AN2644 datasheet(28/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

28/64

DCMA at medium load

In this "above-resonance" DCM submode it is possible to identify eight fundamentals

subintervals. The relevant waveforms are illustrated in the timing diagram of Figure 18.

Two new subintervals, namely (t2, t3) and (t6, t7), appear just after the deadtimes of the half-

bridge leg transitions (t1, t2), (t5, t6), respectively. The other six phases are exactly identical

to those of the CCM above-resonance mode.

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. Note that the

operating point of Q2 is in the third quadrant, current is flowing from the source to

drain. D1 is nonconducting but the voltage across the secondary windings is still

too low to let D2 conduct, then still IR = I(Lp) is a portion of a sinusoid having a

frequency f = fR2. This phase ends when D2 starts conducting at t=t3.

t6 → t7. Q1 is ON and Q2 is OFF. At t=t6 IR is diverted from DQ1 to the RDS(on) of

Q1, so that no significant energy is lost during the turn-on transient. Note that the

operating point of Q1 is in the third quadrant, current is flowing from the source to

drain. D2 is nonconducting but the voltage across the secondary windings is still

too low to let D1 conduct, then still IR = I(Lp) is a portion of a sinusoid having a

frequency f = fR2. This phase ends when D1 starts conducting at t=t7.

Remarks

In this "above-resonance" DCM submode the multiresonant nature of the LLC

converter shows up. In a switching cycle there are two time intervals just after bridge-

leg transitions during which no current is flowing on the secondary side (hence this is

DCM operation), then the entire transformer's primary inductance Ls+Lp resonates and

the second resonance frequency fR2 appears. At the transitions of the half-bridge leg

the resonant current IR is still slightly greater than I(Lp) in absolute value, so it takes a

very small portion of the deadtimes for the two currents to equal each other. Then, the

first one starts as IR equals I(Lp) slightly after t1 and ends at t= t3. The second one

starts slightly after t5 and ends at t= t7.

As shown in the diagrams of Figure 18, the tank circuit current is still lagging the

impressed voltage, so that they have the same sign at half-bridge leg transitions.

Furthermore, the switched currents IR(t1) and IR(t5) are large enough to complete the

HB node swing well within the deadtimes (t1, t2) and (t5, t6) respectively. However, as

compared to CCM mode, the duration of the energy taking phase (t0, t1) is shorter, the

external recirculation phase is longer and the displacement angle

ϕ:

Equation 11

gets close to

π/2. Using ac terminology, the active energy is lower and the reactive

energy is higher.

The secondary rectifiers D1 and D2 start conducting during the conduction period of

Q1 and Q2, respectively. Both the initial current and also its di/dt are zero, thus they

have a soft turn-on. D1 and D2 cease to conduct when tank circuit's current IR equals

Lp's current I(Lp). In this case this is almost synchronous with either switch turn-off.

Again, only when IR equals I(Lp) and no current is flowing through the secondary

rectifier previously conducting can the voltage across the primary winding of the

transformer reverse and hence also the voltage across the secondary rectifiers. In the

ϕ 2π

–

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