LLC resonant converters

1. An apparatus comprising:

a resonant inductor coupled to a switching network and a transformer;

a resonant capacitor coupled to the switching network and the transformer;

a first parallel inductor implemented as a magnetizing inductance; and

a second parallel inductor and a first parallel capacitor connected in series to form a parallel resonant device, wherein a first inductance of the first parallel inductor is at least ten times greater than a second inductance of the second parallel inductor so that between the first parallel inductor and the second parallel inductor, the second parallel inductor becomes a dominant element during a resonant process.

2. The apparatus of claim 1 , wherein:

the second parallel inductor, the resonant inductor and the resonant capacitor form a resonant tank.

3. The apparatus of claim 2 , wherein:

the resonant inductor and the resonant capacitor are connected in series and coupled between a first output terminal of the switching network and a first terminal of a primary side winding of the transformer; and the second parallel inductor is coupled between the first output terminal of the switching network and a second output terminal of the switching network.

4. The apparatus of claim 2 , wherein:

the resonant inductor and the resonant capacitor are connected in series and coupled between a first output terminal of the switching network and a first terminal of a primary side winding of the transformer; and

the second parallel inductor is coupled between a common node of the resonant inductor and the resonant capacitor, and a second output terminal of the switching network.

5. A system comprising:

an input power source;

a first pair of switches coupled between two terminals of the input power source;

a second pair of switches coupled between the two terminals of the input power source;

a resonant tank connected in series with a primary side of a transformer, wherein:

a parallel inductor of the resonant tank is of an inductance less than ten percent of a magnetizing inductance of the transformer;

a parallel capacitor connected in series with the parallel inductor to form a parallel resonant device;

a first common node of the first pair of switches is coupled to a first terminal of the resonant tank; and

a second common node of the second pair of switches is coupled to a second terminal of the resonant tank;

a rectifier coupled to a secondary side of the transformer; and

an output filter coupled to the rectifier.

6. The system of claim 5 , wherein the resonant tank comprises:

a resonant inductor connected in series with a resonant capacitor and coupled between the first common node and a first terminal of the primary side of the transformer; and

the parallel inductor coupled between the first common node and the second common node.

7. The system of claim 5 , wherein the resonant tank comprises:

a resonant inductor connected in series with a resonant capacitor and coupled between the first common node and a first terminal of the primary side of the transformer; and

the parallel inductor coupled to a common node of the resonant inductor and the resonant capacitor.

8. The system of claim 5 , wherein:

the transformer is a non center-tapped transformer; and

the rectifier is a full-wave rectifier.

9. The system of claim 5 , wherein:

the first pair of switches, the second pair of switches, the transformer and the rectifier form a full bridge converter.

10. A converter comprising:

a switching network coupled to a dc input power source;

a transformer providing isolation between a primary side and a secondary side of the converter;

a resonant tank coupled to the switching network, wherein the resonant tank comprises:

a resonant inductor coupled to the switching network and the transformer;

a resonant capacitor coupled to the switching network and the transformer,

wherein the resonant inductor and the resonant capacitor are on different sides of the transformer;

a first parallel inductor implemented as a magnetizing inductance; and

a second parallel inductor and a first parallel capacitor connected in series to form a parallel resonant device, wherein a first inductance of the first parallel inductor is at least ten times greater than a second inductance of the second parallel inductor;

a rectifier coupled to the secondary side; and

an output filter coupled to the rectifier.

11. The converter of claim 10 , wherein:

the switching network is configured to operate at a switching frequency approximately equal to a resonant frequency of the resonant tank.