AN2644 datasheet(22/64 Pages) STMICROELECTRONICS

22 / 64 page

The LLC resonant half-bridge converter

AN2644

22/64

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=t3.

t3 → t4. Q1 is OFF and Q2 is ON. Tank circuit current, which is zero at t=t3

becomes negative. D1 is non-conducting 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 Q2 is switched off at t=t4,

IR reaches its minimum value, after that it starts increasing. Note that at t=t4

IR=I(Lp) and then I(D2)=0.

t4 → t5. This is the deadtime during which both Q1 and Q2 are OFF. At t=t4

I(Q2)=-I(Lp)=-IR is greater than zero and provides the energy to let the node HB

swing from 0 to Vin, so that the body diode of Q1, DQ1, is injected. This allows IR

to flow back to the input source. The voltage across Lp reverses to a Vout and its

current slope changes sign. D1 starts conducting while D2 is reverse biased with

a negative voltage approximately equal to 2·Vout (plus the contribution from LL2,

here not shown). This phase ends when Q1 is switched on at t=t5.

Figure 16.

Operation at resonance (f = fR1): main waveforms

t5 → t6. Q1 is ON and Q2 is OFF. At t=t5 IR is diverted from DQ1 to the RDS(on) of

Q1, so that no significant energy is lost during the turn-on transient. Note that now

the operating point of Q1 is in the third quadrant, current is flowing from the source

to drain. This phase, along with the preceding one can be referred to as the

"external energy recirculation phase": current is negative (coming out of the input

terminal) despite that the impressed voltage is positive so that the input energy is

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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