WO2013173160A2

WO2013173160A2 - Switched mode power converter for high voltage applications

Info

Publication number: WO2013173160A2
Application number: PCT/US2013/040303
Authority: WO
Inventors: Chenghua Zhu
Original assignee: General Electric Company
Priority date: 2012-05-15
Filing date: 2013-05-09
Publication date: 2013-11-21
Also published as: CN103427633A; WO2013173160A3

Abstract

A dual-buck boost switching converter topology for use in high voltage power factor control applications is disclosed. The new topology includes dual buck converters cascaded with a boost converter. The dual buck converters are configured to equally share the input voltage such that voltage stresses on the buck converter components is significantly reduced. Voltage sharing is realized through the use of multiple magnetically coupled energy storage inductors and a pair of sharing diodes. The sharing diodes provide current paths for balancing voltages applied to the buck converter components.

Description

SWITCHED MODE POWER CONVERTER FOR HIGH VOLTAGE APPLICATIONS

BACKGROUND Field

[0001] The present disclosure generally relates to DC-DC converters for power factor control, and more particularly to buck-boost converter topologies for high input voltage applications.

Description of Related Art

[0002] Many new electronic products require regulated input power with near unity power factor for efficient and proper operation. It is common to use switched mode DC-DC converters to provide power factor control (PFC) as part of the input power conditioning. Switched mode converters convert one direct current (DC) voltage to another by chopping the input voltage with switches, storing energy from the chopped voltage in magnetics, and then releasing the stored energy to the output in a controlled fashion. Adjusting the duty cycle of the switches, i.e. changing the percentage of each switching cycle during which the switches conduct, allows regulation of the output power. When a DC-DC converter receives its input power from the local mains supply, it is important for the converter to have good power factor control (PFC) to maintain a high (close to unity) power factor and to have a distortion free input current waveform. A variety of converter topologies and control methods have been developed to achieve PFC. For example, active power factor correction is where feedback and feedforward techniques are used in a switched mode converter to ensure input current waveforms track closely with input voltage waveforms. PFC is typically provided using a boost converter topology because a boost converter is easy to implement. Buck Boost topologies are gaining popularity because they offer inherent benefits for PFC. Most converters and active power factor correction apparatus are designed for use with source voltages between 90V-305V, to be compatible with typical 50Hz and 60Hz mains voltages available in India, Japan, North America, United Kingdom, and Europe. However, many industrial applications use a higher voltage range of around 347V-480V as is available from widely used 480V AC systems. These higher voltage applications are rarely considered by power supply designers because switching devices and other components for use in higher voltage applications are more costly to manufacture and, due to lower demand, are more difficult to obtain. Thus there is a need for a new high voltage switched mode converter topology that has good PFC and uses less costly and readily available lower voltage components.

[0003] Referring to Figure 1, there can be seen a schematic of a cascaded buck-boost converter 100 as is known in the art. The cascaded buck-boost converter 100 includes a conventional buck converter topology 110 followed by a conventional boost converter topology 120. In the cascaded buck boost topology 100, both the buck 110 and boost 120 converters share a single energy storage device, which is typically an inductor LI 07. The buck converter 110 includes a buck switch M101, typically implemented with a semiconductor switching device such as a metal oxide semiconductor field effect transistor (MOSFET) or an insulated gate bipolar transistor (IGBT), coupled to a freewheeling diode D103, with the energy storage inductor LI 07 coupled to a common node 108 between the buck switch M101 and freewheeling diode D103. In operation, the buck converter 110 receives an input voltage at the buck switch M101. The buck switch M101 is closed to allow the input voltage VIN to push current through the inductor LI 07 and store energy in the magnetics of the inductor. Once a sufficient current is flowing, the buck switch Ml 01 is opened and the free-wheeling diode D103 provides a current path so energy stored in the inductor L107 can be transferred to the boost stage 120. The boost converter 120 is coupled to the output side of the energy storage inductor LI 07. The boost converter 120 includes a boost switch Ml 02, which is typically implemented using a semiconductor switching device such as a MOSFET, coupled to a flyback diode D104 with an energy storage device LI 07 coupled to the common node 109 between the boost switch Ml 02 and the flyback diode D104. The boost converter operates by closing the boost switch Ml 02 which shorts the output of the inductor LI 07 to ground, creating a current through the inductor LI 07. The boost switch Ml 02 is then opened to allow the inductor current to flow through the flyback diode D104 where it charges an output capacitor CI 06 and transfers energy to the load 130. In the context of the present disclosure, a switch or switching device that is closed or "turned on" is one that will allow current to flow through it and a switch or switching device that is open or "turned off is one that blocks the flow of current. When a cascaded buck boost converter 100 is used with high voltage input power, such as for example 347V-480V, the full input voltage is applied to the buck switch Ml 01 and the free-wheeling diode D103 causing high voltage stresses which results in the need for costly high-voltage components.

