Secondary side hybrid converter apparatus and method

1. An apparatus comprising:

an isolated power converter coupled to an input dc power source, wherein the isolated power converter comprises:

a primary switching network operating at a fixed switching frequency;

a secondary resonant tank including a dc blocking capacitor; and

a rectifier having two input terminals coupled to the secondary resonant tank, wherein the rectifier comprises two legs, and wherein a first leg comprises two switching elements connected in series and a second leg comprises two elements connected in series, and wherein a common node of the two switching elements is connected to a first input terminal of the rectifier and a common node of the two elements of the second leg is connected to a second input terminal of the rectifier;

an output capacitor coupled between a first output terminal of the rectifier and a load; and

a dc/dc converter coupled between a second output terminal of the rectifier and the load, wherein the dc/dc converter has an input directly connected to the first leg and an output directly connected to the second leg.

2. The apparatus of claim 1 , wherein:

the dc/dc 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 dc/dc converter is configured to operate at a PWM mode.

4. The apparatus of claim 3 , wherein:

the dc/dc converter is configured to operate at a buck converter mode in response to a first input voltage and operate at a boost converter mode in response to a second input voltage, wherein the first input voltage is higher than the second input voltage.

5. The apparatus of claim 1 , wherein:

an average voltage across the dc blocking capacitor is approximately equal to one half of a difference between a voltage across the load and a voltage of the second output terminal of the rectifier.

6. The apparatus of claim 1 , wherein:

a gain of the isolated power converter is approximately equal to 1.

7. The apparatus of claim 1 , wherein:

the dc/dc converter comprises an input capacitor, a first switch, a second switch, a third switch, a fourth switch and an inductor.

8. The apparatus of claim 7 , wherein:

the input capacitor is coupled between the second output terminal of the rectifier and ground;

the first switch and the second switch are connected in series and further coupled between two terminals of the input capacitor of the dc/dc converter;

the third switch and the fourth switch are connected in series and further coupled between two terminals of the output capacitor; and

the inductor is between a common node of the first switch and the second switch, and a common node of the third switch and the fourth switch.

9. The apparatus of claim 7 , wherein:

the rectifier comprises a first secondary switch, a second secondary switch, a third secondary switch and a fourth secondary switch; and

the secondary resonant tank comprises a resonant inductor connected in series with the dc blocking capacitor.

10. The apparatus of claim 9 , wherein:

the secondary resonant tank comprises:

two inputs coupled to a secondary side transformer winding of the isolated power converter;

a first output coupled to a common node of the first secondary switch and the second secondary switch; and

a second output coupled to a common node of the third secondary switch and the fourth secondary switch;

the first secondary switch and the second secondary switch are connected in series and further coupled between two terminals of the input capacitor of the dc/dc converter; and

the third secondary switch and the fourth secondary switch are connected in series and further coupled between two terminals of the output capacitor.

11. A system comprising:

an isolated power converter coupled to an input dc power source, wherein the isolated power converter is an unregulated inductor-inductor-capacitor (LLC) resonant converter and the unregulated LLC resonant converter comprises:

a secondary resonant tank including a dc blocking capacitor; and

a rectifier having two input terminals coupled to the secondary resonant tank, wherein the rectifier comprises a first leg having two switching elements connected in series and a second leg having two elements connected in series, and wherein a bottom terminal of the first leg is directly connected to a bottom terminal of the second leg;

an output capacitor coupled between a first output terminal of the rectifier and a load, wherein the first output terminal of the rectifier is an upper terminal of the second leg; and

a dc/dc converter coupled between a second output terminal of the rectifier and the load, wherein the second output terminal of the rectifier is an upper terminal of the first leg, and wherein:

the dc/dc converter is configured to operate at a buck converter mode when an input voltage from the input dc power source is higher than a voltage threshold; and

the dc/dc converter is configured to operate at a boost converter mode when the input voltage from the input dc power source is lower than the voltage threshold.

12. The system of claim 11 , wherein:

a power delivered by the dc/dc converter is a fraction of a power delivered by the isolated power converter.

13. The system of claim 11 , wherein the LLC resonant converter is configured to:

operate in a duty cycle approximately equal to 50%; and

operate at a fixed frequency.

14. The system of claim 11 , wherein:

the dc/dc converter is a regulated four-switch buck-boost converter.

15. The system of claim 11 , wherein:

the secondary resonant tank comprises a resonant inductor connected in series with the dc blocking capacitor; and

the rectifier comprises a first secondary switch, a second secondary switch, a third secondary switch and a fourth secondary switch, and wherein:

the first secondary switch and the second secondary switch are connected in series and further coupled between two input terminals of the dc/dc converter; and

the third secondary switch and the fourth secondary switch are connected in series and further coupled between two output terminals of the dc/dc converter.

16. A method comprising:

providing a serial hybrid converter comprising:

an isolated power converter coupled to an input dc power source, wherein the isolated power converter comprises:

a primary switching network operating at a fixed switching frequency;

a secondary resonant tank including a dc blocking capacitor; and

a rectifier having two input terminals coupled to the secondary resonant tank, and wherein the rectifier comprises a first leg having two switching elements connected in series and a second leg having two elements connected in series;

an output capacitor coupled between a first output terminal of the rectifier and a load; and

a dc/dc converter coupled between a second output terminal of the rectifier and the load, wherein an upper terminal of the first leg is connected to the load through the dc/dc converter and an upper terminal of the second leg is directly connected to the load;

configuring the dc/dc converter to operate at a buck converter mode in response to a first input voltage; and

configuring the dc/dc converter to operate at a boost converter mode in response to a second input voltage, wherein the first input voltage is higher than the second input voltage.

17. The method of claim 16 , further comprising:

configuring the isolated power converter to operate at a fixed switching frequency, wherein the isolated power converter is an unregulated inductor-inductor-capacitor (LLC) resonant converter.

18. The method of claim 16 , further comprising:

configuring the dc/dc converter to operate at the buck converter mode when an input voltage is higher than a threshold; and

configuring the dc/dc converter to operate at the boost converter mode when the input voltage is lower than the threshold.

19. The method of claim 16 , wherein:

the dc/dc converter is a four-switch buck-boost power converter coupled between a second output terminal of the rectifier and the load, and wherein a power delivered by the dc/dc converter is a fraction of a power delivered by the isolated power converter.

20. The method of claim 16 , further comprising:

configuring the dc/dc converter to operate as a regulated power converter; and

configuring the isolated power converter to operate as an unregulated power converter.