AN2644 datasheet(8/64 Pages) STMICROELECTRONICS

8 / 64 page

The LLC resonant half-bridge converter

AN2644

8/64

For certain reasons that will be clarified in the following sections, the LLC resonant

converter is normally operated in the region where the input impedance of the resonant tank

has inductive nature, i.e. it increases with frequency. This implies that power flow can be

controlled by changing the operating frequency of the converter in such a way that a

reduced power demand from the load produces a frequency rise, while an increased power

demand causes a frequency reduction.

The Half-bridge Driver switches the two power MOSFETs Q1 and Q2 on and off in phase

opposition symmetrically, that is, for exactly the same time. This is commonly referred to as

"50% duty cycle" operation even if the conduction time of either power MOSFET is slightly

shorter than 50% of the switching period. In fact, a small deadtime is inserted between the

turn-off of either switch and the turn-on of the complementary one. The role of this deadtime

is essential for the operation of the converter. It goes beyond ensuring that Q1 and Q2 will

never cross-conduct and will be clarified in the next sections as well. For the moment it will

be neglected, and the voltage applied to the resonant tank will be a square-wave with 50%

duty cycle that swings all the way from 0 to Vin. Before going any further, however, it is

important to make one concept clear.

Figure 4.

LLC resonant half-bridge with split resonant capacitor

A few paragraphs above, the impedance of the tank circuit was mentioned. Impedance is a

concept related to linear circuits under sinusoidal excitation, whereas in this case the

excitation voltage is a square wave.

However, as a consequence of the selective nature of resonant tanks, most power

processing properties of resonant converters are associated with the fundamental

component of the Fourier expansion of voltages and currents in the circuit. This applies in

particular to the input square wave and is the foundation of the First Harmonic

Approximation (FHA) modeling methodology presented in [2] as a general approach and

used in [4] and [5] for the LLC resonant converter specifically. This approach justifies the

usage of the concept of impedance as well as those coming from complex ac circuit

analysis.

Coming back to the input square wave excitation, it has a DC component equal to Vin/2. In

the LLC resonant tank the resonant capacitor Cr is in series to the voltage source and under

steady state conditions the average voltage across inductors must be zero. As a result, the

DC component Vin/2 of the input voltage must be found across Cr which consequently plays

the double role of resonant capacitor and DC blocking capacitor.

It is possible to see, especially at higher power levels, a slightly modified version of the LLC

resonant half-bridge converter, where the resonant capacitor is split as illustrated in

Figure 4. This configuration can be useful to reduce the current stress in each capacitor

Vin

Cr / 2

a:1:1

Cr / 2

Vin

Cr / 2

a:1:1

Cr / 2

AN2644 Datasheet(PDF) 8 Page - STMicroelectronics

AN2644 Datasheet(HTML) 8 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Datasheet Download

Link URL