[0004] Accordingly, it would be desirable to provide switched mode power converter topologies that solve at least some of the problems identified above.

SUMMARY OF THE INVENTION

[0005] As described herein, the exemplary embodiments overcome one or more of the above or other disadvantages known in the art. [0006] One aspect of the present disclosure relates to a dual-buck boost converter that includes first and second input capacitors coupled in series across an input voltage and forming a mid-rail at the common capacitor node between the two series coupled input capacitors. A first buck converter that includes a first buck switch and a first freewheeling diode is coupled in the forward path with the first buck switch coupled to the positive input voltage, and the freewheeling diode coupled to the mid-rail. A second buck converter that includes a second buck switch and a second freewheeling diode is coupled in the return path with the second buck switch coupled to the negative input voltage, and the second freewheeling diode coupled to the mid-rail. A first inductor is coupled to a common node of the first buck converter and a second inductor is coupled to a common node of the second buck converter. The second inductor is also coupled to the negative output voltage. A pair of sharing diodes is coupled in series across the two inductors, and a common node between the sharing diodes the mid-rail. A boost converter, including a boost switch and a flyback diode, is coupled across the two inductors with the flyback diode is coupled to the positive output terminal.

[0007] Another aspect of the present disclosure relates to a switching converter configured to receive an input voltage at positive and negative input terminals and provide an output voltage at positive and negative output terminals. The converter includes a first buck switch and a second buck switch coupled in series with the positive input terminal. A first freewheeling diode and a second freewheeling diode coupled in series in a first circuit branch which couples the second buck switch to the negative input terminal. A first end of a first inductor is coupled to the second buck switch and a first end of a second inductor is coupled to the negative input terminal. A sharing capacitor is configured to couple a first common node between the first and the second buck switch, to a second common node between the first and the second freewheeling diodes. First and second sharing diodes are coupled in series in a second circuit branch across a second end of the first inductor and the second end of the second inductor, with a third common node between the first sharing diode and the second sharing diode coupled to the second common node. A boost converter including a boost switch coupled to a flyback diode is cascaded after the buck stage. The boost switch is coupled in parallel with the second circuit branch and the flyback diode is coupled to the positive output terminal.

[0008] Another aspect of the present disclosure relates to a switching converter configured to receive an input voltage at a positive input terminal and a ground terminal, and provide an output voltage at a positive output terminal and the ground terminal. The converter includes a first buck switch and a second buck switch coupled in series to the positive input terminal. A first freewheeling diode and a second freewheeling diode coupled in series in a first circuit branch that couples the second buck switch to the ground terminal. A first inductor and a second inductor coupled in series and coupled to the second buck switch. A sharing capacitor, a first sharing diode, and a third inductor coupled in series forming a second circuit branch that couples a common node between the first and second buck switches to the ground terminal, and a common node between the first sharing diode and the sharing capacitor is coupled to a common node between the first and second freewheeling diodes. A second sharing diode coupled to a common node between the first and second inductor, and coupled to the common node between the first and second freewheeling diodes. A boost switch coupled to the second inductor and to the ground terminal. A flyback diode is coupled to the boost switch and the positive output terminal. To facilitate sharing the first inductor, second inductor, and third inductor are all magnetically coupled. [0009] Another aspect of the present disclosure relates to a switching converter with a common ground that is configured to receive an input voltage at positive input terminal and a ground terminal, and provide an output voltage at a positive output terminal and the ground terminal. The converter includes a first buck switch and a second buck switch coupled in series with the positive input terminal. A first freewheeling diode and a second freewheeling diode are coupled in series in a first circuit branch that couples the second buck switch to the ground terminal, and an energy storage inductor is coupled to the second buck switch. A sharing capacitor, a first sharing diode, and a second inductor are coupled in series forming a second circuit branch that couples the common node between the first and second buck switches to the ground terminal. A common node between the first sharing diode and the sharing capacitor is coupled to a common node between the first and second freewheeling diodes, and a third inductor is coupled in series with a second sharing diode forming a third circuit branch that is coupled in parallel with the sharing capacitor. A boost switch is coupled to the energy storage inductor and to the ground terminal, and a flyback diode couples the boost switch to the positive output terminal. Voltage sharing is facilitated by magnetically coupling the three inductors.

