By hvhipot 
The existing (non resonant) SMPS are relatively lighter, smaller, and more efficient than previous linear power supplies. The opportunity cost obtained is “higher circuit complexity, more switching losses,” “higher switching stress,” and “EMI (electromagnetic interference). For these “hard switching” types of converters, if we want to design more efficient converters (i.e. lighter, smaller size, and higher power density)
You have no choice but to increase the ‘switching frequency’. Therefore, in this situation, the opportunity cost mentioned above is facing a greater problem.
On the other hand, if using “soft switches (ZVS or ZCS)”, the above problems still exist when using high “switching frequencies”.
It can be minimized. So far, various “softswitch” technologies have been developed, among which the most famous and commonly used ones are the applications of so-called “resonant circuits” and “converter topologies”. The following are some of the most basic resonant circuit concepts among various resonant converters.
The above figure shows the simplest and most commonly used “LC series resonant circuit” among various types of resonant circuits. The driving voltage Vd is distributed between the LC resonant network impedance and the load impedance. By changing the frequency of the driving voltage Vd, the impedance of the resonant network can be altered. The input voltage is distributed between this impedance and the recoil load, and used as a voltage divider.
Due to this principle, the DC gain of the LC series resonant converter is always<1. Therefore, under light loads, compared to resonant networks, the impedance on the load side becomes very large, and most of the input voltage is applied to the load. This means that when the output is light, it makes voltage stability very difficult. In theory, the frequency should be increased to infinity to maintain the no-load voltage.
By inserting a shunt inductor in parallel with the aforementioned LC series resonant circuit, a further improved circuit called LLC resonant circuit can be obtained. To address the issue of “shunt” and improve the “LC series resonant circuit”, due to the innovative combination of inductance and “magnetizing inductance”, when searching for a new circuit – pure circuit, from an analytical perspective, it is the same as the existing “LC series resonant circuit”.
The reason for doing so may be that inserting the parallel inductor may help the circuit work, but at higher input voltages, I believe reducing switching losses is more effective than improving efficiency through conduction losses. However, this strange circuit configuration has been proven to have many advantages, such as stable output voltage for wide input voltage/load variations, wide ZVS range, and relatively simple circuit configuration.
In addition, the parallel inductance actually uses the excitation inductance of the transformer, so in the circuit design diagram, it looks like an LC series resonant type. On the other hand, in the series resonant type, the magnetizing inductance is much larger than the LC series resonant inductance (Lr), but the magnetizing inductance of the LLC resonant circuit is only 3 to 8 times that of Lr, which is composed of the air gap of the transformer.