Abstract:
A resonant converter comprising: a controllable current source; a resonant tank circuit coupled to the current source; and an isolated buck-type converter coupled to the resonant tank circuit, the isolated buck-type converter having an output, wherein the resonant tank circuit enables switches in the isolated buck-type converter to switch under soft-switching conditions. In some embodiments, the controllable current source is a switch-mode-type current source. In some embodiments, the isolated buck-type converter comprises a half-bridge converter. In some embodiments, the isolated buck-type converter comprises a full-bridge converter. In some embodiments, the isolated buck-type converter comprises a push-pull converter.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of converter topology. More particularly, the present invention relates to a two stage resonant DC/DC converter. 
     BACKGROUND OF THE INVENTION 
     In DC/DC converters, a DC input voltage is converted to a lower DC output voltage. Normally, the output voltage needs to be precisely regulated and input to output isolation is necessary in order to meet safety requirements. 
       FIG. 1  is a schematic diagram of a prior art two stage converter  100 . The two stage converter  100  comprises a power factor correction (PFC) boost converter  120  and an isolated buck-type converter  140 . The PFC boost converter  120  provides a high voltage DC current to the isolated buck-type converter  140 . The isolated buck-type converter  140  converts the high voltage DC current into a low-voltage DC current. 
     In this and other prior art converters, the switches of the second stage work under hard switching conditions, resulting in high switching losses, and thereby affecting the total efficiency of the converter and limiting the switching frequency. Additionally, the second stage needs a current-limiting circuit to provide over-current protection during abnormal conditions, such as during an output short circuit. This need for over-current protection increases the complexity of the control circuit. 
     What is needed in the art is a simplified DC/DC converter design that reduces switching losses. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a resonant converter comprises a controllable current source, a resonant tank circuit coupled to the current source, and an isolated buck-type converter coupled to the resonant tank circuit. The isolated buck-type converter has an output. The resonant tank circuit enables switches in the isolated buck-type converter to switch under soft-switching conditions. 
     In some embodiments, the controllable current source is a switch-mode-type current source. In some embodiments, the resonant converter further comprises a power factor correction (PFC) boost converter coupled to an input of the controllable current source, wherein the PFC boost converter is configured to provide a voltage to the input of the controllable current source. In some embodiments, the PFC boost converter is configured to provide a DC input voltage to the input of the controllable current source, and the isolated buck-type converter is configured to provide a DC output voltage to the output of the isolated buck-type converter. In some embodiments, the isolated buck-type converter comprises one of the group consisting of: a half-bridge converter, a full-bridge converter and a push-pull converter. 
     In some embodiments, the isolated buck-type converter includes a push-pull converter that comprises: a transformer having a first primary winding, a second primary winding, a first secondary winding, and a second secondary winding, wherein the controllable current source is coupled to a node between the first and second primary windings to form a primary center tap; a first primary switch coupled between the first primary winding and the controllable current source; and a second primary switch coupled between the second primary winding and the controllable current source. 
     In some embodiments, the push-pull converter further comprises a first secondary diode coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary diode coupled between the second secondary winding and the output of the isolated buck-type converter. 
     In some embodiments, the push-pull converter further comprises a first primary inductor coupled between the first primary winding and the first primary switch, and a second primary inductor coupled between the second primary winding and the second primary switch. 
     In some embodiments, the push-pull converter further comprises a first secondary inductor coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary inductor coupled between the second secondary winding and the output of the isolated buck-type converter. 
     In some embodiments, the push-pull converter further comprises a first secondary switch coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary switch coupled between the second secondary winding and the output of the isolated buck-type converter. 
     In some embodiments, the isolated buck-type converter includes a full-bridge converter that comprises: a transformer having a first primary winding, a first secondary winding, and a second secondary winding; a first primary switch coupled between a first terminal of the first primary winding and the controllable current source; a second primary switch coupled between a second terminal of the first primary winding and the controllable current source; a third primary switch coupled between the first terminal of the first primary winding and the controllable current source, wherein the first primary switch and the third primary switch are coupled to the first terminal of the first primary winding through a common node; and a fourth primary switch coupled between the second terminal of the first primary winding and the controllable current source, wherein the second primary switch and the fourth primary switch are coupled to the second terminal of the first primary winding through a common node. 