[0010] These and other aspects and advantages of the exemplary embodiments will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Additional aspects and advantages of the invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Moreover, the aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings :

[0012] Figure 1 illustrates a cascaded buck-boost converter topology as is known in the art.

[0013] Figure 2 illustrates an embodiment of a dual-buck boost converter topology incorporating aspects of the present disclosure.

[0014] Figure 3 illustrates charge and discharge current paths for the dual-buck boost converter topology of Figure 2 incorporating aspects of the present disclosure.

[0015] Figure 4 illustrates another embodiment of a dual-buck boost converter topology incorporating aspects of the present disclosure.

[0016] Figure 5 illustrates an embodiment of a dual-buck boost converter topology with a common ground incorporating aspects of the present disclosure.

[0017] Figure 6 illustrates an alternate common ground dual-buck boost converter topology incorporating aspects of the present disclosure.

[0018] Figures 7 illustrate results of simulations of the dual-buck boost converter topology incorporating aspects of the present disclosure.

[0019] Figures 8 illustrate results of simulations of the dual-buck boost converter topology incorporating aspects of the present disclosure where switching times are offset by 500ns. [0020] Figures 9 illustrate results of simulations of the dual-buck boost converter topology incorporating aspects of the present disclosure where switching times are offset by 500ns.

[0021] Figures 10 illustrate results of simulations of the dual-buck boost converter topology incorporating aspects of the present disclosure where the buck switches have different parasitic parameters.

[0022] Figures 11 illustrate results of simulations of the dual-buck boost converter topology incorporating aspects of the present disclosure where the input capacitors have different values.

[0023] Figures 12 illustrate results of simulations of the dual-buck boost converter topology incorporating aspects of the present disclosure where the inductors have differences leakage inductances.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

[0024] Figure 2 illustrates a schematic of an exemplary embodiment of an improved converter that significantly reduces voltages applied to the switching devices. The aspects of the disclosed embodiments are directed to a dual buck boost converter as a PFC circuit for high input voltage range applications that reduces voltage stress on buck switches and diodes.

