AN2644 datasheet(27/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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loop operation) the voltage conversion ratio diminishes. For a given Vout (closed-loop

operation) operating frequency needs to get back closer to resonance.

The secondary rectifiers D1 and D2 start conducting as Q1 and Q2 turn-on

respectively. The initial current is zero and also its di/dt is low, thus they have a soft

turn-on. D1 and D2 cease to conduct when tank circuit's current IR equals Lp's current

I(Lp). The transformer's secondary current is a·[IR-I(Lp)], then the equality IR = I(Lp)

means that the secondary rectifiers current is zero as well. Unlike when operating at

resonance, this does not happen synchronously with the half-bridge leg transitions

(IR < I(Lp) at t=t1 and IR > I(Lp) at t=t4). However as the leg transition occurs IR and

I(Lp) are "forced" to become equal, then the current of the conducting diode goes to

zero. The physical reason for that is the presence of Ls. When there is a transition of

the half-bridge leg the resulting voltage change is not immediately directly impressed

on the transformer (C can be considered as a short circuit during transitions, i.e. the

voltage across it can be considered constant) but falls across Ls that acts as a "shock

absorber". This leaves I(Lp) unchanged but pushes IR towards I(Lp), forcing a rate of

change that is approximately:

Equation 10

Only when IR equals I(Lp) and no current is flowing through the secondary rectifier

previously conducting can the voltage across the primary winding of the transformer

reverse and hence also the voltage across the secondary rectifiers. In the end, also in

this case they are reverse-biased only when their current has gone to zero, thus

ensuring ZCS.

Figure 18.

Operation above resonance (f > fR1): main waveforms in DCMA operation

at medium load

--------

a V

out

⋅

---------------------------------

–

out

⋅

-----------------------------------------------

⎩⎭

⎪⎪

⎨⎬

⎪⎪

⎧⎫

≈

∈(t1, t2)

∈(t4, t5)

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

Q1 OFF

Q2 ON

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 ON

Q2 OFF

Q1 OFF

Q2 ON

Q1 ON

Q2 OFF

AN2644 Datasheet(PDF) 27 Page - STMicroelectronics

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