Power distribution architecture with series-connected bus converter

Abstract

Apparatus for power conversion are provided. One apparatus includes a power converter including an input circuit and an output circuit. The power converter is configured to receive power from a source for providing power at a DC source voltage V.sub.S. The power converter is adapted to convert power from the input circuit to the output circuit at a substantially fixed voltage transformation ratio K.sub.DC=V.sub.OUT/V.sub.IN at an output current, wherein V.sub.IN is an input voltage and V.sub.OUT is an output voltage. The input circuit and at least a portion of the output circuit are connected in series across the source, such that an absolute value of the input voltage V.sub.IN applied to the input circuit is approximately equal to the absolute value of the DC source voltage V.sub.S minus a number N times the absolute value of the output voltage V.sub.OUT, where N is at least 1.

Claims

1. An apparatus comprising: a switching power converter including an input circuit and an output circuit, the switching power converter being constructed and arranged to convert power from the input circuit to the output circuit at a substantially fixed ratio, K.sub.DC, at an output current, wherein an input voltage V.sub.IN is applied to the input circuit and an output voltage V.sub.OUT is produced by the output circuit of the switching power converter, and wherein the substantially fixed ratio can be represented as K.sub.DC=V.sub.OUT/V.sub.IN; the input circuit and the output circuit being coupled by a transformer, the transformer including at least a first winding and a second winding; the input circuit including a first primary power switch and the first winding; the output circuit including at least one secondary power switch and the second winding; the input circuit and at least a portion of the output circuit of the switching power converter being connected in series across a source such that an absolute value of the input voltage V.sub.IN applied to the input circuit is approximately equal to the absolute value of a DC source voltage V.sub.S minus a number N times the absolute value of the output voltage V.sub.OUT, where N is at least 1; a series resonant circuit including the first winding and at least one resonant capacitor connected in series with the first winding, the series resonant circuit having a characteristic resonant frequency and a characteristic resonant period, the first primary power switch being connected to drive the series resonant circuit; and a switch controller adapted to operate the first primary power switch in a series of converter operating cycles, each converter operating cycle characterized by two power transfer intervals of essentially equal duration each interval having a duration less than the characteristic resonant period, during which one or more primary power switches are ON and power is transferred from the input circuit to the output circuit via the transformer.

2. The apparatus of claim 1, wherein the switching power converter is a self-contained assembly adapted to be installed as a unit.

3. The apparatus of claim 1, wherein both the input circuit and the output circuit comprise full-bridge circuits.

4. The apparatus of claim 1, wherein the output circuit includes a plurality of power switches, wherein the plurality of power switches in the output circuit are controlled to turn ON and OFF at times of essentially zero current.

5. The apparatus of claim 1, wherein the output circuit includes a plurality of power switches, wherein the plurality of power switches in the output circuit are controlled to turn ON and OFF at times of essentially zero voltage.

6. The apparatus of claim 1, wherein the input circuit includes a plurality of power switches, wherein the plurality of power switches in the input circuit are controlled to turn ON and OFF at times of essentially zero voltage.

7. The apparatus of claim 1, wherein the power transfer interval is essentially equal to half of the characteristic resonant period.

8. The apparatus of claim 1, wherein the output current is a sinusoidal half wave.

9. The apparatus of claim 1, wherein the first primary power switch is switched with a first duty cycle and the secondary power switch is switched with a second duty cycle, wherein the first duty cycle and the second duty cycle are fixed and essentially equal.

10. The apparatus of claim 1, wherein the switch controller operates the first primary power switch in the series of converter operating cycles at an operating frequency, wherein the operating frequency is a function of the characteristic resonant frequency.

11. The apparatus of claim 1, wherein the output circuit comprises two secondary windings, each secondary winding comprising a same number of secondary turns.

12. The apparatus of claim 11, wherein: the first winding is characterized by a first number of turns; the second winding is characterized by a second number of turns; and the substantially fixed ratio, K.sub.DC, is a function of a ratio of the first number of turns to the second number of turns.

