AN2644 datasheet(11/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

11/64

considerably lower than that in a flyback converter, as shown in Table 1, this is one of the

few real drawbacks of the topology.

Additional details concerning power losses will be discussed in Analysis of power losses on

page 41.

To complete the general picture on the LLC resonant converter, there is another aspect that

needs to be addressed concerning parasitic components which affect the behavior of the

circuit.

The first parasitic element to consider is the capacitance of the midpoint of the half-bridge

structure, the node common to the source of the high-side power MOSFET and to the drain

of the low-side power MOSFET. Its effect is that the transitions of the half-bridge midpoint

will require some energy and take a finite time to complete. This is linked to the previously

mentioned deadtime inserted between the turn-off of either switch and the turn-on of the

complementary one, and will be discussed in more detail in Section 2.2.

The second parasitic element to consider is the distributed capacitance of transformer's

windings. This capacitance, which exists for both the primary and the secondary windings,

in combination with windings' inductance, originates what is commonly designated as the

transformer "self-resonance". In addition to this capacitance one needs to consider also the

junction capacitance of the secondary rectifiers, which adds up to that of the secondary

windings and lowers the resulting self-resonance frequency (loaded self-resonance).

The effect of all this parasitic capacitance can be modeled with a single capacitor CP

connected in parallel to LM as illustrated in Figure 7. The resonant tank, as a consequence,

turns from LLC to LLCC. This 4th- order tank circuit features a third resonance frequency at

the transformer's loaded self-resonance (fLSR > fR1). When the operating frequency is

considerably lower than fLSR the effect of CP is negligible. However, at frequencies greater

than fR1 and if the load impedance is high enough, its effect starts making itself felt,

eventually resulting in reversing the transferable power vs. frequency relationship as

frequency approaches fLSR. Power now increases with the switching frequency, feedback

becomes positive and the converter loses control of the output voltage. The onset of this

"feedback reversal" in closed-loop operation is revealed by a sudden frequency jump to its

maximum value as the load falls below a critical value (i.e. the frequency exceeds a critical

value) and a simultaneous output voltage rise.

In some way, either appropriately choosing the operating frequency range (<< fLSR) or

increasing fLSR, the converter must work away from feedback reversal. This usually sets the

practical upper limit to a converter's operating frequency range.

Table 1.

Output stress for LLC resonant half-bridge vs. PWM topologies @ 50% duty cycle

Output current form factors

Forward - ZVS AHB

LLC resonant HB

Flyback (CCM-DCM boundary)

Peak-to-DC ratio

≈1.05÷1.15

Rms-to-DC ratio

≈1

AC-to-DC ratio

≈0.03÷0.09

---

≈

1.57

-----------

≈

1.11

---

1.63

≈

------

–

≈

0.48

---

–

1.29

≈

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