Datasheet

Cr Lr

n:1:1

Cr Lr

Vge

Voe

UCC25600

SLUS846B –SEPTEMBER 2008– REVISED JULY 2011

www.ti.com

APPLICATION INFORMATION

Principal of Operation

The soft-switching capability, high efficiency and long holdup time make the LLC resonant converter attractive for

many applications, such as digital TV, ac/dc adapters and computer power supplies. Figure 12 shows the

schematic of the LLC resonant converter.

Figure 12. LLC Resonant Converter

The LLC resonant converter is based on the Series Resonant Converter (SRC). By utilizing the transformer

magnetizing inductor, zero-voltage switching can be achieved over a wide range of input voltage and load. As a

result of multiple resonances, zero-voltage switching can be maintained even when the switching frequency is

higher or lower than resonant frequency. This simplifies the converter design to avoid the zero-current switching

region, which can lead to system damage. The converter achieves the best efficiency when operated close to its

resonant frequency at a nominal input voltage. As the switching frequency is lowered the voltage gain is

significantly increased. This allows the converter to maintain regulation when the input voltage falls low. These

features make the converter ideally suited to operate from the output of a high-voltage boost PFC pre-regulator,

allowing it to hold up through brief periods of ac line-voltage dropout.

Due to the nature of resonant converter, all the voltages and currents on the resonant components are

approximately sinusoidal. The gain characteristic of LLC resonant converter is analyzed based on the First

Harmonic Approximation (FHA), which means all the voltages and currents are treated as sinusoidal shape with

the frequency same as switching frequency.

According to the operation principle of the converter, the LLC resonant converter can be draw as the equivalent

circuit as shown in Figure 13.

Figure 13. LLC Resonant Converter Equivalent Circuit