13. The apparatus of claim 12, wherein the first and second windings are connected to form a center-tap winding with two terminal ends and a center tap.

14. The apparatus of claim 13, wherein the second winding is the center-tap winding with the two terminal ends and the center tap.

15. The apparatus of claim 14, wherein N is equal to approximately 2, such that the input voltage V.sub.IN is approximately equal to the DC source voltage VS minus 2 times the output voltage V.sub.OUT.

16. The apparatus of claim 15, wherein each of the two terminal ends of the second winding are selectively connected to a common terminal through a respective secondary power switch and the center tap is connected to supply the output voltage V.sub.OUT with respect to the common terminal.

17. The apparatus of claim 16, wherein the input circuit includes a second primary power switch, and the first and second primary power switches are connected together to drive a first end of the series resonant circuit.

18. The apparatus of claim 17, wherein the input circuit further comprises at least two additional primary power switches connected together to drive a second end of the series resonant circuit.

19. An method comprising: providing a switching power converter including an input circuit and an output circuit, the switching power converter being constructed and arranged to convert power from the input circuit to the output circuit at a substantially fixed ratio, K.sub.DC, at an output current, wherein an input voltage V.sub.IN is applied to the input circuit and an output voltage V.sub.OUT is produced by the output circuit of the switching power converter, and wherein the substantially fixed ratio can be represented as K.sub.DC=V.sub.OUT/V.sub.IN; the input circuit and the output circuit being coupled by a transformer, the transformer including at least a first winding and a second winding; the input circuit including a first primary power switch and the first winding; the output circuit including at least one secondary power switch and the second winding; the input circuit and at least a portion of the output circuit of the switching power converter being connected in series across a source such that an absolute value of the input voltage V.sub.IN applied to the input circuit is approximately equal to the absolute value of a DC source voltage V.sub.S minus a number N times the absolute value of the output voltage V.sub.OUT, where N is at least 1; providing a series resonant circuit including the first winding and at least one resonant capacitor connected in series with the first winding, the series resonant circuit having a characteristic resonant frequency and a characteristic resonant period, the first primary power switch being connected to drive the series resonant circuit; and providing a switch controller adapted to operate the first primary power switch in a series of converter operating cycles, each converter operating cycle characterized by two power transfer intervals of essentially equal duration each interval having a duration less than the characteristic resonant period, during which one or more primary power switches are ON and power is transferred from the input circuit to the output circuit via the transformer.

20. The method of claim 19, wherein the switching power converter is a self-contained assembly adapted to be installed as a unit.

21. The method of claim 19, wherein both the input circuit and the output circuit comprise full-bridge circuits.

22. The method of claim 19, wherein N is equal to approximately 2, such that the input voltage V.sub.IN is approximately equal to the DC source voltage VS minus 2 times the output voltage V.sub.OUT.

23. The method of claim 19, wherein the input circuit includes a second primary power switch, and the first and second primary power switches are connected together to drive a first end of the series resonant circuit.

24. The apparatus of claim 19, wherein the input circuit further comprises at least two additional primary power switches connected together to drive a second end of the series resonant circuit.

25. An apparatus comprising: a switching power converter including an input circuit and an output circuit, the switching power converter being constructed and arranged to convert power from the input circuit to the output circuit at a substantially fixed ratio, K.sub.DC, at an output current, wherein an input voltage V.sub.IN is applied to the input circuit and an output voltage V.sub.OUT is produced by the output circuit of the switching power converter; the input circuit and the output circuit being coupled by a transformer, the transformer including at least a first winding and a second winding; the input circuit including a first primary power switch and the first winding; the output circuit including at least one secondary power switch and the second winding; the input circuit and at least a portion of the output circuit of the switching power converter being connected in series across a source; a series resonant circuit including the first winding and at least one resonant capacitor connected in series with the first winding, the series resonant circuit having a characteristic resonant frequency and a characteristic resonant period, the first primary power switch being connected to drive the series resonant circuit; and a switch controller adapted to operate the first primary power switch in a series of converter operating cycles, each converter operating cycle characterized by two power transfer intervals of essentially equal duration each interval having a duration less than the characteristic resonant period, during which one or more primary power switches are ON and power is transferred from the input circuit to the output circuit via the transformer.

