PDF

Resonant LLC Converter: Operation and Design

Application Note AN 2012-09

V1.0 September 2012

1. At resonant frequency operation, fs=fr.

2. Above resonant frequency operation fs>fr.

3. Below Resonant frequency operation, fs<fr.

Figure 2.4

Despite the aforementioned three modes; which will be explained in details later in this section; the converter

has only two possible operations within the switching cycle, as described below. And each of the modes

pointed out above may contain one or both of these operations.

1) Power delivery operation, which occurs twice in a switching cycle; first, when the resonant tank is

excited with a positive voltage, so the current resonates in the positive direction in the first half of the

switching cycle, the equivalent circuit of this mode is shown in Figure 2.5, and second occurance is

when the resonant tank is excited with negative voltage, so the current resonates in the negative

direction in the second half of the switching cycle, the equivalent circuit of this mode is shown in

Figure 2.6.

During the power delivery operations, the magnetizing inductor voltage is the positive/negative

reflected output voltage and the magnetizing current is charging/discharging respectively.

The difference between the resonant current and the magnetizing current passes through the

transformer and rectifier to the secondary side, and power is delivered to the load.

Figure

2) Freewheeling operation, which can occurs following the power delivery operation only if the resonant

current reaches the transformer magnetizing current, this only happens when fs<fr, causing the

transformer secondary current to reach zero and the secondary side rectifier to disconnect,

consequently the magnetizing inductor will be free to enter the resonance with the resonant inductor

and capacitor, the frequency of this second resonance is smaller than the original resonant

frequency fr, especially at high values of m where Lm>>Lr, thus the primary current during the

freewheeling operation will only change slightly, and can be approximated to be unchanged for

simplicty. The equivalent circuits of the freewheeling operation in both halves of the switching cycle

are shown in Figure 2.7 and Figure 2.8.

0.1 1 10

K .2 m, Fx,( )

K 10 m, Fx,( )

At resonance

Below resonance

Above

resonance

Cr Lr