What is LLC in power supply?

LLC resonant converters have become a hot topic in power electronics because they can meet the demanding performance requirements set by modern power supply designs. The LLC is one of a significantly larger family of resonant converter topologies, all of which are based on resonant tanks. Resonant tanks are circuits made up of inductors and capacitors that oscillate at a specific frequency, called the resonant frequency.

Because they allow for higher switching frequencies (fSW) and reduce switching losses, these switch-mode DC/DC power converters are often used in high-power, high-efficiency applications. LLC resonant converters are ideal for power supply applications with delicate systems (i.e. high-end consumer electronics), or power-demanding operations (i.e. charging electric vehicles).

An LLC converter is made up of 4 blocks: the power switches, resonant tank, transformer, and diode rectifier (see Figure 1). First, the MOSFET power switches convert the input DC voltage into a high-frequency square wave. This square wave then enters the resonant tank, which eliminates the square wave’s harmonics and outputs a sine wave of the fundamental frequency. The sine wave is transferred to the secondary of the converter through a high-frequency transformer, which scales the voltage up or down, according to the application. Lastly, the diode rectifier converts the sine wave into a stable DC output.

The LLC converter’s ability to maintain high efficiency even at very high power comes from its resonant nature. The resonant nature of LLC converters enable soft switching in both the primary and secondary sides, increasing efficiency by reducing switching losses.

In addition, an LLC topology saves board space. LLC topologies do not have an output inductor, which means that all inductors can be more easily integrated into a single magnetic structure to save area and cost. When all of a circuit’s inductive elements are placed in the same structure, electromagnetic compatibility is improved, since it is easier and cheaper to shield one structure than three.