on the switching network under all input conditions.
When designing an LLC resonant half-bridge converter, keep in mind:
At any time, fn_min in the Mg/fn chart requires a ridge above the gain curve. This is to ensure that the metal oxide semiconductor field effect transistor (MOSFET) can maintain the ZVS state. The efficiency of LLC-SRC can only be optimized at one operating point. When fsw = fo, the series Lr and Cr become zero impedance states (Figure 4); the converter has the highest efficiency at that point. You need to decide which line/load conditions you want to optimize and make sure your switching frequency is the resonant frequency under those conditions.
Plasma and LCD TVs have now entered thousands of households. The switching power supply design of these two appliances is special, only active or passive PFC mode can be used, and it needs to be able to work in a non-heating and ventilation environment for a long time. This requires the power supply not only to have high power density and smooth electromagnetic interference signals, but also to use components as little as possible. In these respects, the half-bridge LLC resonant converter has many advantages.
Half-bridge LL resonant capacitor and resonant inductor configuration
Both single resonant capacitors and split resonant capacitors are present in the half bridge converter. For a single resonant capacitor configuration, its input current ripple and root mean square (RMS) values ​​are high and the rms current through the resonant capacitor is large. This solution requires a high voltage (600~1,500V) resonant capacitor. However, this solution also has the advantages of small size and simple wiring.
The split resonant capacitor has a smaller input current ripple and rms value than a single resonant capacitor. The resonant capacitor handles only half of the rms current, and the capacitance of the capacitor used is only half that of a single resonant capacitor. When using clamp diodes (D3 and D4) for simple, inexpensive overload protection, the resonant capacitor can operate at a lower rated voltage of 450V. As the name suggests, the half-bridge LLC converter contains two inductors (excitation inductance Lm and series resonant inductor Ls). Depending on the location of the resonant inductor, the resonant tank also includes two different configurations, one for the discrete solution and the other for the integrated solution. Both solutions have their own advantages and disadvantages, and the way LLC works with these two solutions is slightly different.
It is purposeful to mount the resonant inductor outside the transformer. It helps designers increase design flexibility, giving designers the flexibility to set Ls and Lm values; in addition, EMI radiation is lower. However, the disadvantage of this solution is that the insulation between the primary and secondary windings of the transformer becomes complicated, and the cooling conditions of the windings deteriorate, and more components need to be assembled.
In another integrated solution, the leakage inductance of the transformer is used as the resonant inductor (LLK = LS). This solution requires only one magnetic component and will make the switching power supply smaller in size. In addition, the transformer windings have better cooling conditions and insulation between the primary and secondary windings is facilitated. However, this solution is relatively less flexible (the range of available LS inductors is limited) and its EMI radiation is stronger, while there is a strong proximity between the primary and secondary windings.
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