AN2644 datasheet(34/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

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They cease to conduct before the transitions of the half-bridge. The operation is DCM,

then ZCS occurs by definition.

Essentially, the reason why D2 does not conduct during (t1, t2) and D1 does not during

(t5, t6) is that during these subintervals the condition expressed by Equation 12 is met.

The waveforms in DCMB1 are very similar, only the portion of sinusoid at f = fR2 decays

to zero more quickly. While DCMB2 is characterized by the duration of the intervals (t1,

t2) and (t5, t6) that gets longer as frequency is reduced, in DCMB1 the duration of these

intervals starts decreasing quite rapidly to reach zero at the boundary with capacitive

mode operation. The "border belt" adjacent to the capacitive region is included in the

region where DCMB1 occurs, thus it can be a good design practice to limit the

operation of the converter to the DCMB1-DCMB2 boundary.

2.3.4

Capacitive-mode operation below resonance (fR2 < f < fR1, R<Rcrit)

In this operating mode, unique to below-resonance operation and corresponding to the

CCMB operating mode defined in [3], it is possible to distinguish six fundamental time

subintervals within a switching cycle, as illustrated in the timing diagrams of Figure 22.

In this case it is convenient to define t0 as the instant when Q1 is switched on.

t0 → t1. The tank current IR(t0) as Q1 is switched on is already positive: this

means that in the just finished deadtime it was flowing through the body diode of

Q2, DQ2. Actually, this is confirmed by the voltage VHB of the half-bridge midpoint

which was previously zero and is abruptly pulled up to Vin exactly at t=t0 with a

large dv/dt. The body diode DQ2 is then reverse-recovered and a large current

spike flows through Q1 and DQ2 until the latter recovers completely. Functionally

this is similar to a cross-conduction of the half-bridge leg. Very high di/dt may

occur and, as a result, large voltage spikes are generated across the parasitic

inductances experiencing this large di/dt. The voltage VHB may have large

positive overshoots. During this phase when energy is taken from the input source,

the tank current reaches its maximum value and then decays. The phase ends

when IR=I(Lp) and, then, I(D1)=0 at t=t1.

t1 → t2. Unlike what happens in the below-resonance DCMB2 submode, where

there was insufficient voltage across the resonant capacitor to forward-bias D2,

here Vc is much larger and D2 starts conducting immediately at t=t1. IR starts

decreasing, becoming lower than I(Lp) and eventually crosses zero before Q1

turns off, which marks the end of the this phase at t=t2.

t2 → t3. This is the deadtime during which both Q1 and Q2 are OFF. During this

phase IR is already negative and further decreasing. Since Q1 is off, it is flowing

through DQ1. Note that the voltage of the half-bridge midpoint VHB does not

change. This phase ends when Q2 turns on at t=t3.

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