Parallel hybrid converter apparatus and method

1. An apparatus comprising:

an isolated power converter having an input connected to an input dc power source, a first output and a second output, wherein the isolated power converter comprises a first secondary winding, a first resonant tank, a first secondary switch network and a first output capacitor connected in cascade, and a second secondary winding, a second resonant tank, a second secondary switch network and a second output capacitor connected in cascade; and

a non-isolated power converter having an input connected to the second output of the isolated power converter, wherein the first output of the isolated power converter and an output of the non-isolated power converter are connected in series.

2. The apparatus of claim 1 , wherein:

the non-isolated power converter is a four-switch buck-boost converter; and

the isolated power converter is an inductor-inductor-capacitor (LLC) resonant converter.

3. The apparatus of claim 1 , wherein:

the non-isolated power converter is connected between the second output capacitor and a third output capacitor, and wherein the first output capacitor and the third output capacitor are connected in series and further connected to a load.

4. The apparatus of claim 1 , wherein:

a power delivered by the second secondary switch network is a fraction of a power delivered by the first secondary switch network.

5. The apparatus of claim 1 , wherein the second secondary switch network comprises:

a first switch and a second switch connected in series between two terminals of the second output capacitor; and

a first capacitor and a second capacitor connected in series between the two terminals of the second output capacitor, and wherein the second secondary winding is coupled to a common node of the first switch and the second switch, and a common node of the first capacitor and the second capacitor, respectively.

6. The apparatus of claim 1 , wherein:

the isolated power converter is an LLC resonant converter configured to operate at a switching frequency approximately equal to a resonant frequency of the LLC resonant converter.

7. The apparatus of claim 1 , wherein:

the isolated power converter is configured as an unregulated power converter.

8. The apparatus of claim 1 , wherein the isolated power converter comprises a transformer comprising:

a primary winding connected to the input of the isolated power converter through a primary switch network;

the first secondary winding coupled to the first output of the isolated power converter; and

the second secondary winding coupled to the second output of the isolated power converter.

9. A system comprising:

a primary switch network configured to be connected to a power source;

a transformer having a first secondary winding, a second secondary winding and a primary winding, the primary winding being connected to the primary switch network;

a first rectifier connected to the first secondary winding through a first resonant tank;

a second rectifier connected to the second secondary winding through a second resonant tank;

an output filter connected to the first rectifier; and

a power converter connected to the second rectifier, wherein outputs of the output filter and the power converter are connected in series.

10. The system of claim 9 , wherein:

the primary switch network, the transformer, the first resonant tank, the second resonant tank, the first rectifier and the second rectifier form an inductor-inductor-capacitor (LLC) resonant converter.

11. The system of claim 9 , wherein:

a power flowing through the second rectifier is a fraction of a power flowing through the first rectifier.

12. The system of claim 9 , wherein:

the primary switch network is a full-bridge.

13. The system of claim 9 , wherein:

the power converter is a buck-boost converter.