AN2644 datasheet(31/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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illustrated in the timing diagram of Figure 21. Again, t0 is the instant when, with Q1

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

Figure 20.

Operation below resonance (fR2 < f < fR1 , R>Rcrit): main waveforms in

DCMAB operation

t0 → t1. Q1 is ON and Q2 is OFF. This is the "energy taking" phase, when current

flows from the input source to the tank circuit, so that energy is positive. The

operating point of Q1 is in the first quadrant (current is flowing from drain to

source). D2 is nonconducting and its reverse voltage is approximately 2·Vout (plus

the contribution from LL2, here not shown). D1 is conducting as well, so the

voltage across Lp is a Vout. Lp, then, is not participating in resonance and Cr is

resonating with Ls only. IR is a portion of a sinusoid having a frequency f = fR1.

During this phase, which ends when IR equals I(Lp) and, then, I(D1)=0 at t=t1, IR

reaches its maximum value, after that it starts decaying.

t1 → t2. Q1 is ON and Q2 is OFF. At t=t1 I(D1) becomes zero and IR equals I(Lp),

that is before the conduction time of Q2 ends. Both D1 and D2 are nonconducting

and Lp, no longer shunted by the load reflected to the primary side, goes

effectively in series to Ls and participates to resonance. IR is a portion of a

sinusoid having a frequency f = fR2. Depending on the tank circuit's parameters

and on the operating conditions, this portion can be similar to a straight line, as

shown in the diagrams of Figure 21. This phase ends when Q1 is switched off at

t=t2.

t2 → t3. This is the deadtime during which both Q1 and Q2 are OFF. At t=t2

I(Q1)=I(Lp)=IR is greater than zero and provides the energy to let the node HB

swing from Vin to ground, so that the body diode of Q2, DQ2, is injected. This

allows IR to flow. The voltage across Lp reverses to -a·Vout. D2 starts conducting

while D1 is reverse biased with a negative voltage approximately equal to 2·Vout

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

AN2644 Datasheet(PDF) 31 Page - STMicroelectronics

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