AN2644 datasheet(29/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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end, also in this case they are reverse-biased only when their current has gone to zero,

thus ensuring ZCS.

Essentially, the reason why D2 does not conduct during (t2, t3) and D1 does not during

(t6, t7) is that the voltage across the transformer (given by Vin - Vc) is not large enough

so that the voltage developed across Lp (given by the inductive divider ratio Lp/(Ls+Lp))

and reflected to the secondary side can forward-bias D2 or D1. In formulae, this is

expressed as:

Equation 12

DCMAB at light load

In this "above-resonance" DCM sub-mode it is possible to identify ten fundamental

subintervals. The relevant waveforms are illustrated in the timing diagram of Figure 19.

Two more new subintervals, namely (t1, t2), (t6, t7) appear just before the deadtimes of the

half-bridge leg transitions (t2, t3), (t7, t8), respectively. The remaining eight phases are

exactly identical to those of the DCMA above-resonance mode.

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

zero before Q1 turns off at t=t2, when this phase ends, and remains zero until t=t2.

During this interval the Equation 12 is met and the current IR = I(Lp) is a portion of

a sinusoid having a frequency f = fR2.

t6 → t7. Q1 is OFF and Q2 is ON. At t=t6 IR equals I(Lp) and then I(D2) becomes

zero before Q2 turns off at t=t7, when this phase ends, and remains zero until t=t7.

During this interval the Equation 12 is met and the current IR = I(Lp) is a portion of

a sinusoid having a frequency f = fR2.

Remarks

The transition between this DCMAB mode and the previous DCMA is marked by the

IR = I(Lp) condition occurring exactly at t = t1 of Figure 18.

In this "above-resonance" DCM submode, in a switching cycle there are two time

intervals (t1, t4), (t6, t9) during which no current flows on the secondary side (hence this

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

resonates and fR2 appears. Considering each half-cycle, nonconductive intervals

appear at the beginning and the end.

As shown in the diagrams of Figure 19, 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(t2) and IR(t6) are large enough to complete the

HB node swing well within the deadtimes (t2, t3) and (t7, t8) respectively. As compared

to DCMA mode, the duration of the energy taking phase (t0, t1) is even shorter, the

external recirculation phase is longer and the displacement angle

ϕ:

Equation 13

is even closer to

π/2. Most of the energy in the tank circuit is reactive.

As to the secondary rectifiers, they 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

in t

() Vc t

()

–

()

--------------------

out

⋅

≤

ϕ 2π

–

-----------------

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