     In some embodiments, the full-bridge converter further comprises a first secondary diode coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary diode coupled between the second secondary winding and the output of the isolated buck-type converter. 
     In some embodiments, the full-bridge converter further comprises a primary inductor coupled between the first terminal of the first primary winding and the common node of the first primary switch and the third primary switch. 
     In some embodiments, the full-bridge converter further comprises a secondary inductor coupled between a common node between the first and second secondary windings and the output of the isolated buck-type converter. 
     In some embodiments, the full-bridge converter further comprises a first secondary switch coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary switch coupled between the second secondary winding and the output of the isolated buck-type converter. 
     In some embodiments, the isolated buck-type converter includes a half-bridge converter that comprises: a transformer having a first primary winding, a first secondary winding, and a second secondary winding; a first primary switch coupled between a first terminal of the first primary winding and the controllable current source; a second primary switch coupled between the first terminal of the first primary winding and the controllable current source, wherein the first primary switch and the second primary switch are coupled to the first terminal of the first primary winding through a common node. 
     In some embodiments, the half-bridge converter further comprises a first secondary diode coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary diode coupled between the second secondary winding and the output of the isolated buck-type converter. 
     In some embodiments, the half-bridge converter further comprises a primary inductor coupled between the first terminal of the first primary winding and the common node of the first primary switch and the second primary switch. 
     In some embodiments, the half-bridge converter further comprises a secondary inductor coupled between a common node between the first and second secondary windings and the output of the isolated buck-type converter. 
     In some embodiments, the half-bridge converter further comprises a first secondary switch coupled between the first secondary winding and the output of the isolated buck-type converter, and a second secondary switch coupled between the second secondary winding and the output of the isolated buck-type converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a prior art two stage converter. 
         FIG. 2  is a schematic diagram of one embodiment of a two stage resonant converter in accordance with the principles of the present invention. 
         FIG. 3  is a schematic diagram of one embodiment of a two stage resonant converter employing a push-pull converter in accordance with the principles of the present invention. 
         FIG. 4A  is a waveform diagram of one embodiment of the first stage of a two stage resonant converter in accordance with the principles of the present invention. 
         FIG. 4B  is a waveform diagram of one embodiment of the second stage of a two stage resonant converter in accordance with the principles of the present invention. 
         FIG. 5  is a schematic diagram of another embodiment of a two stage resonant converter employing a push-pull converter in accordance with the principles of the present invention. 
         FIG. 6  is a schematic diagram of yet another embodiment of a two stage resonant converter employing a push-pull converter in accordance with the principles of the present invention. 
         FIG. 7  is a schematic diagram of one embodiment of a two stage resonant converter employing a full-bridge converter in accordance with the principles of the present invention. 
         FIG. 8  is a schematic diagram of another embodiment of a two stage resonant converter employing a full-bridge converter in accordance with the principles of the present invention. 
         FIG. 9  is a schematic diagram of yet another embodiment of a two stage resonant converter employing a full-bridge converter in accordance with the principles of the present invention. 
         FIG. 10  is a schematic diagram of one embodiment of a two stage resonant converter employing a half-bridge converter in accordance with the principles of the present invention. 
         FIG. 11  is a schematic diagram of another embodiment of a two stage resonant converter employing a half-bridge converter in accordance with the principles of the present invention. 
         FIG. 12  is a schematic diagram of yet another embodiment of a two stage resonant converter employing a half-bridge converter in accordance with the principles of the present invention. 
         FIG. 13  is a schematic diagram of one embodiment of a controllable DC current source in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein can be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
       FIG. 2  is a schematic diagram of one embodiment of a two stage resonant converter  200  in accordance with the principles of the present invention. The two stage resonant converter  200  comprises a power factor correction (PFC) boost converter  220  coupled to an input of a controllable current source  230 , which is coupled to a resonant tank circuit and isolated buck-type converter  240 . The PFC boost converter  220  provides a high voltage DC current to the controllable current source  230 . The controllable current source  230  provides a constant DC current to the resonant tank circuit and isolated buck-type converter  240 , which converts the constant DC current into a low-voltage DC current. The isolated buck-type converter  240  provides this low-voltage DC current to its output. In some embodiments, the controllable current source  230  is a switch-mode-type current source. In some embodiments, the isolated buck-type converter  240  comprises one of the group consisting of a half-bridge converter, a full-bridge converter, and a push-pull converter. 