[0025] In the embodiment shown in Figure 2, the dual-buck boost converter 200 topology includes a pair of buck regulators 110a and 110b sharing a pair of coupled energy storage inductors LI, L2 with a single cascaded boost stage 120. Power is provided to the converter 200 through a pair of input terminals 202, 204 which are configured to receive an input power designated as VIN. Two input capacitors CI, C2 are coupled in series in a first circuit branch across the input terminals 202, 204 forming a common node 212 between them. The circuit node between two series connected components is where the components are connected together and is referred to herein as a common node. For example node 212 is the common node of capacitors CI and C2. The common node 212 of the input capacitors CI, C2 is coupled to a mid-rail 206, which has a mid-rail voltage, Vmid, maintained at about half the input voltage, that is, at VIN/2. As will be discussed further below, the mid-rail 206 helps facilitate sharing of the input voltage between the two buck converters 110a, 110b. One of the buck converters 110a is coupled in the forward path of the dual-buck boost converter 200 between the positive input terminal 202 and the mid-rail 206 and the second buck converter is coupled in the return path between the mid-rail 206 and the negative input terminal 204. The term forward path refers to the path of positive current flowing from the input VIN to the output VOUT and load 130. The load 130 can comprise any suitable load, such as for example a fluorescent lamp or other electroluminescent device. For example the forward path of converter 200 starts at the positive input terminal 202, proceeds through the switching device Ml, inductor LI, and diode D9, and flows to the positive output terminal 208. Similarly, the return path is the path of current returning from the output or load 130 to the input VIN. For example the return path in converter 200 starts at the negative output terminal 210, proceeds through inductor L2 and switching device M2, and flows to the negative input terminal 204. Each buck converter 110a, 110b includes a respective buck switch Ml, M2 and a diode Dl, D3. The first converter 110a in the forward path includes switch Ml and diode Dl and the second converter 110b in the return path includes switch M2 and diode D3. Each buck converter 110a, 110b is connected in series with a respective energy storage inductor, LI or L2. The inductor LI is connected to the common node 214 between the buck switch Ml and diode Dl and inductor L2 is connected to the common node 216 between the buck switch M2 and the diode D2. The buck switches Ml, M2 of the first 110a and second 110b converters are connected to the positive 202 and negative 204 input terminals respectively, thereby applying a switched input voltage to the inductors LI and L2 and producing a buck stage output voltage, Vbuck, across the load side of the two inductors LI, L2. The freewheeling diode Dl, D3 of each buck converter 110a, 110b, is connected to the mid-rail 206 so that each freewheeling diode Dl, D3 is exposed to a reduced voltage of VIN-Vmid (that is, the arithmetic difference between VIN and Vmid), rather than the full input voltage VIN. The boost converter 120 including boost switch M3 and flyback diode D9 is coupled to the output Vbuck of the two buck converters 110a, 110b, such that the boost switch M3 selectively shorts the two inductors LI, L2. The boost converter's flyback diode D9 is coupled in the forward path after the boost switch M3 to allow current to flow to the load 130. An output capacitor C3 is connected in parallel with the load 130 to smooth and maintain the output voltage VOUT while the flyback diode D9 prevents the output capacitor C3 from discharging through the boost switch M3 while the boost switch M3 is turned ON. To provide charge and discharge paths for the input capacitors CI, C2, a pair of sharing diodes D2, D4, are connected in series with each other between the supply and return paths and in parallel with the boost switch M3. The common node 218 between the sharing diodes D2, D4, is coupled to the mid-rail 206 so that current can flow to or from the input capacitors CI, C2. In the exemplary embodiment shown in Figure 2, the switching devices Ml, M2, M3 are MOSFETs. Alternatively, other types of semiconductor switching devices including field effect transistors (FETs), insulated gate bipolar transistors (IGBTs), or any suitable semiconductor switching device that is capable of switching the current on and off at the desired frequencies and voltages can be used. [0026] The switches Ml, M2, and M3, may be controlled using any conventional control method as may be used with boost PFC. In one embodiment, an insulated driver circuit (not shown) can be used. Generally the three switches Ml, M2, M3, are controlled to turn on and off synchronously. When all three switches Ml, M2, M3, are on (i.e. they are conducting) current from the input VIN flows through the two coupled inductors LI, L2 to store energy. When the switches Ml, M2, M3 are turned off, the stored energy in the coupled inductors LI, L2 charges the output capacitor C3 and is transferred to the load 130.

[0027] Ideally the mid-rail 206 is maintained at approximately one-half of the input voltage (that is, at approximately VIN/2), and the voltage across the input capacitors CI, C2, freewheeling diodes Dl, D3, and the buck switches Ml, M2 are all balanced at about half of the input voltage (VIN/2). In the exemplary dual-buck boost converter topology 200 described above, two measures are used to realize voltage sharing on the two buck switches Ml, M2 and maintain the mid-rail voltage, Vmid, at about half the input voltage. The inductors LI, L2 are magnetically coupled such that they can act as a transformer with a one-to-one (1 : 1) turns ratio, while sharing diodes D2, D4 provide charge and discharge paths for the input capacitors CI, C2. These two measures combine to maintain the mid-rail voltage, Vmid, at around half the input voltage, VIN/2, even when non-ideal conditions exist, for example, when variances exist in the switching times of the switching devices Ml, M2; when there are differing parasitic parameters in the switching devices Ml, M2; when there are differences in the amount of capacitance provided by the input capacitors CI, C2; and when there are variations in the parasitic inductance of the energy storage inductors LI, L2.

[0028] The input capacitors CI, C2 hold the mid-rail 206 at about one-half the input voltage (VIN/2) while the switching devices Ml, M2 are turned off. Therefore the voltage across the first input capacitor CI (Vcl) needs to be equal to the voltage across the second input capacitor CI (Vc2) which should be about half the input voltage: Vcl=Vc2=VIN/2. If Vcl is lower than Vc2 for any reason, a charging path for the capacitor C 1 is provided while the switch M2 is turned on. Referring to Figure 3, the charging path, designated by numeral 302 , is through the sharing diode D2, the inductor LI, and the body diode of switching device Ml . Also, while the switching device M2 is on, the coupling between inductors LI and L2 works as a transformer and transfers energy from input capacitor C2 to input capacitor CI thereby discharging input capacitor C2 and charging input capacitor CI . In a similar fashion, if Vc2 is lower than Vcl, a charging path 304 for input capacitor C2 is provided while the upper switch Ml is on. This charging path 304 allows input capacitor C2 to be charged through sharing diode D4, inductor L2, and the body diode of switching device M2. In this case, i.e. Vc2 is lower than Vcl, the coupling of inductors LI and L2 acts to transfer energy from input capacitor CI to input capacitor C2. Complementary discharge paths for the two input capacitors CI, C2 are also provided. Discharge path for capacitor CI is through the sharing diode D4, body diode of the buck switch Ml, inductor L2 and the input voltage VIN. A similar discharge path for input capacitor C2 is through the sharing diode D2, inductor LI, switch Ml and the input voltage VIN. In this fashion, the voltages Vcl and Vc2 always remain balanced and the voltage stresses on the switching devices Ml, M2 are clamped to Vcl and Vc2. As a result, the voltage stresses on the buck switches Ml, M2 are controlled at around half of the input voltage VIN (approximately VIN/2).

