AN2644 datasheet(36/64 Pages) STMICROELECTRONICS

36 / 64 page

The LLC resonant half-bridge converter

AN2644

36/64

Remarks

In the below-resonance capacitive mode operation, since current is continuously

flowing on the secondary side, this is a CCM mode (as already stated, it corresponds to

the CCMB mode defined in [3].

The physical reason of this CCMB operation is, as previously pointed out, the large

voltage developed across the resonant capacitor. A large energy level must be

circulating in the tank circuit and this is consistent with R<Rcrit condition in order for the

capacitive mode to take place. Capacitive operating mode is then likely to occur under

heavy load, overload or short circuit conditions and appropriate countermeasures must

be taken to handle this.

Note that in all the inductive modes the absolute value of the switched currents (i.e. the

current IR when either power MOSFET is switched off) is always greater than (in CCM

mode above resonance) or equal to (in all DCM modes) the current I(Lp). In the

capacitive mode, instead, the absolute value of the switched currents is lower than that

in Lp. This consideration is useful when analyzing the transitions of the node HB.

Unlike the other modes, in capacitive mode the secondary rectifiers D1 and D2 start

conducting during the conduction period of Q2 and Q1, respectively. The initial current

is zero and its di/dt is low, thus they have a soft turn-on. Also in this case the voltage

across the secondary rectifiers reverses as a result of their currents going to zero.

Then, ZCS is maintained even under these conditions and it is possible to conclude

that the secondary rectifiers are soft-switched at both turn-on and turn-off under all

operating conditions and that this property is inherent in the topology. In fact no

assumption has been made about the tank circuit's parameters.

Figure 23.

No-load operation (cutoff): main waveforms

I(D2) = D2 current

V(D2) = D2 anode voltage

I(Q2) = Q2 current

I(Lp) = Lp (magnetizing) current

Vc = Resonant capacitor voltage

LVG = Q2 gate

I(D1) = D1 current

V(D1) = D1 anode voltage

I(Q1) = Q1 current

IR = Tank circuit’s current

VHB = Node HB voltage

HVG= Q1 gate

Q1 OFF

Q2 ON

Q1 ON

Q2 OFF

Q1 ON

Q2 OFF

I(D2) = D2 current

V(D2) = D2 anode voltage

I(Q2) = Q2 current

I(Lp) = Lp (magnetizing) current

Vc = Resonant capacitor voltage

LVG = Q2 gate

I(D1) = D1 current

V(D1) = D1 anode voltage

I(Q1) = Q1 current

IR = Tank circuit’s current

VHB = Node HB voltage

HVG= Q1 gate

I(D2) = D2 current

V(D2) = D2 anode voltage

I(Q2) = Q2 current

I(Lp) = Lp (magnetizing) current

Vc = Resonant capacitor voltage

LVG = Q2 gate

I(D1) = D1 current

V(D1) = D1 anode voltage

I(Q1) = Q1 current

IR = Tank circuit’s current

VHB = Node HB voltage

HVG= Q1 gate

Q1 OFF

Q2 ON

Q1 ON

Q2 OFF

Q1 ON

Q2 OFF

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