AN2644 datasheet(20/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

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reverse recovery is not invoked. This topic will be addressed in Section 2.3, where it will be

shown that this property is inherent in the topology, hence it occurs regardless of converter's

design or operating conditions.

2.3

Fundamental operating modes

The LLC resonant half-bridge converter features a considerable number of different

operating modes, which stem from its multiresonant nature. Essentially, the term

"multiresonant" means that the configuration of the resonant tank may change within a

single switching cycle. We have seen that there are two resonant frequencies, one (the

higher) associated to either of the secondary rectifiers conducting, the lower one associated

to both rectifiers non-conducting. Then, depending on the input-to-output voltage ratio, the

output load and the characteristics of the resonant tank circuit, the secondary rectifiers can

be always conducting (with the exception of a single point in time), which is referred to as

CCM (Continuous Conduction Mode) like in PWM converters, or there can be finite time

intervals during which neither of the secondary rectifiers is conducting. This will obviously be

called a DCM (Discontinuous Conduction Mode) operating mode.

Different kinds of CCM and DCM operating modes exist, although not all of them can be

seen in a given converter, some are not even recommended, like those associated with

capacitive mode operation. However, in all CCM modes the parallel inductance Lp is always

shunted by the load resistance reflected back to the primary side, so that it never

participates in resonance, rather it acts as an additional load to the remaining LC resonant

circuit. Similarly, in all DCM modes, there will be some finite time intervals where Lp, being

no longer shunted from the secondary side, becomes part of resonance.

In the following we will consider four fundamental operating modes and use the

nomenclature defined in [3]:

Operation at resonance, when the converter works exactly at f = fR1;

Above-resonance operation, when the converter works at a frequency f > fR1. Moving

away from resonance, we will consider three sub-modes:

CCMA operation at heavy load;

DCMA operation at medium load;

DCMAB operation at light load;

Below-resonance operation, when the converter works at a frequency fR2 < f < fR1 with

a load resistor R > Rcrit. Moving away from resonance, we will consider two sub-modes:

DCMAB operation at medium-light load;

DCMB operation at heavy load;

Below-resonance operation, when the converter works at a frequency fR2 < f < fR1 with

a load resistor R < Rcrit (capacitive mode), corresponding to the CCMB operating mode

defined in [3];

In addition, two extreme operating conditions will be considered:

No-load operation (cutoff)

Output short-circuit operation

It is interesting to point out that, unlike PWM converters where DCM operation is invariably

associated to light load operation and CCM to heavy load operation, in the LLC resonant

converter this combination does not hold.

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