[0029] The dual-buck boost converter 200 provides good voltage sharing among the two buck switches Ml, M2 in the presence of component mismatches and switching time variances. Tests were run on a dual-buck boot converter with the topology shown in Figure 2 using an input voltage of 690V DC and a switching frequency of 100 KHz. The results of these tests and simulations are illustrated in Figures 7-12. Figure 7 illustrates the results when the three switches Ml, M2 and M3 are controlled to turn on and off substantially simultaneously. In this example the circuit components and switching times are ideally matched and diodes D2 and D4 do not conduct and the switches Ml and M2 share the voltage. The graph 702 of Figure 7 illustrates the driving signal 701 for each of the three switches Ml, M2 and M3. Graph 704 illustrates the voltage waveform 705 and the current waveform 703 on the switches Ml, M2 and M3. Graph 706 illustrates the current waveform 707 through the diodes D2 and D4.

[0030] The graphs of Figures 8-9 illustrate that the voltage sharing remained essentially equal when the switching times of the two buck switches Ml, M2 were offset from each other by 500 nanoseconds either way. In Figure 8, the switch Ml is turned on about 500 nanoseconds before switch M2 and M3. Curve 801 represents the driving signal to switch Ml, while curve 803 represents the driving signal to switch M2 and M3. Graph 804 illustrates the voltage and current waveforms on the swithes Ml, M2 and M3. Curve 805 represents the voltage waveform on switch Ml, while curve 806 represents the voltage waveform on switches M2 and M3. Curve 807 represents the current waveform on switch Ml, while curve 808 represents the current waveform on switches M2 and M3. Graph 810 illustrates the current waveform through diodes D2 and D4. Curve 811 illustrates the current waveform through diode D2, while curve 812 illustrates the current waveform through diode D4. In this simulation, the voltage stress of switches Ml and M2 are substantially balanced, and close of one-half of the input voltage VIN (or approximately VIN/2). In Figure 9, as is shown in graph 902 Ml is turned on behind M2 and M3 by approximately 500 nanoseconds. Curve 901 represents the driving signal of switch Ml, while curve 903 represents the driving signal of switches M2 and M3. In graph 904, curve 905 represents the voltage waveform on switch Ml, while curve 906 represents the voltage waveform on switch M2. Curve 907 represents the current waveform on switch Ml, while curve 908 represents the current waveform on switch M2. In graph 910, curve 911 represents the current waveform through diode D2, while curve 912 represents the current waveform through diode D4. As illustrated, the voltage stress on switches Ml and M2 are relatively balanced, and close to approximately one-half of the input voltage VIN (or approximately VIN/2). . In the simulation represented in Figure 10, an additional 500 picofarad parasitic capacitance was added between the drain and source of switching device Ml . Curve 1001 in graph 1002 illustrates the driving signal of the switches Ml, M2 and M3. In graph 1004, curve 1003 illustrates the voltage waveform on the switches, while curve 1005 represents the current waveform on the switches. In graph 1008, curve 1007 is the current waveform through diode D2 and curve 1009 is the current waveform through diode D4. As illustrated, the voltage stress on switches Ml and M2 are relatively balanced, and close to approximately one-half of the input voltage VIN (or approximately VIN/2). Referring to Figure 11, in this simulation, the values of CI and C2 are set to be about 0.1 microfarad (μΡ) and 0.3 microfarad (μΡ), respectively. Graph 1102 illustrates the driving signal 1101 applied to each of the three switches Ml, M2, M3. Graph 1104 illustrates the voltage waveform 1103 and the current waveform 1105 on the switches Ml, M2 and M3. Graph 1106 illustrates the current waveform 1107 through the diodes D2 and D4. In this example, switches Ml and M2 are switched on and off substantially simultaneously. These tests show that the voltage sharing is independent of the values of input capacitor CI and C2, and therefore mismatches in capacitance value of these two components have little or no effect on the voltage sharing. Figure 12 illustrates the simulation test results when the leakage inductance of inductor L2 was set to twice the leakage inductance of inductor LI . In this example, the leakage inductance of inductor LI is approximately 10 microhenries (μΗ) while the leakage inductance of inductor L2 is approximately 20 microhenries (μΗ). Graph 1202 illustrates the driving signal 1201 applied to the three switches Ml, M2 and M3. Graph 1204 illustrates the voltage waveform 1203 and the current waveform 1205 on the switches Ml, M2 and M3. Graph 1206 illustrates the current waveform 1207 through the diodes D2 and D4. In this simulation, switches Ml and M2 are switched on and off substantially simultaneously. As can be seen from the graphs 1204 and 1206, the different leakage inductance values have little or no effect on voltage sharing.

