AN2644 datasheet(21/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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To illustrate the above-mentioned operating modes we will refer to the reference converter

shown in Figure 15. The discussion will start from the inspection of the main waveforms in a

switching cycle, highlighting each subinterval where the circuit assumes a topological state

and deducing the properties of the converter when operated in that mode from those

waveforms. Half-bridge leg transitions are considered instantaneous. Their features have

been already discussed.

Figure 15.

Reference LLC converter for the analysis of the fundamental operating

modes

2.3.1

Operation at resonance (f = fR1)

In this operating mode it is possible to distinguish six fundamental time subintervals within a

switching cycle, which are illustrated in Figure 16.

The first subinterval and, then, the instant t0 can be chosen quite arbitrarily. We fix t0 as 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, when current

flows from the input source to the tank circuit, so that energy is positive and both

refills the resonant tank and supplies the load. The operating point of Q1 is in the

first quadrant (current is flowing from drain to source). D2 is reverse-biased with a

voltage -2·Vout (it is actually larger because of the contribution from the secondary

leakage inductance LL2). D1 is conducting, so Lp is shorted by the output load

reflected back to the primary side and the voltage across it is fixed at 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 Q1 is switched off at t=t1, IR reaches its maximum value, after that it starts

decaying. Note that at t=t1 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

I(Q1)=I(Lp)=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. The voltage across Lp reverses to -a·Vout and the slope of its current

changes sign. D2 starts conducting while D1 is reverse biased with a negative

voltage approximately equal to 2·Vout (plus the contribution from LL2, here not

shown). This phase ends when Q2 is switched on at t=t2.

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 now

Driver

CTRL

Isolated

feedback

22 nF

200 uH

500 uH

Vout

Vin

Coss1

100 pF

Coss2

100 pF

8.33:1:1

24 Vdc

300W

I(D1)

I(D2)

I(Q1)

I(Q2)

I(Lp)

360 to 420

Vdc

Cout

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