LLC Resonant Converters Raise the Power-Efficiency Bar (.PDF Download)

Switch-mode power supplies (SMPS) are known for their superior efficiency. And while they achieve a significant level of efficiency, the quest to improve it even further continues to march on. One development helping to attain that goal is LLC resonant converters. These unique SMPS use resonant circuit techniques to reduce switching losses.

However, to receive any gains from resonant techniques requires a sophisticated controller. Now such controllers are available, and they feature additional power savings that further boost efficiency.

What the Devil is an LLC Resonant Converter?

Switching transistors are vulnerable to significant losses, which occur during the turn-on and turn-off intervals. When the transistor is off, no power is dissipated. When the transistor is on, its low on-resistance keeps power dissipation to a minimum. During switching times, though, the transistors pass through their linear region where their resistance is higher, which means dissipation of power. Luckily the transition periods are short. Reducing the switching time substantially cuts down the power consumption. 

The switching time is determined by the transistor’s specifications, but also by other circuit characteristics. Also, keep in mind that sharp pulse edge transitions produce transients that generate noise and electromagnetic interference (EMI). Therefore, a key goal in design is to reduce the switching time by using a higher switching frequency. Though EMI still occurs at faster switching rates, there’s a beneficial reduction in power consumption. Using LLC resonant techniques can bring about that desired reduction in power consumption.

LLC, of course, refers to the use of two inductances (L) and a capacitor (C). This combination establishes resonance at the switching frequency. As a result, the switching transistors see a sine wave and are enabled to switch at the zero-crossing points or near zero. This has the effect of reducing switching losses in the transistors.

The benefits of going this route are that it allows for higher switching frequencies, which, in turn, reduce the size of transformers and filters (and related components) as well as minimize switching-transistor heat dissipation and the need for large heat sinks. All of these benefits are achieved while increasing the circuit’s overall efficiency.