[0031] Figure 4 illustrates an alternate topology for an embodiment of a dual-buck boost converter 400 where both buck switches M401, M402 are placed in series in the supply path. The principle of operation of this converter 400 is similar to the converter 200 described above and shown in Figure 2 and 3. A single input capacitor C401 is placed in parallel across the input terminals 410, 420, with the two buck switches M401, M402 placed in series in the supply path of the converter (i.e. the supply path of converter 400 proceeds from the positive input terminal 410, through the first buck switch Ml , through the second buck switch M2, through the inductor LI, and through the flyback diode D9 to the positive output terminal 440). A single sharing capacitor C402 is used to couple a mid-rail 430 to the common node 412 between the two series connected buck switches M401, M402. The remainder of this exemplary converter embodiment 400 is the same as the previous embodiment 200 described above. Similar to the converter topology 200, magnetically coupled inductors LI, L2 are used to divide the input voltage VIN while sharing diodes D2, D4 provide charge and discharge paths respectively, to maintain the voltage on the sharing capacitor C402 at about half of the input voltage. When the three switches M401, M402, M3 are closed, the voltage of the mid-rail 430 is about half the input voltage (VIN/2). The sharing capacitor C402 is thereby charged to a voltage equal to the supply voltage VIN minus the mid-rail 430 voltage resulting in a voltage on the sharing capacitor C2 of about VIN/2. When the switches are turned off the voltage across the second buck switch M402 is clamped to the voltage of the sharing capacitor C2. Thus providing generally equal sharing of the input voltage across the two buck switches M401, M402.

[0032] Figure 5 illustrates an alternate embodiment of a converter topology for a dual- buck boost converter 500 with the two buck switches M401, M402 placed in series in the supply path of the converter 500. In contrast with the previously discussed converter 400 of Figure 4, this converter 500 topology couples both the first and second energy storage inductors LI, L2 in series in the converter's supply path after the sharing diode D502 as shown in Figure 5, and a third inductor L3, which is magnetically coupled to the first two inductors LI, L2 is used to set the mid-rail 530 voltage. In this embodiment there is no inductor in the return path thereby creating a common ground 550 between the input voltage and the output voltage. All three inductors LI, L2, L3 have about the same number of turns resulting in a turns ratio of one-to- one-to-one (1 : 1 : 1) between the three inductors LI, L2, L3, so that transformer coupling can keep the voltage across each of the three inductors generally equal. Any imbalances in the voltage drop across the first inductor LI and the second inductor L2 are removed through transformer action, thereby maintaining the voltage across each of the inductors LI, L2 at half of the input voltage. In the embodiment illustrated in Figure 5, the third inductor L3 is coupled in series with the second sharing diode D504, and its voltage is also set to half the input voltage through transformer coupling with each of the first two inductors LI, L2. When the three switches M501, M502, M3 are closed, the inductor L3 sets the voltage of the mid-rail 530 to about half the input voltage (VIN/2), resulting in a voltage on the sharing capacitor of about half the input voltage (VIN/2). The voltage across the sharing capacitor C402 is held at about half of the input voltage VIN/2 by the two sharing diodes D502, D504 similar to the previous two topologies thereby providing good voltage sharing across the two buck switches M401 and M402. As in the above embodiments the sharing diodes D502, D504 provide a charge path and a discharge path, respectively, for the sharing capacitor C402. By placing both energy storage inductors, LI, L2 in the forward path the converter topology 500 provides an additional advantage of having a common ground between the negative input terminal 420 and the negative output terminal 540.

