Auxiliary resonant apparatus for LLC converters
A resonant tank comprises a series resonant inductor coupled to a switching network and a transformer, a series resonant capacitor coupled to the switching network and the transformer, a first parallel inductor implemented as a magnetizing inductance of the transformer, a second parallel inductor implement as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor and a switch connected in series with the second parallel inductor.
1. An apparatus comprising:
a series resonant inductor coupled to a switching network and a transformer;
a series resonant capacitor coupled to the switching network and the transformer;
a first parallel inductor implemented as a magnetizing inductance of the transformer;
a second parallel inductor implemented as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor; and
a switch connected in series with the second parallel inductor, wherein a terminal of the switch is directly connected to a terminal of the second parallel inductor, and wherein a first terminal of the second parallel inductor and the first parallel inductor are separated by a resonant element, and a second terminal of the second parallel inductor and the first parallel inductor are separated by the switch.
2. The apparatus of claim 1 , wherein:
the first inductance is at least ten times greater than the second inductance.
3. The apparatus of claim 1 , wherein:
the switch, the second parallel inductor and drain-to-source capacitances of the switching network form an auxiliary resonant tank; and
the series resonant inductor and the series resonant capacitor form a main resonant tank.
4. The apparatus of claim 3 , wherein:
the series resonant inductor and the series 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 and the switch are connected in series to form a series inductor-switch circuit coupled between the first output terminal of the switching network and a second output terminal of the switching network.
5. The apparatus of claim 3 , wherein:
the series resonant inductor and the series 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 and the switch are connected in series to form a series inductor-switch circuit coupled between a common node of the series resonant inductor and the series resonant capacitor, and a second output terminal of the switching network.
6. The apparatus of claim 1 , wherein:
the switch is formed by two back-to-back connected switching elements.
7. The apparatus of claim 1 , wherein:
the switch is so controlled that the switch is turned on during a short period prior to a zero voltage transition.
8. The apparatus of claim 7 , wherein:
the short period is less than 50% of a switching period of the switching network.
9. A system comprising:
an input power source;
a switching network comprising:
a first pair of switches coupled between two terminals of the input power source; and
a second pair of switches coupled between the two terminals of the input power source;
a resonant tank connected between the switching network and a primary side of a transformer, 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;
a first parallel inductor implemented as a magnetizing inductance of the transformer;
a second parallel inductor implemented as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor; and
a switch connected in series with the second parallel inductor, wherein a first terminal of the second parallel inductor and the first parallel inductor are separated by a resonant element, and a second terminal of the second parallel inductor and the first parallel inductor are separated by the switch;
a rectifier coupled to a secondary side of the transformer; and
an output filter coupled to the rectifier.
10. The system of claim 9 , wherein the resonant tank comprises:
the resonant inductor connected in series with the resonant capacitor and coupled between a first common node of the first pair of switches and a first terminal of the primary side of the transformer; and
the second parallel inductor connected in series with the switch to form an inductor-switch circuit coupled between the first common node and a second common node of the second pair of switches.
11. The system of claim 9 , wherein the resonant tank comprises:
the resonant inductor connected in series with the resonant capacitor and coupled between a first common node of the first pair of switches and a first terminal of the primary side of the transformer; and
the second parallel inductor connected in series with the switch to form an inductor-switch circuit coupled between a common node of the resonant inductor and the resonant capacitor, and a second common node of the second pair of switches.
12. The system of claim 9 , wherein:
the transformer is a center-tapped transformer; and
the rectifier is a synchronous rectifier.
13. The system of claim 9 , wherein:
the transformer is a non center-tapped transformer; and
the rectifier is a full-wave rectifier.
14. A method comprising:
providing a resonant tank coupled between a switching network comprising a first switch, a second and a transformer, wherein the resonant tank comprises:
a series inductor;
a series capacitor connected in series with the series inductor; and
a parallel inductor connected in series with a switch, wherein a magnetizing inductance of the transformer is greater than an inductance of the parallel inductor and a current flowing through the parallel inductor is equal to a current flowing through the switch, and wherein a first terminal of the parallel inductor and the transformer are separated by a resonant tank element, and a second terminal of the parallel inductor and the transformer are separated by the switch;
configuring the switching network to operate at a switching frequency approximately equal to a resonant frequency of the resonant tank;
prior to a zero voltage transition, during a dead time turning on the switch; and
after the zero voltage transition, turning off the switch.
15. The method of claim 14 , further comprising:
coupling a switching network to a dc input power source;
coupling a primary side of a converter and a secondary side of the converter through a transformer providing isolation between the primary side and the secondary side of the converter;
coupling a rectifier to the secondary side; and
coupling an output filter to the rectifier.
16. The method of claim 14 , wherein:
the switch is formed by two back-to-back connected metal oxide semiconductor transistors.
17. The method of claim 14 , wherein:
the dead time is approximately equal to one tenth of a switching period of the switching network.
18. The method of claim 14 , wherein:
the magnetizing inductance is at least ten times greater than the inductance of the parallel inductor.