AN2644 datasheet(30/64 Pages) STMICROELECTRONICS

30 / 64 page

The LLC resonant half-bridge converter

AN2644

30/64

soft turn-on. They cease to conduct before the transitions of the half-bridge. The

operation is DCM, then ZCS occurs by definition.

Figure 19.

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

operation at light load

2.3.3

Operation below resonance (fR2 < f < fR1, R>Rcrit)

In this operating mode the converter still exhibits its usual frequency vs. load characteristic.

We will consider two submodes where, in a closed-loop regulated system, DCM operation

gets deeper and deeper as the output load is increased and frequency is moved away from

resonance.

DCMAB at medium-light load

This "below-resonance" DCM submode is exactly equal to the above-resonance DCMAB

submode previously considered. The waveforms are shown in Figure 20 and it is possible to

see that they match those shown in Figure 19.

Note only that in this case the condition IR = I(Lp) at t=t1 occurs at a considerably higher

power level than in the case of the above-resonance DCMAB submode (2:1 in the example

shown).

DCMB at heavy load

This "below-resonance" DCM submode, which is unique to below resonance operation,

actually comprises two submodes (DCMB2 & DCMB1) as frequency goes away from

resonance. Their waveforms do not differ much, only those of DCMB2 will be shown. It is

possible to identify eight fundamentals subintervals and the relevant waveforms are

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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