AN2644 datasheet(37/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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2.4

No-load operation

The ability to operate under no-load conditions is another peculiar characteristic of the LLC

resonant half-bridge converter. The typical waveforms in this operating mode, called also

"cutoff" mode, which can occur at frequencies both above, below and at the resonant

frequency fR1, as demonstrated in [3], are illustrated in the timing diagrams of Figure 23.

Note that it is IR=I(Lp), then I(D1)=I(D2)=0, throughout the entire switching cycle. IR is made

by portions of sinusoid at f=fR2, but looks very much like a triangular waveform. In order for

this operation to occur, the voltage developed across Lp and reflected to the secondary side

has to be lower than the output voltage throughout the entire switching cycle, so that either

secondary rectifier cannot be forward-biased. In other words, Equation 12 has to be met for

∈(t0, t6). The ability of the converter to operate with no-load can be easily deducted by

Equation 12 itself. However big the peak value of Vin(t)-Vc(t) is and provided a·Vout is not

zero, it is possible to find a value of Lp that meets condition (

α). In the end, no-load

operation is not an intrinsic property of the LLC resonant converter (like ZCS for the

secondary rectifiers) but it can be achieved with an appropriate design of the tank circuit.

The difference with respect to the LC series resonant converter is apparent. If Lp

→ ∞ the

LLC converter turns into the LC one but Vc

→ 0 because there is no current through the

resonant tank and the only possible equilibrium condition is Vin = a·Vout. If the input and

output voltage have a different ratio, output voltage regulation will be impossible.

Seen from a different standpoint, under no load conditions and at a frequency considerably

higher than fR1, the resonant capacitor Cr "disappears" (Vc(t) ≈ Vin/2) and the output voltage

is given by the inductive divider made up by Ls and Lp as shown by the equivalent circuit of

Figure 24. Then, if the voltage conversion ratio is greater than the inductive divider ratio,

regulation will be possible at some finite frequency, otherwise it will not be. From Equation

12, substituting Vin(t) = Vin and, then, Vc(t) = Vin/2, it is possible to find a necessary condition

in order for the converter to be able to regulate at zero load:

Equation 15

Lp, then, plays a key role. It not only makes zero load operation possible but allows soft-

switching under these conditions too, as already discussed. The price to pay for that is the

considerable tank current IR=I(Lp) circulating in the circuit and illustrated in Figure 23. This

circulation is not lossless. Power is dissipated in power MOSFET, the resonant capacitor

and the transformer.

This prevents the LLC resonant converter from achieving extremely low input power levels at

no load, unless appropriate countermeasures are taken. The most effective way to reduce

no-load consumption to a very low level is to let the converter operate intermittently ("burst-

mode" or "pulse-skipping" operation). In this way, the average value of the tank current can

be reduced at an almost negligible value. Furthermore, the average switching frequency will

be considerably lowered thus minimizing the residual turn-off switching losses.

2.5

Overload and short circuit operation

The equivalent schematic of the converter under short circuit conditions is illustrated in

Figure 25. The inductance Lp is actually shunted by an extremely low impedance, so that

the LLC circuit reduces to a series LC circuit. The transformer will work as a "current

transformer", so that the output current will be a times the primary (input) current Iin. It is

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out

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≤

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