       FIG. 3  is a schematic diagram of one embodiment of a two stage resonant converter  300  employing a push-pull converter in accordance with the principles of the present invention. The two stage resonant converter  300  comprises a controllable DC current source  330  and a transformer  340 . The transformer comprises a first primary winding P 1 , a second primary winding P 2 , a first secondary winding S 1 , and a second secondary winding S 2 . The controllable current source  330  is coupled to a node  342  between the first and second primary windings P 1 , P 2  to form a primary center tap. A first primary switch  344  is coupled between the first primary winding P 1  and the controllable current source  330 . A second primary switch  346  is coupled between the second primary winding P 2  and the controllable current source  330 . 
     In some embodiments, a first secondary diode  356  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary diode  358  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  354 . In some embodiments, an output capacitor  360  is coupled in parallel between the transformer  340  and the output of the isolated buck-type converter. In some embodiments, a first primary inductor  352  is coupled between the first primary winding P 1  and the first primary switch  344 , and a second primary inductor  350  is coupled between the second primary winding P 2  and the second primary switch  346 . In some embodiments, a resonant capacitor  348  is coupled in parallel between the controllable DC current source  330  and the transformer  340 . Together with the first primary inductor  352  and the second primary inductor  350 , resonant capacitor  348  forms a resonant tank circuit. 
       FIG. 4A  illustrates a waveform diagram of one embodiment of the first stage of a two stage resonant converter in accordance with the principles of the present invention.  FIG. 4B  illustrates a waveform diagram of one embodiment of the second stage of the two stage resonant converter in accordance with the principles of the present invention. For the purposes of discussing  FIGS. 4A-B , an example is provided using the two stage resonant converter  300  of  FIG. 3  with the controllable DC current source  1300  of  FIG. 13 , which will be discussed in further detail below. 
     In  FIG. 4A , the signals from bottom to top are: the gate drive of switch  1340  (Vg-Q 1 ), the drain current of switch  1340  (Id-Q 1 ), the current of diode  1330  (ID 1 ), and the current of inductor  1350  (I-L 1 ). When switch  1340  (Q 1 ) is turned on, the input voltage Vin is applied to first stage diode  1330  (D 1 ) and first stage diode  1330  (D 1 ) turns off. First stage switch  1340  (Q 1 ) conducts the inductor current. In this period of time, energy is transferred from input power source  1310  (Vin) to the second stage and stored in the first stage inductor  1350  (L 1 ) in the mean time. After first stage switch  1340  (Q 1 ) turns off, first stage diode  1330  (D 1 ) conducts the inductor current, and the stored inductor energy keeps transferring to the second stage. 
     In  FIG. 4B , the signals from bottom to top are: the gate drive of switch  346  (Vg-Q 3 ), the gate drive of switch  344  (Vg-Q 2 ), the drain current of switch  344  (Id-Q 2 ), the current of diode  358  (I-D 3 ), and the drain to source voltage of switch  344  (Vds-Q 2 ). At time point T 0 , switch  344  (Q 2 ) turns on and switch  346  (Q 3 ) is off. Diode  358  (D 3 ) and diode  356  (D 2 ) are both off, so the transformer secondary side is open. The current in the primary side of the transformer is the magnetizing current, and it flows through switch  344  (Q 2 ), first primary inductor  352  (Lr 2 ) and first primary winding P 1 , and discharges the output capacitance of MOSFET switch  344  (Q 2 ). At the turn on point, the drain current of switch  344  (Id-Q 2 ) flows through the MOSFET body diode, and the voltage across switch  344  (Vds-Q 2 ) is approximately zero, making switch  344  (Q 2 ) turn on at ZVS (zero voltage switching) condition. The turn on loss of MOSFET switch  344  (Q 2 ) is low. At time point T 1 , the drain current of switch  344  (Id-Q 2 ) reaches zero, the body diode of MOSFET switch  344  (Q 2 ) turns off with zero current switching, and the current changes direction and shifts to the positive path (drain to source) of MOSFET switch  344  (Q 2 ). 