26. The apparatus of claim 25, wherein the input circuit includes a second primary power switch, and the first and second primary power switches are connected together to drive a first end of the series resonant circuit.

27. The apparatus of claim 26, wherein the input circuit further comprises at least two additional primary power switches connected together to drive a second end of the series resonant circuit.

28. The apparatus of claim 25, wherein: the first winding is characterized by a first number of turns; the second winding is characterized by a second number of turns; and the first number of turns is equal to the second number of turns.

29. The apparatus of claim 28, wherein the first and second windings are connected to form a center-tap winding with two terminal ends and a center tap.

30. The apparatus of claim 29, wherein the terminal ends of the first and second windings are selectively connected to a common terminal through respective secondary power switches.

31. The apparatus of claim 30, wherein the center tap is connected to supply the output voltage V.sub.OUT with respect to the common terminal.

32. The apparatus of claim 29, wherein the bus converter is a self-contained assembly adapted to be installed as a unit.

33. The apparatus of claim 32, wherein the output circuit includes a plurality of power switches, wherein the plurality of power switches in the output circuit are controlled to turn ON and OFF at times of essentially zero voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic block diagram of a prior art IBA power distribution system according to an illustrative embodiment.

(2) FIG. 2 shows a functional block diagram of a series-connected DC Transformer according to an illustrative embodiment.

(3) FIG. 3 shows a schematic diagram of a new power distribution architecture according to an illustrative embodiment.

(4) FIG. 4 shows a schematic diagram of an isolated SAC-based DC Transformer according to an illustrative embodiment.

(5) FIG. 5 shows a schematic diagram of a series-connected SAC-based DC Transformer according to an illustrative embodiment.

(6) FIG. 6 shows a schematic diagram of a series-connected SAC-based DC Transformer having a center-tapped winding in the output circuit according to an illustrative embodiment.

(7) FIG. 7 shows a schematic diagram of a series-connected DC Transformer for receiving power from a negative input source and delivering power at a positive output voltage according to an illustrative embodiment.

DETAILED DESCRIPTION

(8) Power Distribution Architecture

(9) A power distribution system 50 is shown in FIG. 3 having a primary power source 51 delivering power via a connection 52 to a front-end power-processing unit 53. The primary power source 51 may be an AC utility line, and the front end unit 53 may be a power conversion stage that converts power from the power source 51 delivering power at a relatively high but safe DC voltage to a power distribution bus 55, e.g. the DC voltage may vary from a minimum, e.g. 38 Volts, to a maximum, e.g. 55 Volts. Preferably, the front-end unit 53 provides voltage step down and isolation and may optionally provide power factor correction, regulation, or both. An optional backup power system 54 is shown connected to the power distribution bus 55 to provide power in the event of a loss of power from the primary power source 52. The backup power system may include batteries, a charger for maintaining the batteries, and a switchover mechanism that connects the batteries to the bus in response to predetermined events, such as a decline in voltage or loss of power from the output of the front end 53 or the primary power source 51.

(10) One or more bus converters, e.g. bus converters 56, 57, may be connected to the power distribution bus 55 downstream from the front end 53 as shown in the example of FIG. 3 to convert power received from the relatively high voltage power distribution bus 55 for delivery to a respective lower voltage bus. As shown, bus converters 56 and 57 respectively supply power to buses 58 and 59 at voltages, e.g. at or near the requisite load voltages, that are lower than the voltage of the power distribution bus 55, providing step-down voltage transformation. The bus converters 56, 57 are generally separated by a distance from their respective regulators 60, 61. For example, in a typical system, one or more system circuit boards housed in a common enclosure may each include one or more bus converters, preferably located near the edge of, or other location on, the board where power connections are made to the board. A down-stream regulator receiving power from the bus converter(s) may be preferably located adjacent to the circuitry, e.g. a processor, ASIC, or other circuitry, to which it or they supply power. The physical distance separating the bus converter and a respective down-stream regulator in such an example may range from as much as a dimension of the system circuit board, i.e. a diagonal dimension where the bus converter and regulator located at opposite corners, a length or width dimension where they are located at opposite edges, a half-length or width where one is situated closer to the middle and the other is at an edge, etc. In another example, a bus converter may be located off of the system board in which case the electrical distance could be greater than a dimension of the system board. Naturally, the distance separating the bus converter and a respective down-stream regulator will depend on the system layout. However, a bus converter housed in a self-contained assembly adapted to be installed as a unit at a location remote from the down-stream regulator(s) may be separated by a distance from a down-stream regulator regardless of their respective mounting locations at the system level.

