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PSUs 101: A Detailed Look Into Power Supplies

LLC Resonant Converter

We already mentioned the LLC resonant converter, but we haven't said what this circuit actually does in order to offer increased efficiency. So let's explain how it works. For starters, there are three popular resonant topologies: series resonant converter (SRC), parallel resonant converter (PRC) and series parallel resonant converter (SPRC). The LLC resonant converters are members of the SRC family.

An LLC resonant converter consists of two inductors (one parallel to the load) and one capacitor. To understand how it works, you have to know what the resonant frequency of a circuit is. Briefly, resonant frequency is the frequency at which the inductive reactance of the circuit becomes equal to the capacitive reactance. The impedance of both the inductor and the cap cancel each other out, and as such, energy dissipation on them is minimal.

In an LLC resonant converter, there are two resonant frequencies (Fr1 and Fr2), since the capacitor can resonate with the two inductors that cannot resonate with each other. The switching frequency (Fsw) of the main switchers can vary between Fr1 and Fr2 (LLC region), with increased frequencies at light loads and decreased frequencies at higher loads, or can be higher than Fr1 (SRC region). In every case, all attempts are made to achieve the best conditions for zero-voltage switching (ZVS) of the main switches to effectively minimize energy loss.

Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.