     From T 1  on, diode  358  (D 3 ) turns on and begins to conduct current. The voltage of transformer secondary winding S 2  is clamped to Vo. Accordingly, the voltage of transformer primary winding P 1  is clamped to N*Vo, with N being the turns ratio of primary winding to secondary winding. Resonant capacitor  348  (Cr) is resonant with first primary inductor  352  (Lr 2 ), and the drain current of switch  344  (Id-Q 2 ) increases from zero. Current Id-Q 2  can be divided into two portions, the resonant portion, which equals Id 3 /N and transfers to the secondary side though the transformer, and the magnetizing portion. At T 2  point, the resonant portion reduces to zero. Accordingly the secondary diode  358  (D 3 ) turns off at ZCS (zero current switching condition) condition, and the switching loss is reduced. From T 2  to T 3 , diode current is zero, so the transformer secondary side is “open.” On the primary side, only the magnetizing current is remaining. 
     At T 3 , switch  344  (Q 2 ) is turned off by the drive signal. This is a near ZCS turn off because only a small magnetizing current flow through switch  344  (Q 2 ). T 3  to T 4  is a “dead time”, during which both switch  344  (Q 2 ) and switch  346  (Q 3 ) are off. On the primary side of the transformer, the magnetizing current consists of two parts: (1) the drain current of switch  344  (Id-Q 2 ), which flows from Q 2 ′s drain to source and charges the output capacitance of switch  344  (Q 2 ); and (2) the drain current of switch  346  (Id-Q 3 ), which flows from Q 3 &#39;s source to drain and discharges the output capacitance of switch  346  (Q 3 ). At time point T 4 , the drain current of switch  344  (Id-Q 2 ) has reduced to zero and all the magnetizing current has flown through the body diode of MOSFET switch  346  (Q 3 ). Switch  346  (Q 3 ) turns on by the drive signal at ZVS condition. The next half cycle will repeat the similar work mechanism. 
       FIG. 5  is a schematic diagram of another embodiment of a two stage resonant converter  500  employing a push-pull converter in accordance with the principles of the present invention. The two stage resonant converter  500  comprises a controllable DC current source  530  and a transformer  540 . The transformer  540  comprises a first primary winding P 1 , a second primary winding P 2 , a first secondary winding S 1 , and a second secondary winding S 2 . The controllable current source  530  is coupled to a node  542  between the first and second primary windings P 1 , P 2  to form a primary center tap. A first primary switch  544  is coupled between the first primary winding P 1  and the controllable current source  530 , and a second primary switch  546  is coupled between the second primary winding P 2  and the controllable current source  530 . 
     In some embodiments, a first secondary diode  556  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary diode  558  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, a first secondary inductor  552  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary inductor  554  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, a resonant capacitor  548  is coupled in parallel between the controllable DC current source  530  and the transformer  540 . Together with the first secondary inductor  552  and the second secondary inductor  554 , resonant capacitor  548  forms a resonant tank circuit. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  550 . In some embodiments, an output capacitor  560  is coupled in parallel between the transformer  540  and the output of the isolated buck-type converter. In some embodiments, a ground terminal  562  is coupled between the transformer  540  and the output of the isolated buck-type converter. 
       FIG. 6  is a schematic diagram of yet another embodiment of a two stage resonant converter  600  employing a push-pull converter in accordance with the principles of the present invention. The two stage resonant converter  600  comprises a controllable DC current source  630  and a transformer  640 . The transformer  640  comprises a first primary winding P 1 , a second primary winding P 2 , a first secondary winding S 1 , and a second secondary winding S 2 . The controllable current source  630  is coupled to a node  642  between the first and second primary windings P 1 , P 2  to form a primary center tap. A first primary switch  644  is coupled between the first primary winding P 1  and the controllable current source  630 , and a second primary switch  646  is coupled between the second primary winding P 2  and the controllable current source  630 . 