(11) The output of each bus converter 56, 57 may, in turn, provide power via its respective bus 58, 59 to a respective plurality of regulators, preferably at or near the point of load, such as point-of-load switching voltage regulators 60, 61. It should be understood that although two bus converters 56, 57 are shown in the example of FIG. 3, any number of bus converters, e.g. one, may be used. Similarly, although regulators 60 and 61 are shown in FIG. 3 as comprising a plurality of individual regulators, any suitable number of regulators, e.g. one, may be connected to a particular bus converter within the constraints of the physical devices used. The regulators 60, 61 may supply power to respective loads (not shown). The loads can be a variety of devices, including integrated circuits and electromechanical devices (such as storage and cooling devices).

(12) The bus converters 56, 57 shown in the system of FIG. 3, however, preferably do not provide galvanic isolation between their respective output busses 58, 59 and the power distribution bus 55 as described in additional detail below.

(13) Series-Connected DC Transformer

(14) Referring to FIG. 2, a functional block diagram of a series-connected power conversion system 20 suitable for use as a bus converter in the power distribution system 50 of FIG. 3 is shown. The power conversion system 20 includes an input 21 for receiving power from a source at a source voltage, V.sub.S, and an output 22 for delivering power to a load at an output voltage, V.sub.O, that is less than V.sub.S, and a DC Transformer 25. The DC Transformer 25 may be implemented preferably using the Sine-Amplitude Converter (“SAC”) topologies and timing architectures described in Vinciarelli, Factorized Power Architecture and Point of Load Sine Amplitude Converters, U.S. Pat. No. 6,930,893 and in Vinciarelli, Point of Load Sine Amplitude Converters and Methods, U.S. Pat. No. 7,145,786 both assigned to VLT., Inc. and incorporated here in their entirety by reference (hereinafter the “SAC Patents”). Alternatively, other converter topologies, such as hard-switching, fixed ratio DC-DC converters, may be used. The DC Transformer 25 converts power received from its input 23 (distinguished from the input 21 of the bus converter 20) at an input voltage, V.sub.IN, for delivery to its output 24 at an output voltage, V.sub.OUT, using an essentially fixed voltage gain or voltage transformation ratio.

(15) The voltage gain or voltage transformation ratio of a system as defined generally herein is the ratio of its output voltage to its input voltage at a specified current such as an output current. For the system 20 in FIG. 2, the voltage transformation ratio may be expressed as K.sub.SYS=V.sub.O/V.sub.S @ I.sub.L. Similarly, the voltage transformation ratio of the DC Transformer 25 may be stated as K.sub.DC=V.sub.OUT/V.sub.IN @ I.sub.O. Note that the system output voltage, V.sub.O, and the DC Transformer output voltage, V.sub.OUT, are the same in the configuration shown. However, the input 23 and output 24 of the DC Transformer 25 are shown in a series-connected configuration across the system input 21. As a result, the input voltage, V.sub.IN, to the DC Transformer input 23 is less than the input voltage, V.sub.S, to the system input 21 by an amount equal to the output voltage:
V.sub.IN=V.sub.S−V.sub.O.  (1)

(16) Similarly as shown in FIG. 2, the current, I.sub.L, drawn by the load from the system output 22 is greater than the current produced at the output 24 of the DC Transformer 25 by an amount equal to the input current:
I.sub.O=I.sub.L−I.sub.IN.  (2)

