By hvhipot 
Resonant networks are typically composed of multiple passive inductors or capacitors, due to differences in the number of components and connection methods. The common and practical topology structures of resonant converters can be roughly divided into two categories: load resonant type and switch resonant type. Load resonant converter is an early proposed structure that focuses on improving the power supply voltage conversion ratio characteristics. According to the resonance mode of the resonant element, it can be divided into series resonant (also known as series resonant test device) converters, parallel resonant converters, and series parallel resonant converters produced by the combination of the two.
Series resonance: Due to its series voltage division method, its DC gain is always less than 1, similar to a BUCK converter. In order to stabilize the output voltage under light load, the switching frequency must be increased. Under light load or no-load conditions, the output voltage cannot be adjusted, and the increase in input voltage will cause the operating frequency of the system to become higher and higher than the resonant frequency. As the resonant frequency increases, the impedance of the resonant cavity also increases, which means that more and more energy circulates inside the resonant cavity without being transmitted to the secondary output; However, in load series resonance, the current flowing through the power device decreases as the load becomes lighter, resulting in reduced on state losses.
Parallel resonance: The output terminal can be open circuited but not short circuited, which will damage the resonant capacitor, and excessive primary loop current will have an impact on the switching devices and power supply; When loaded lightly, there is no need to significantly change the frequency to stabilize the output voltage. Compared with series resonance, the converter has a larger operating range and can operate up to no-load; When the load is light, the input current does not change much, and the on state loss of the switch tube is relatively fixed. The efficiency is relatively low under light load, and it is more suitable for working in situations where the load is relatively constant at rated power.
In order to achieve miniaturization, traditional LC series resonant switching power supplies are forced to increase their operating frequency to reduce the size of filtering inductors and switching transformers. However, the increase in frequency leads to increased switching losses, decreased efficiency, and increased switching noise.
LLC series resonant converters mainly use current resonance and voltage resonance only during the switch from ON to OFF and OFF to ON periods. Its switching waveform is a sine wave, so when voltage is applied to the switching element, no large current will flow; Moreover, by utilizing the parasitic capacitance of switching elements to achieve zero voltage switching (ZVS), high-frequency, high-efficiency, and extremely low-noise converters can be made.