[0033] Figure 6 illustrates an alternate embodiment of a converter topology 600 providing voltage sharing by the buck switches M501, M502 as well as a common ground between the input voltage VIN and the output voltage VOUT. The converter topology 600 shown in Figure 6 is similar to the converter 500 except it uses a single energy storage inductor L601 in the supply path. The energy storage inductor L601 is magnetically coupled to the two additional inductors L602, L603 with a turns ratio of about one-to-one half-to-one half (L601-to- L602-to-L603, 1 : 1/2: 1/2). With this turns ratio, transformer action sets the voltage across each of the secondary inductors L602, L603 to half the voltage across the energy storage inductor L601. Thus, when the boost switch is closed, the voltage across the secondary inductors L602, L603 is about half the input voltage VIN/2. A sharing diode D602, D604 is placed in series with each of the secondary inductors providing charge and discharge paths for the sharing capacitor C402, thereby setting the voltage of the sharing capacitor to half the input voltage VIN (VIN/2) while the switches M501, M502, M3 are closed. When the switches M501, M502, M3 are opened, the sharing capacitor C402 clamps the voltage across the second buck switch M502 to half the input voltage VIN (about VIN/2) thereby balancing the voltage stresses among the buck switches M501, M502. [0034] The aspects of the disclosed embodiments are directed to a dual-buck boost switching converter topology for use in high voltage power factor control applications. The new topology includes dual buck converters cascaded with a boost converter. The dual buck converters are configured to equally share the input voltage such that voltage stresses on the buck converter components is significantly reduced. Voltage sharing is realized through the use of multiple magnetically coupled energy storage inductors and a pair of sharing diodes. The sharing diodes provide current paths for balancing voltages applied to the buck converter components. Dual-buck boost converters of the types disclosed may be advantageously used in lamp drive applications to provide ballasting of gas discharge lamps. Multi-stage lamp drive applications requiring a voltage or other types of power regulation can also benefit from use of the dual-buck boost topologies described herein.

[0035] Thus, while there have been shown, described and pointed out, fundamental novel features of the invention as applied to the exemplary embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Moreover, it is expressly intended that all combinations of those elements, which perform substantially the same function in substantially the same way to achieve the same results, are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

CLAIMS What is claimed is:

1. A dual-buck boost converter configured to receive an input voltage at positive and negative input terminals and provide an output voltage at positive and negative output terminals, the converter comprising:

a first buck converter;

a second buck converter;

a common node between the first buck converter and the second buck converter, the common node comprising a mid-rail between the positive and negative input terminals; and a boost converter cascaded with the first buck converter and the second buck converter on the common node.

2. The dual- buck boost converter of claim 1, comprising:

first and second input capacitors coupled in series in a first circuit branch coupled across the positive and negative input terminals;

a common capacitor node between the two series coupled input capacitors forming the mid-rail; and wherein

the first buck converter comprises a first buck switch coupled to a first freewheeling diode at a first common node, wherein the first buck switch is coupled to the positive input terminal, and the freewheeling diode is coupled to the mid-rail; the second buck converter comprises a second buck switch coupled to a second freewheeling diode at a second common node, wherein the second buck switch is coupled to the negative input terminal, and the second freewheeling diode is coupled to the mid-rail;

a first inductor coupled to the first common node of the first buck converter and a second inductor coupled to the second common node of the second buck converter, wherein the second inductor is further coupled to the negative output terminal;

first and second sharing diodes coupled in series in a second circuit branch coupled across the first inductor and the second inductor, wherein a common sharing diode node formed between the first and second sharing diodes is coupled to the mid-rail; and

wherein the boost converter comprises a boost switch and a flyback diode, wherein the boost switch is coupled in parallel with the second circuit branch and the flyback diode is coupled to the positive output terminal.

3. The dual-buck boost converter of claim 2 wherein the first and second inductors are magnetically coupled.

4. The dual-buck boost converter of claim 2 further comprising an output capacitor coupled across the positive and negative output terminals.

5. The dual-buck boost converter of claim 2 wherein the first and second buck switch and the boost switch each comprise a MOSFET.

6. The dual-buck boost converter of claim 1 further comprising an electroluminescent device coupled to the positive and negative output terminals.