(17) The system voltage transformation ratio, K.sub.SYS, using the series-connected DC Transformer 25, may be expressed as a function of the DC Transformer voltage transformation ratio, K.sub.DC:
K.sub.SYS=K.sub.DC/(K.sub.DC+1)  (3)

(18) The above equation (3) may be rearranged to express the DC Transformer 25 voltage transformation ratio, K.sub.DC, required in a series-connected system as a function of the system voltage transformation ratio, K.sub.SYS:
K.sub.DC=K.sub.SYS/(1−K.sub.SYS)  (4)

(19) Referring to FIG. 4, an isolated SAC that may be utilized for DC Transformer 25, according to one embodiment, is shown having a full-bridge input circuit, including switches S1, S2, S3, and S4, connected to drive the resonant circuit including capacitor C and the input winding, having N1 turns, with the input voltage V.sub.IN. The isolated SAC is shown having a full-bridge output circuit, including switches S5, S6, S7, and S8, connected to rectify the voltage impressed across the output winding, having N2 turns, and delivering the output voltage, V.sub.o. The voltage transformation ratio of the SAC will be essentially a function of the turns ratio: K.sub.DC==V.sub.O/V.sub.IN=N2/N1.

(20) A series-connected SAC 200 is shown in FIG. 5. By way of comparison, the series-connected SAC 200 uses the same full-bridge input circuit topology, including switches S1, S2, S3, and S4, driving the resonant circuit including capacitor C and the input winding, having N1 turns, with the input voltage V.sub.IN. SAC 200 also uses the same full-bridge output topology, including switches S5, S6, S7, and S8, connected to rectify the voltage impressed across the output winding, having N2 turns, and delivering the output voltage, V.sub.O. The voltage transformation ratio of the series-connected SAC 200 from the input circuit to output circuit is also essentially a function of the transformer turns ratio N2/N1 and the same as the isolated SAC 25 in FIG. 4: K.sub.DC=V.sub.O/V.sub.IN=N2/N1. However, when evaluated in terms of the system, i.e. using V.sub.S applied across the series-connected input and output, the voltage transformation ratio becomes: K.sub.SYS=V.sub.O/V.sub.S=N2/(N2+N1).

(21) Many contemporary applications use a voltage transformation ratio equal to ⅕ requiring an odd transformer turns ratio (N2/N1=⅕) which is generally not optimal. Referring to equation (4) above, the K.sub.SYS=⅕ bus converter may be implemented using a K.sub.DC=¼ series-connected topology (e.g. as shown in FIGS. 2, 4, and 5), allowing the use of an even, i.e. 1:4, turns ratio in the transformer. An even transformer turns ratio may provide greater transformer layout flexibility and efficiency.

(22) Note that the series-connected converter 200 may be implemented by connecting an off-the-shelf isolated DC Transformer, such as the isolated converter shown in FIG. 4, as shown in FIG. 2. Alternatively, the converter 200 may be implemented as series-connected input and output circuits, e.g. as shown in FIGS. 5, 6, and 7 discussed below, in an integrated converter, optionally providing greater power density eliminating the isolation imposed design constraints, eliminating control circuit bias currents from flowing through to the output and the potential need for an output clamp, and providing system-ground referenced control circuitry (not shown) for interface signals that are referenced to ground rather than the output for the reconfigured off-the-shelf isolated converter.

(23) Connecting the input and output of the DC Transformer 25 in series eliminates galvanic isolation between the input and output of the series-connected bus converter 20, which is counterintuitive. However, when used in the architecture of FIG. 3, isolation is deployed at an intermediate stage where the isolation may be superfluous. The architecture of FIG. 3, therefore, trades isolation at this stage for efficiency gain and reduced component stress. If isolation is required, e.g. for safety reasons, in the architecture of FIG. 3, it may preferably be provided by an upstream power conversion stage such as the front-end converter 53.

(24) Efficiency

(25) The power processed by the isolated SAC shown in FIG. 4 may be compared with that of the series-connected SAC 200 (FIG. 5) by summing the product of maximum voltage across (V.sub.n) and average current (I.sub.n) through each switch (n=1 through 8).