     In some embodiments, a first primary inductor  652  is coupled between the first primary winding P 1  and the first primary switch  644 , and a second primary inductor  650  is coupled between the second primary winding P 2  and the second primary switch  646 . In some embodiments, a resonant capacitor  648  is coupled in parallel between the controllable DC current source  630  and the transformer  640 . Together with the first primary inductor  652  and the second primary inductor  650 , resonant capacitor  648  forms a resonant tank circuit. In some embodiments, a first secondary switch  658  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary switch  660  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  654 . In some embodiments, an output capacitor  664  is coupled in parallel between the transformer  640  and the output of the isolated buck-type converter. In some embodiments, a ground terminal  662  is coupled between the transformer  640  and the output of the isolated buck-type converter. 
       FIG. 7  is a schematic diagram of one embodiment of a two stage resonant converter  700  employing a full-bridge converter in accordance with the principles of the present invention. 
     The two stage resonant converter  700  comprises a controllable DC current source  730  and a transformer  740 . The transformer  740  comprises a first primary winding P 1 , a first secondary winding S 1 , and a second secondary winding S 2 . A first primary switch  742  is coupled between a first terminal of the first primary winding P 1  and the controllable current source  730 . A second primary switch  744  is coupled between a second terminal of the first primary winding P 1  and the controllable current source  730 . A third primary switch  746  is coupled between the first terminal of the first primary winding P 1  and the controllable current source  730 . A fourth primary switch  748  is coupled between the second terminal of the first primary winding P 1  and the controllable current source  730 . The first primary switch  742  and the third primary switch  746  are coupled to the first terminal of the first primary winding P 1  through a common node  750 . The second primary switch  744  and the fourth primary switch  748  are coupled to the second terminal of the first primary winding P 1  through a common node  752 . 
     In some embodiments, a first secondary diode  760  coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary diode  762  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  758 . In some embodiments, an output capacitor  764  is coupled in parallel between the transformer  740  and the output of the isolated buck-type converter. In some embodiments, a primary inductor  756  is coupled between the first terminal of the first primary winding P 1  and the common node  750  of the first primary switch  742  and the third primary switch  746 . In some embodiments, a resonant capacitor  754  is coupled in parallel between the controllable DC current source  730  and the transformer  740 . Together with the primary inductor  756 , resonant capacitor  754  forms a resonant tank circuit. 
       FIG. 8  is a schematic diagram of another embodiment of a two stage resonant converter  800  employing a full-bridge converter in accordance with the principles of the present invention. The two stage resonant converter  800  comprises a controllable DC current source  830  and a transformer  840 . The transformer  840  comprises a first primary winding P 1 , a first secondary winding S 1 , and a second secondary winding S 2 . A first primary switch  842  is coupled between a first terminal of the first primary winding P 1  and the controllable current source  830 . A second primary switch  844  is coupled between a second terminal of the first primary winding P 1  and the controllable current source  830 . A third primary switch  840  is coupled between the first terminal of the first primary winding P 1  and the controllable current source  830 . A fourth primary switch  848  is coupled between the second terminal of the first primary winding P 1  and the controllable current source  830 . The first primary switch  842  and the third primary switch  840  are coupled to the first terminal of the first primary winding P 1  through a common node  850 . The second primary switch  844  and the fourth primary switch  848  are coupled to the second terminal of the first primary winding P 1  through a common node  852 . 
     In some embodiments, a first secondary diode  858  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary diode  860  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, a secondary inductor  862  is coupled between a common node, between the second terminal of the first secondary winding S 1  and first terminal of the second secondary winding S 2 , and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  856 . In some embodiments, an output capacitor  864  is coupled in parallel between the transformer  840  and the output of the isolated buck-type converter. In some embodiments, a resonant capacitor  854  is coupled in parallel between the controllable DC current source  830  and the transformer  840 . Together with the secondary inductor  862 , resonant capacitor  854  forms a resonant tank circuit. 