7. A switching converter configured to receive an input voltage at positive and negative input terminals and provide an output voltage at positive and negative output terminals, the converter comprising:

a first buck switch and a second buck switch coupled in series with the positive input terminal and coupled together at a first common node;

a first circuit branch coupling the second buck switch to the negative input terminal; a first freewheeling diode and a second freewheeling diode coupled in series in the first circuit branch and coupled together at a second common node;

a first inductor having a first end coupled to the second buck switch;

a second inductor having a first end coupled to the negative input terminal;

a sharing capacitor coupling the first common node between the first and the second buck switch to the second common node between the first and the second freewheeling diodes;

first and second sharing diodes coupled in series in a second circuit branch coupled across a second end of the first inductor and the second end of the second inductor, wherein a third common node between the first sharing diode and the second sharing diode is coupled to the second common node; and

a boost converter comprising a boost switch coupled to a flyback diode, wherein the boost switch is coupled in parallel with the second circuit branch and the flyback diode is coupled to the positive output terminal.

8. The switching converter of claim 7 wherein the first and second inductors are magnetically coupled.

9. The switching converter of claim 7 wherein the first and second inductors are coupled with a turns ratio of about one-to-one.

10. The switching converter of claim 7 further comprising an output capacitor coupled across the positive and negative output terminals.

11. The dual-buck boost converter of claim 7 wherein the first and second buck switch and the boost switch each comprise a MOSFET.

12. The switching converter of claim 7 further comprising an input capacitor coupled across the positive and negative input terminals.

13. The switching converter of claim 7 further comprising an electroluminescent device coupled to the positive and negative output terminals.

14. A switching converter configured to receive an input voltage at a positive input terminal and a ground terminal, and provide an output voltage at a positive output terminal and the ground terminal, the converter comprising:

a first buck switch and a second buck switch coupled in series to the positive input terminal; a first freewheeling diode and a second freewheeling diode coupled in series in a first circuit branch, wherein the first circuit branch couples the second buck switch to the ground terminal;

a first inductor and a second inductor coupled in series and coupled to the second buck switch;

a sharing capacitor, a first sharing diode, and a third inductor coupled in series forming a second circuit branch, wherein the second circuit branch couples a common node between the first and second buck switches to the ground terminal, and wherein a common node between the first sharing diode and the sharing capacitor is coupled to a common node between the first and second freewheeling diodes;

a second sharing diode coupled to a common node between the first and second inductor, and coupled to the common node between the first and second freewheeling diodes;

a boost switch coupled to the second inductor and to the ground terminal; and

a flyback diode coupled to the boost switch and the positive output terminal,

wherein the first inductor, second inductor, and third inductor are magnetically coupled.

15. The switching converter of claim 14 wherein the first inductor, second inductor, and third inductor have a turns ratio of about one-to-one-to-one.

16. The switching converter of claim 14 further comprising an output capacitor coupled across the positive output terminal and the ground terminal.

17. The dual-buck boost converter of claim 14 wherein the first and second buck switch and the boost switch each comprise a MOSFET.

18. The switching converter of claim 14 further comprising an input capacitor coupled across the positive input terminal and the ground terminal.

19. The switching converter of claim 14 further comprising an electroluminescent device coupled to the positive and negative output terminals.

20. A switching converter configured to receive an input voltage at positive input terminal and a ground terminal, and provide an output voltage at a positive output terminal and the ground terminal, the converter comprising:

a first buck switch and a second buck switch coupled in series with the positive input terminal;

a first freewheeling diode and a second freewheeling diode coupled in series in a first circuit branch, wherein the first circuit branch couples the second buck switch to the ground terminal;

an energy storage inductor coupled to the second buck switch;

a sharing capacitor, a first sharing diode, and a second inductor coupled in series forming a second circuit branch, wherein the second circuit branch couples the common node between the first and second buck switches to the ground terminal, and wherein a common node between the first sharing diode and the sharing capacitor is coupled to a common node between the first and second freewheeling diodes; a third inductor coupled in series with a second sharing diode forming a third circuit branch coupled in parallel with the sharing capacitor;

a boost switch coupled to the energy storage inductor and to the ground terminal; and a flyback diode coupled to the boost switch and the positive output terminal,

21. The switching converter of claim 20 wherein the first inductor, second inductor, and third inductor have a turns ratio of about one-to-one half-to-one half.

22. The switching converter of claim 20 further comprising an output capacitor coupled across the positive output terminal and the ground terminal.

23. The switching converter of claim 20 further comprising an input capacitor coupled across the positive input terminal and the ground terminal.

24. The switching converter of claim 20 further comprising an electroluminescent device coupled to the positive and negative output terminals.