(26) $\begin{array}{cc}{P}_{\mathrm{Processed}}=\underset{n=1}{\overset{n=8n}{.Math.}}\left(\mathrm{Vn}*\mathrm{In}\right)& \left(5\right)\end{array}$

(27) Each input switch (S1, S2, S3 and S4) in the full bridge input circuits (FIGS. 4, 5) is subjected to the input voltage, V.sub.IN, (distinguished from the source voltage V.sub.S) and an average of one half of the input current, I.sub.IN. The sinusoidal nature of the current in the SAC topology represents a difference between the RMS and average currents, which is unimportant for the following comparison between two converters using the same topology. The power processed by the input circuits is:
P.sub.IN=2*V.sub.IN*I.sub.IN  (6)

(28) Similarly, each output switch (S5, S6, S7 and S8) in the full bridge output circuit of FIG. 4 will be subjected to the full output voltage, V.sub.O, and will carry an average of one half of the output current, I.sub.O. Note that the output current in the case of the isolated converter is equal to the load current, I.sub.L and in the case of the series-connected converter (discussed below) is not. The power processed by the output circuits may therefore be reduced to:
P.sub.OUT=2*V.sub.O*I.sub.O  (7)

(29) Combining equations (6) and (7) and making the appropriate substitutions using K.sub.DC=V.sub.O/V.sub.IN and the corollary I.sub.IN=K.sub.DC*I.sub.O, the total power processed by the converters reduces to:
P=4*V.sub.O*I.sub.O  (8)

(30) In the isolated converter of FIG. 4, the output current equals the load current (I.sub.O=I.sub.L), therefore, the power processed by the isolated converter, P.sub.ISO, may be reduced to the following function of load power, P.sub.Load=V.sub.O*I.sub.L:
P.sub.ISO=4*P.sub.Load  (9)

(31) Neglecting fixed losses in the converter, the input current may be expressed as a function of the output current and voltage transformation ratio as follows:
I.sub.IN=I.sub.O*K.sub.DC  (10)

(32) Combining equations (2), (4), and (10), the output current of the series-connected converter may be expressed as a function of load current and voltage transformation ratio as follows:
I.sub.O-Series=I.sub.L*(1−K.sub.SYS)  (11)

(33) Substituting equation (11) into equation (8) produces the total power processed by the series-connected converter as a function of load power (P.sub.Load=V.sub.O*I.sub.L) and system voltage transformation ratio:
P.sub.SERIES=4*P.sub.Load*(1−K.sub.SYS)  (12)

(34) Accordingly, the efficiency advantage of the series-connected converter over the isolated converter—the ratio of equations (12) and (9)—reduces to:
P.sub.SERIES/P.sub.ISO=(1−K.sub.SYS)  (13)

(35) From equation (13) it can be seen that the series-connected converter offers a significant efficiency advantage. Consider a typical example for comparison, using a bus converter to convert power from a nominal 50 Volt power distribution bus for delivery to a 10 volt load (K.sub.SYS=⅕) at 100 amps: the series-connected converter processes only 80% of the power, offering a 20% efficiency savings compared to the isolated converter.

(36) In a typical isolated DC Transformer, like most DC-DC converters, the control circuitry is configured to operate from power drawn from the input producing a quiescent component of the input current. Use of such a converter, e.g. an off-the-shelf DC Transformer, in a series-connected configuration could, therefore, allow the quiescent input current to flow unregulated into a load connected to the output, which would be problematic while the power train is not operating and, therefore, incapable of regulating the output voltage. It may, for that reason, be desirable to clamp the output voltage using a zener diode, such as zener diode 26 in FIG. 2, or other clamp circuit or device appropriately scaled in breakdown voltage and power dissipation to carry the quiescent input current, protecting the load and perhaps the output circuitry of the converter. Integrating the series-connected input and output circuitry into a non-isolated converter topology such as shown in FIGS. 5, 6, and 7 affords the opportunity to configure the control circuitry to draw power from the input to ground preventing that component of the input current from flowing out to the load. Additionally, a DC blocking capacitor may be used in the power train to avoid leakage current from flowing from the input to the output. One or both of the above measures may be used to avoid the need to clamp the output.