       FIG. 9  is a schematic diagram of yet another embodiment of a two stage resonant converter  900  employing a full-bridge converter in accordance with the principles of the present invention. The two stage resonant converter  900  comprises a controllable DC current source  930  and a transformer  940 . The transformer  940  comprises a first primary winding P 1 , a first secondary winding S 1 , and a second secondary winding S 2 . A first primary switch  942  is coupled between a first terminal of the first primary winding P 1  and the controllable current source  930 . A second primary switch  944  is coupled between a second terminal of the first primary winding P 1  and the controllable current source  930 . A third primary switch  946  is coupled between the first terminal of the first primary winding P 1  and the controllable current source  930 . A fourth primary switch  948  is coupled between the second terminal of the first primary winding P 1  and the controllable current source  930 . The first primary switch  942  and the third primary switch  946  are coupled to the first terminal of the first primary winding P 1  through a common node  950 . The second primary switch  944  and the fourth primary switch  948  are coupled to the second terminal of the first primary winding P 1  through a common node  952 . 
     In some embodiments, a primary inductor  956  is coupled between the first terminal of the first primary winding P 1  and the common node  950  of the first primary switch  942  and the third primary switch  946 . In some embodiments, a resonant capacitor  954  is coupled between the controllable DC current source  930  and the transformer  940 . Together with the primary inductor  956 , resonant capacitor  954  forms a resonant tank circuit. In some embodiments, a first secondary switch  960  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary switch  962  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  958 . In some embodiments, an output capacitor  968  is coupled in parallel between the transformer  940  and the output of the isolated buck-type converter. In some embodiments, a ground terminal  964  is coupled between the transformer  940  and the output of the isolated buck-type converter. 
       FIG. 10  is a schematic diagram of one embodiment of a two stage resonant converter  1000  employing a half-bridge converter in accordance with the principles of the present invention. The two stage resonant converter  1000  comprises a controllable DC current source  1030  and a transformer  1040 . The transformer  1040  comprises a first primary winding P 1 , a first secondary winding S 1 , and a second secondary winding S 2 . A first primary switch  1042  is coupled between a first terminal of the first primary winding P 1  and the controllable current source  1030 . A second primary switch  1044  is coupled between the first terminal of the first primary winding P 1  and the controllable current source  1030 . The first primary switch  1042  and the second primary switch  1044  are coupled to the first terminal of the first primary winding P 1  through a common node  1046 . 
     In some embodiments, a first secondary diode  1058  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary diode  1060  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  1056 . In some embodiments, an output capacitor  1062  is coupled in parallel between the transformer  1040  and the output of the isolated buck-type converter. In some embodiments, a primary inductor  1054  is coupled between the first terminal of the first primary winding P 1  and the common node  1046  of the first primary switch  1042  and the second primary switch  1044 . In some embodiments, a first resonant capacitor  1048  and a second resonant capacitor  1050  are coupled between the controllable DC current source  1030  and the transformer  1040 . In some embodiments, first resonant capacitor  1048  and second resonant capacitor  1050  are coupled to the second terminal of the first primary winding P 1  through a common node  1052 . Together with the primary inductor  1054 , first resonant capacitor  1048  and second resonant capacitor  1050  form a resonant tank circuit. 
       FIG. 11  is a schematic diagram of another embodiment of a two stage resonant converter  1100  employing a half-bridge converter in accordance with the principles of the present invention. The two stage resonant converter  1100  comprises a controllable DC current source  1130  and a transformer  1140 . The transformer  1140  comprises a first primary winding P 1 , a first secondary winding S 1 , and a second secondary winding S 2 . A first primary switch  1142  is coupled between a first terminal of the first primary winding P 1  and the controllable current source  1130 . A second primary switch  1144  is coupled between the first terminal of the first primary winding P 1  and the controllable current source  1130 . The first primary switch  1142  and the second primary switch  1144  are coupled to the first terminal of the first primary winding P 1  through a common node  1146 . 
