AN2644 datasheet(32/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

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(plus the contribution from LL2, not shown here). This phase ends when Q2 is

switched on at t=t3.

t3 → t4. Q1 is OFF and Q2 is ON. At t=t3 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

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

to drain. D2 keeps on conducting and the voltage across Lp is -a·Vout, so that Lp

is not participating in resonance and Cr is resonating with Ls only. IR is a portion of

a sinusoid having a frequency f = fR1. This phase ends when IR=0 at t=t4.

Figure 21.

Operation below resonance (fR2 < f < fR1, R>Rcrit): main waveforms in

DCMB2 operation

t4 → t5. Q1 is OFF and Q2 is ON. The tank circuit current, which is zero at t=t4

becomes negative. D1 is nonconducting and its reverse voltage is approximately

2·Vout (plus the contribution from LL2, here not shown). Lp's current has a

negative slope, so the voltage across Lp must be negative. Since the diode D2 is

conducting this voltage will be equal to -a·Vout. Lp, then, is not participating in

resonance, Cr is resonating with Ls only and IR is a portion of a sinusoid having a

frequency f = fR1. During this phase, which ends when IR equals I(Lp) and,

thereby, I(D2) is zero at t=t15. IR reaches its minimum value, after that it starts

increasing.

t5 → t6. This phase mirrors (t1, t2). At t=t5 I(D2) becomes zero and IR equals I(Lp),

that is before the conduction time of Q1 ends. Both D1 and D2 are nonconducting

and Lp, no longer shunted by the load reflected to the primary side, goes

effectively in series to Ls and participates to resonance. IR is now a portion of a

sinusoid having a frequency f = fR2. This phase ends when Q2 is switched off at

t=t6.

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 OFF

Q2 ON

Q1 ON

Q2 OFF

Q1 ON

Q2 OFF

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 OFF

Q2 ON

Q1 ON

Q2 OFF

Q1 ON

Q2 OFF

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 OFF

Q2 ON

Q1 ON

Q2 OFF

Q1 ON

Q2 OFF

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