AN2644 datasheet(35/64 Pages) STMICROELECTRONICS

35 / 64 page

AN2644

The LLC resonant half-bridge converter

35/64

Figure 22.

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

waveforms

t3 → t4. The tank current IR(t3) as Q2 is switched on is already negative and

flowing through the body diode of Q1, DQ1. The voltage of the half-bridge

midpoint, VHB, is abruptly pulled down to ground at t=t3 with a large dv/dt. The

body diode DQ1 is then reverse-recovered and a large current spike flows through

Q2 and DQ1 until the latter recovers completely. Again there is a condition

functionally equivalent 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

negative undershoots. During this phase energy is internally recirculated, the tank

current reaches its minimum value and then increases. The phase ends when

IR=I(Lp) and, then, I(D2)=0 at t=t4.

t4 → t5. This phase mirrors exactly (t1, t2). The voltage across the resonant

capacitor is so large that D1 is forward-biased and, thereby, starts conducting

immediately at t=t4. IR starts increasing, becoming higher than I(Lp) and

eventually crosses zero before Q2 turns off, which marks the end of this phase at

t=t5.

t5 → t6. This is the deadtime during which both Q1 and Q2 are OFF. During this

phase IR is already positive and increasing. Since Q2 is off, it is flowing through

DQ2. Note again that the voltage of the half-bridge midpoint VHB does not

change. This phase ends when Q2 turns on at t=t3 and another switching cycle

begins.

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 ON

Q2 OFF

5 t6

Q1 OFF

Q2 ON

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