     In some embodiments, a first secondary diode  1156  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary diode  1158  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  1154 . In some embodiments, an output capacitor  1162  is coupled in parallel between the transformer  1140  and the output of the isolated buck-type converter. In some embodiments, a second inductor  1160  is coupled between a common node of the first and second secondary windings S 1 , S 2  and the output of the isolated buck-type converter. In some embodiments, a first resonant capacitor  1148  and a second resonant capacitor  1150  are coupled between the controllable DC current source  1130  and the transformer  1140 . In some embodiments, first resonant capacitor  1148  and second resonant capacitor  1150  are coupled to the second terminal of the first primary winding P 1  through a common node  1152 . Together with the secondary inductor  1160 , first resonant capacitor  1148  and second resonant capacitor  1150  form a resonant tank circuit. 
       FIG. 12  is a schematic diagram of yet another embodiment of a two stage resonant converter  1200  employing a half-bridge converter in accordance with the principles of the present invention. The two stage resonant converter  1200  comprises a controllable DC current source  1230  and a transformer  1240 . The transformer  1240  comprises a first primary winding P 1 , a first secondary winding S 1 , and a second secondary winding S 2 . A first primary switch  1242  is coupled between a first terminal of the first primary winding P 1  and the controllable current source  1230 . A second primary switch  1244  is coupled between the first terminal of the first primary winding P 1  and the controllable current source  1230 . The first primary switch  1242  and the second primary switch  1244  are coupled to the first terminal of the first primary winding P 1  through a common node  1246 . 
     In some embodiments, a first secondary switch  1258  is coupled between the first secondary winding S 1  and the output of the isolated buck-type converter, and a second secondary switch  1260  is coupled between the second secondary winding S 2  and the output of the isolated buck-type converter. In some embodiments, the output of the isolated buck-type converter is coupled to a load resistor  1256 . In some embodiments, an output capacitor  1264  is coupled in parallel between the transformer  1240  and the output of the isolated buck-type converter. In some embodiments, a ground terminal  1262  is coupled between the transformer  1240  and the output of the isolated buck-type converter. In some embodiments, a primary inductor  1254  is coupled between the first terminal of the first primary winding P 1  and the common node  1246  of the first primary switch  1242  and the second primary switch  1244 . In some embodiments, a first resonant capacitor  1248  and a second resonant capacitor  1250  are coupled between the controllable DC current source  1230  and the transformer  1240 . In some embodiments, first resonant capacitor  1248  and second resonant capacitor  1250  are coupled to the second terminal of the first primary winding P 1  through a common node  1252 . Together with the primary inductor  1254 , first resonant capacitor  1248  and second resonant capacitor  1250  form a resonant tank circuit. 
       FIG. 13  is a schematic diagram of one embodiment of a controllable DC current source  1300  in accordance with the principles of the present invention. The controllable DC current source  1300  comprises an input voltage supply  1310 , an input capacitor  1320 , a first stage diode  1330 , a first stage switch  1340 , and a first stage inductor  1350 . Input capacitor  1320  is coupled in parallel with input voltage supply  1310 , which generates an input supply voltage Vin, and with first stage diode  1330 . In some embodiments, first stage switch  1340  is an N-channel MOSFET in enhancement mode. However, it is contemplated that other types of switches can be used as well. A first terminal (or drain) of first stage switch  1340  is coupled to the positive terminal of input voltage supply  1310  and a first terminal of input capacitor  1320 . A third terminal (or source) of first stage switch  1340  is coupled to the cathode terminal of first stage diode  1330  and to a first terminal of first stage inductor  1350 . A second terminal of input capacitor  1320  is coupled to the negative terminal of input voltage supply  1310  and to the anode terminal of first stage diode  1330 . Additionally, the anode terminal of first stage diode  1330  is also coupled to the negative terminal of input voltage supply  1310 . Controllable current source  1300  can be used for any of the controllable DC current sources previously shown and discussed with respect to  FIGS. 2-3  and  5 - 12 . Furthermore, it is contemplated that the present invention can employ alternative embodiments for the controllable current source other than the design illustrated in  FIG. 13 . 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be readily apparent to one skilled in the art that other various modifications may be made and equivalents may be substituted for elements in the embodiments chosen for illustration without departing from the spirit and scope of the invention as defined by the claims.