(37) Configuring the control circuitry to reference the system ground in the integrated converter (rather than the input return in the off-the-shelf isolated converter) easily allows any interface signals to be ground-referenced (rather than output referenced) which is advantageous from the perspective of the system integrator.

(38) Center-Tap Secondary

(39) Another series-connected SAC 210 is shown in FIG. 6. By way of comparison, the series-connected SAC 210 uses the same full-bridge input circuit topology, including switches S1, S2, S3, and S4, driving the resonant circuit including capacitor C and the input winding, having N1 turns, with the input voltage V.sub.IN, as shown in FIG. 5. However, a center-tap output winding, having 2*N2 turns, is used in the output circuit, which includes switches S5, S6, S7, and S8, connected to rectify the voltage impressed across the output windings and delivering the output voltage, V.sub.O. The system voltage transformation ratio of the series-connected SAC 210 (FIG. 6) is essentially a function of the transformer turns ratio: K.sub.SYS=V.sub.O/V.sub.SYS=N2/(N1+2*N2); as is the voltage transformation ratio from input circuit to output circuit: K.sub.DC=V.sub.O/V.sub.IN=N2/N1.

(40) The converter 210 of FIG. 6 differs from the series-connected converter 200 (FIG. 5) in that the input voltage, V.sub.IN, presented to the input circuit is equal to the source voltage, V.sub.S, reduced by twice the output voltage, V.sub.O:
V.sub.IN-210=V.sub.S−2V.sub.O  (14)

(41) as suggested by the addition of N2 turns in the output winding of the transformer. Also, each output switch (S5, S6, S7 and S8) in the converter 210 is subjected to twice the output voltage, V.sub.O, with the upper output switches (S5 and S7) each carrying an average of half of the input current, I.sub.IN, and the lower output switches (S6 and S8) each carrying an average of half of the difference between the load current, I.sub.L, and the input current, I.sub.IN. Using the same analysis as described above, summing the product of maximum voltage across (V.sub.n) and average current (I.sub.n) through each switch (N=1 through 8), the total power processed by the converter 210 of FIG. 6 is:
P.sub.210=2*V.sub.IN*I.sub.IN2*V.sub.O*I.sub.IN+2*V.sub.O*(I.sub.L−I.sub.IN)  (15)

(42) Using the system voltage transformation ratio, K.sub.SYS=V.sub.O/V.sub.S in equation (14), the input voltage may be expressed as:
V.sub.IN-210=V.sub.O*((1/K.sub.SYS)−2)  (16)

(43) Recognizing that in an ideal converter the input power equals the output power V.sub.S*I.sub.IN=V.sub.O*I.sub.L the input current may be expressed as:
I.sub.IN=K.sub.SYS*I.sub.L  (17)

(44) Making the appropriate substitutions into equation (15), the total power processed by series-connected converter 210 (FIG. 6) reduces to:
P.sub.210=4*V.sub.O*I.sub.L*(1−K.sub.SYS)  (18)

(45) which may be further reduced to express the total power processed by the series-connected converter 210 using a center-tap output winding as shown in FIG. 6 as a function of load power (P.sub.Load=V.sub.O*I.sub.L) and system voltage transformation ratio:
P.sub.210=4*P.sub.Load*(1−K.sub.SYS)  (19)

(46) Which is the same result obtained in equation (12) above for the series-connected converter 200 in FIG. 5.

(47) There may be certain advantages of one series-connected topology over the other depending upon the application. For example, the transformer in the converter 200 (FIG. 5) has N2 fewer turns than in the transformer of the converter 210 (FIG. 6) offering reduced winding losses. However, the input switches (S1, S2, S3 and S4) in the converter 210 (FIG. 6) are exposed to lower voltages than in the converter 200 (FIG. 5) which may afford lower switch conduction losses. Also, two of the output switches (S5 and S7) in converter 210 (FIG. 6) carry much less current and may be implemented with smaller and more cost effective devices than in converter 200 (FIG. 5).

