AN2644 datasheet(24/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

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ratio a (n), depends only on the input voltage, not on the load and on the parameters of

the resonant tank. From the design point of view, since Vin and Vout are specified, one

can decide the input voltage where to operate at resonance by choosing the turns ratio

a. Vin/2=a·Vout considered as the 1:1 conversion ratio for the LLC resonant half-bridge.

The logical consequence of LLC converter's ability to operate at resonance

independently of the output load is that the LLC resonant half-bridge can deliver any

power if operated at resonance. This can be seen also in another way. As the

impedance of the series Cr-Ls is zero, the RL load Rac//Lp "sees" the impressed

voltage directly or, in other words, it is supplied by an ideal voltage source. This is a

"singularity" in LLC converter's operation, sometimes referred to as the "load-

independent" point, where the usual energy vs. frequency relationship does not hold.

Actually, the inevitable voltage drops across the resistive elements of the real-world

circuit (such as power MOSFETs' RDS(on), winding resistance, secondary rectifier's

drop, etc.) cause a slight dependence of the frequency on the load. Note that this is a

situation analog to that of CCM-operated PWM converters, where duty cycle is ideally

independent of the load, in reality slightly dependent because of losses.

Energy is taken from the input source only from t0 to t1, the "energy taking" phase, then

for less than half the switching period, while from t1 to t4 energy recirculates internally

to allow energy flow to the load while Q1 is not conducting. This low "duty cycle" of the

input current can be a limiting factor in terms of power handling capability, especially if

the input voltage is low. This consideration suggests the use of the half-bridge topology

in high input voltage applications (e.g. with a PFC front-end, which provides a

400 V input rail). The natural improvement to this limitation is the full-bridge topology,

where phase d) becomes active as well.

Tank circuit current lag

ϕ originates the external energy recirculation phase from t4 to t6

during which energy flow is negative (impressed voltage and input current have

opposite signs). This energy subtracts to that drawn from the input during the energy

taking phase a), hence reducing the net energy flow from the input source to the load

each cycle. This energy can be regarded as reactive energy and cos

ϕ as the input

power factor. Making

ϕ as small as possible (i.e. increasing Lp) would shorten the

duration of the external energy recirculation phase and reduce the amount of reactive

energy, thus improving the energy transfer process. This, however, would also reduce

IR(t1) and IR(t4), hence ϕ can be reduced as long as ZVS is maintained.

Recalling that the current switched at turn-off by Q1, IR(t1) and by Q2, IR(t4) determine

their switching losses, it is straightforward that keeping

ϕ to the minimum value that

ensures ZVS of Q1 and Q2 provides an optimum design. Of course, component

tolerance must be adequately accounted for, thus

ϕ must be larger than the minimum

required and the typical operation will be suboptimal.

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

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 exactly when Q1 and Q2 turn-off, respectively.

These are also the moments when the voltages across D1 and D2 reverse. As a result,

neither D1 nor D2 experience a voltage reversal while conducting a forward current.

Reverse recovery, with all its adverse effects, does not occur. Note that, in this respect,

this is a situation identical to that of a PWM converter operating on the boundary

between CCM and DCM.

2.3.2

Operation above resonance (f > fR1)

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

will consider three submodes where, in a closed-loop regulated system, CCM operation

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