AN2644 datasheet(25/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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progressively turns into DCM operation as the output load is reduced and frequency is

moved away from resonance.

CCMA operation at heavy load

Also in this case it is possible to identify six fundamentals subintervals. The relevant

waveforms are illustrated in the timing diagram of Figure 17. Again, t0 is the instant when,

with Q1 conducting and Q2 open, the tank current IR has a positive-going zero-crossing.

t0 → t1. Q1 is ON and Q2 is OFF. This is the "energy taking" phase. It is identical

to the corresponding phase seen in the operation at resonance with the only

difference that at the end of this phase, at t=t1, it is still IR > I(Lp) and then I(D1)>0.

t1 → t2. This is the deadtime during which both Q1 and Q2 are OFF. At t=t1 IR is

greater than zero and provides the energy to let the node HB swing from Vin to 0,

so that the body diode of Q2, DQ2, is injected. This allows IR to flow; IR slope

changes to a higher negative value, so that it quickly approaches I(Lp), which is

still increasing with the same slope. D1 conducts until IR equals I(Lp), and then

becomes reverse biased, with a negative voltage equal to 2·Vout (plus the

contribution from LL2, here not shown), I(Lp) slope changes sign and D2 starts

conducting. This phase ends when Q2 is switched on at t=t2. Note that the time

Tz needed for IR to hit I(Lp) is related to their values at t=t1 and their slopes after

t=t1 and not to the duration of the deadtime TD. Here it is Tz = TD just by chance.

Figure 17.

Operation above resonance (f > fR1): main waveforms in CCMA operation

at heavy load

t2 → t3. Q1 is OFF and Q2 is ON. At t=t2 IR is diverted from DQ2 to the RDS(on) of

Q2, so that no significant energy is lost during the turn-on transient. This phase,

which ends when IR=0 at t=t3, is identical to the (t2, t3) phase of the operation at

resonance.

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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