(48) Negative Input-Positive Output

(49) Referring to FIG. 7, another series-connected SAC-based converter 215 is shown configured to receive a negative source voltage, V.sub.S, and deliver a positive output voltage. (The topology shown in FIG. 7 may alternatively be adapted to receive a positive source voltage and deliver a negative output voltage.) Converter 215 may be viewed as a variation of the converter 210 (FIG. 6) in which the input and output circuit positions have been rearranged with the output terminal serving as the common terminal. The converter 215 of FIG. 7 differs from the converter 210 (FIG. 6) in that the absolute value of the input voltage, V.sub.IN, presented to the input circuit is equal to the absolute value of the source voltage, V.sub.S, reduced by the absolute value of the output voltage, V.sub.O (compared to twice the output voltage in FIG. 6) because of the polarity change from input to output:
|V.sub.IN-215|=|V.sub.S|−|V.sub.O|  (20)
as also suggested by the transformer configuration. Also, the upper output switches (S5 and S7) each carry an average of half of the output current, I.sub.O, which equals the load current, I.sub.L in FIG. 7, compared to the difference between the load current, I.sub.L, and the input current, I.sub.IN, in FIG. 6. Once again, summing the product of maximum voltage across (V.sub.n) and average current (I.sub.n) through each switch (N=1 through 8) as described above, the total power processed by the converter 215 of FIG. 7 is:
P.sub.215=2*V.sub.IN*I.sub.IN+2*V.sub.O*I.sub.IN+2*V.sub.O*I.sub.L  (21)

(50) which, when reduced using equations (17) and (20), becomes:
P.sub.215=4*P.sub.Load  (22)

(51) A comparison of the power processed by the converter 215 (equation (22); FIG. 7) with the power processed by the isolated converter 25 (equation (9); FIG. 4) may indicate no efficiency advantage, however, the input switches (S1, S2, S3 and S4) in the series-connected converter 215 of FIG. 7 are subjected to lower voltages potentially affording use of better figure of merit switches leading to potential efficiency improvements. Furthermore, the absence of isolation-related design constraints in such an integrated converter may be used to increase power density.

(52) The converters 20 (FIG. 2), 200 (FIG. 5), 210 (FIG. 6), and 215 (FIG. 7) are examples of a class of series-connected converters in which at least a portion of the output circuit is connected in series with the input circuit such that the absolute value of the voltage, V.sub.IN, presented to the input circuit is equal to the absolute value of the source voltage V.sub.S, minus N times the absolute value of the output voltage, V.sub.O, where the value of N is at least 1:
|V.sub.IN|=|V.sub.S|−N*|V.sub.O|  (23)

(53) The value of N will vary depending upon the converter topology used, e.g. a center-tap secondary or not, polarity reversing or not, etc. In the examples described above: N=1 for converters 20 (FIG. 2), 200 (FIG. 5), and 215 (FIG. 7) and N=2 for converter 210 (FIG. 6) as shown in equation 14. Although a full bridge switch configuration is preferred for its superior noise performance, half-bridge switch configurations may also be deployed in the input circuitry, the output circuitry, or both.

(54) The disclosure is described above with reference to drawings. These drawings illustrate certain details of specific embodiments that implement the systems, apparatus, and/or methods of the present disclosure. However, describing the disclosure with drawings should not be construed as imposing on the disclosure any limitations that may be present in the drawings. No claim element herein is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.” Furthermore, no element, component or method step in the present disclosure is intended to be dedicated to the public, regardless of whether the element, component or method step is explicitly recited in the claims.

(55) It should be noted that although the disclosure provided herein may describe a specific order of method steps, it is understood that the order of these steps may differ from what is described. Also, two or more steps may be performed concurrently or with partial concurrence. It is understood that all such variations are within the scope of the disclosure.

(56) The foregoing description of embodiments of the disclosure have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principals of the disclosure and its practical application to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated.