Hybrid power converters combining switched-capacitor and transformer-based stages

A hybrid power converter circuit includes a switched-capacitor power converter stage and a pulse-width modulation (PWM) or resonant output circuit coupled to a switching node of the switched-capacitor power converter stage. In particular, the PWM or resonant output circuit can include a transformer having a primary winding and a secondary winding magnetically coupled to each other, and the secondary winding is coupled to the output node of the power converter. The switched-capacitor power converter stage is coupled between the input node of the power converter and the primary winding of the transformer, and includes capacitors and switches configured to connect the capacitors to the input node during a first phase of operation and connect the capacitors to the primary winding of the transformer of the PWM or resonant output circuit during a second phase of operation.

TECHNICAL FIELD

The present subject matter relates to techniques and equipment for power conversion that combine switched capacitor converters and transformer-based power converters approaches and circuits.

BACKGROUND

Power converters are used to convert electrical power having one voltage level (e.g., 12V) to electrical power having a different voltage level (e.g., 3V). Power converters can also be used to convert power having one type (e.g., alternating current (AC) power) to power having a different type (e.g., direct current (DC) power). Switching mode power converters are widely used because of their high efficiency. Generally, switching mode converters are either inductor and transformer based or capacitor based without a magnetic component.

In the case of a transformer-based architecture, the power converter includes two coils that are inductively coupled to each other, such as two coils sharing a common core formed of magnetic material. A primary coil is coupled to the input circuitry of the transformer, a secondary coil is coupled to the output circuitry of the transformer, and power conversion is provided according to a ratio of turns of the primary and secondary coils. In the case of a capacitor-based converter architecture, the power converter includes multiple switches (e.g., transistors such as field-effect transistors (FETs)) that operate under the control of a controller to selectively charge capacitors in series or in parallel, to provide a desired output power level.

Transformer-based converters are typically used for applications where isolation is required between input and output voltage rails, or application with a big voltage conversion ratio. However, transformer-based converters require large sized magnetic components due to their higher AC losses and limited switching frequency. A need therefore exists for a transformer architecture that can run at higher switching frequency and provide reduced magnetic component sizes.

Moreover, conventional transformer-based converters include forward, flyback, push-pull, half-bridge, and full-bridge PWM converters and various resonant converters. A typical example converts a 48 V input voltage to a 12 V output. For such a converter, because of the higher voltage stress on the input side, power MOSFETs (metal-oxide-semiconductor field effect transistors) on the transformer primary side are high-voltage rated MOSFETs that suffer not only from high switching losses but also have high conduction losses due to a high on-resistance Rds(on). As a result, the maximum switching frequency of the converter is limited due to the higher power dissipation and power device thermal stress. Because of the limited switching frequency, the conventional transformer-based solutions usually need a large size power transformer and large size output inductor and thus provide low converter power density. A need therefore exists for a transformer architecture that can provide reduced high power density while being configured to handle elevated voltage levels.

SUMMARY

The teachings herein alleviate one or more of the above noted problems with conventional power converters.

In accordance with one aspect of the disclosure, a power converter includes a switched-capacitor power converter stage and a pulse-width modulation (PWM) or resonant output circuit coupled to a switching node of the switched-capacitor power converter stage. In one example, the switched-capacitor power converter stage includes a plurality of switches and a plurality of capacitors, and the PWM or resonant output circuit includes a transformer having an input winding that is selectively coupled via switches of the plurality of switches to one or more capacitors of the plurality of capacitors, and that is inductively coupled to an output winding of the transformer.

In accordance with another aspect of the disclosure, a power converter has an input node for receiving a transformer input voltage and an output node for outputting a transformer output voltage. The power converter includes a transformer having a primary winding and at least one secondary winding magnetically coupled to each other, wherein the secondary winding is coupled to the output node of the power converter. The power converter further includes a switched capacitor circuit coupled between the input node and the primary winding of the transformer, the switched capacitor circuit comprising a plurality of capacitors and a plurality of switches configured to connect the capacitors to the input node during a first phase of operation and connect the capacitors to the primary winding during a second phase of operation. In one example, first and second capacitors of the plurality of capacitors of the switched capacitor circuit are coupled in series with each other and the input node during the first phase of operation, and the first and second capacitors are coupled in parallel with each other and to the primary winding during the second phase of operation.

In accordance with a further aspect of the disclosure, a power converter is configured to convert an input voltage to an output voltage. The power converter includes means for dividing the input voltage across a series connection of circuit elements to obtain a divided input voltage having a voltage level lower than the input voltage, and means for transforming the divided input voltage into the output voltage through at least two magnetically coupled coils. The means for dividing the input voltage can include switching means configured to selectively connect the circuit elements in a series connection and a parallel connection. The power converter can further include means for sensing the output voltage and for controlling the switching means according to the sensed output voltage.

DETAILED DESCRIPTION

The various methods and circuits disclosed herein relate to hybrid power converters. The hybrid converters combine a switched-capacitor circuit with a transformer-based output circuit (e.g., a pulse width modulation (PWM) circuit or a resonant frequency modulation circuit) within a same power stage to achieve high power density and high efficiency at high switching frequency. Compared to conventional transformer-based converters, the hybrid power converters have significantly reduced magnetic component sizes. Additionally, a feedback loop can be used to provide output regulation when needed. For high power/current applications, multiple converters can be paralleled with current sharing.

FIG. 1Ais a circuit diagram of an illustrative transformer-based hybrid power converter in accordance with the disclosure. As shown, the transformer-based hybrid power converter100includes a switched-capacitor converter stage101and a transformer-based output circuit103coupled in series between the input node VINand the output node VOUT. The transformer-based output circuit103includes a primary side103aand a secondary side103brespectively connected to primary and secondary coil windings of a transformer.

Various circuit architectures of the switched-capacitor converter stage101and of the primary and secondary sides103aand103bof the transformer-based output circuit103are described in more detail in relation toFIGS. 2A-2GandFIG. 3, below.

As shown inFIG. 1A, the transformer-based hybrid power converter integrates the switched-capacitor converter101stage with the transformer-based output circuit103in a single power converter. The hybrid converter can be a non-regulated, open loop converter running at about 50% duty-cycle. Or, if output voltage regulation is required, a feedback controller105can monitor the level of the output voltage VOUTand regulate operation of the switched-capacitor converter101and/or the transformer-based output circuit103to maintain a desired output voltage level, as shown in the block diagram ofFIG. 1B. In particular, the feedback controller105may measure or sense the output voltage level VOUTat the output node(s), and may control the operation of switches in the switched-capacitor converter101and/or the transformer-based output circuit103to regulate the voltage level VOUTat the desired level. For example, the feedback controller105can control the switching duty-cycle or switching frequency of switches of the switched-capacitor converter101to thereby regulate the voltage level VOUT. Especially, in situations in which the transformer-based output circuit103is a synchronous PWM converter that includes switches, the feedback controller105can control a pulse width of switching of the switches of the transformer-based output circuit103to regulate the voltage level VOUT. In situations in which the transformer-based output circuit103is a resonant converter, the feedback controller105can control the switching frequency of the switches of the output circuit103to regulate the voltage level VOUT. In such situations, a synchronous step-down controller can for example be used as the feedback controller105to control operation of the output circuit103and of the switched-capacitor converter101in a feedback loop.

FIG. 1Ashows one implementation of the primary side circuit of the hybrid power converter. The primary side circuit includes the switched-capacitor converter101and the primary side103aof the transformer-based output circuit103. As shown, the switched-capacitor converter101is a switched-capacitor voltage divider converter. The switched-capacitor voltage divider converter on the primary side can have a ratio of 2:1, from the IN node to the MID node, as illustratively shown inFIG. 1A, but could more generally have a different step-up or step-down ratio (e.g., a 3:1 ratio, a 4:1 ratio, a 3:2 ratio, a 4:3 ratio, or the like) with different switched capacitor topologies such as those shown inFIGS. 4A-4D, for example. InFIG. 1A, the power MOSFETs of the switched-capacitor voltage divider converter are only exposed to voltage stress of VIN/2 because the series interconnection of the capacitors CF and CM between the input node IN and ground (through switches Q1and Q3) serve to divide the input voltage by two.

InFIG. 1A, switches Q3and Q4are shared by the front-end switching capacitor converter101and the transformer-based output circuit103. The voltage of node MID, which is also the voltage across capacitor CM, serves as the input voltage of the transformer-based output circuit103. The output voltage VOUTcan be controlled by the PWM duty-cycle of switch Q3. The primary side103aof the transformer-based output circuit103also includes a transformer primary side winding NP connected to the switched-capacitor converter101at the switching node SW2through a DC-decoupling capacitor C1. The DC-decoupling capacitor C1ensures that the transformer primary side winding NP is only provided with AC voltage in steady state. The secondary side103bof the transformer-based output circuit103includes synchronous rectifier switches Q5and Q6used for increasing high efficiency, though the switches Q5and Q6can alternatively be replaced by power diodes (see, e.g.,FIG. 2D) if a simplified circuit providing lower efficiency is desired.

FIG. 1Cshows simulation waveforms associated with the functioning of the circuit ofFIG. 1A. Specifically waveforms showing the current iCFflowing through flying capacitor CF, the current iC1flowing through capacitor C1and transformer winding NP, the current iQ3flowing through switch Q3, the current iQ4flowing through switch Q4, the voltage VSW2at switching node SW2, the voltage VMIDat node MID, the input voltage VINand the output voltage VOUTare shown through several cycles of operation of the hybrid converter100.

As shown inFIG. 1C, in the switched-capacitor converter101, the flying capacitor current iCFstarts and ends near OA in each switching cycle. Therefore the MOSFETs Q1-Q2run at a zero-current switching condition with minimum switching losses, while the MOSFETs Q3-Q4serve as the switched-capacitor FETs and integrated PWM converter FETs. The MOSFETs Q1-Q4have reduced voltage stress (e.g., the voltage across Q1-Q4remains at or below VIN/2) and therefore lower voltage rated FETs can be used as the MOSFETs Q1-Q4. In general, FETs with lower voltage ratings, such as FETs used as the MOSFETs Q1-Q4, have lower on-resistance than FETs with higher voltage ratings. Furthermore, iQ3=iC1−iCFduring the FET conduction time period, so the Q3and Q4switches current stresses are reduced, as compared to a conventional transformer-based power converter. The reduced voltage and current stresses of the hybrid converter switches allows the converter to be more efficient. As a result, the converter100can run at a much higher switching frequency fswwithout suffering high power losses, resulting in significantly reduced transformer and power inductor size and resulting in a high power density.

In operation of the switched-capacitor converter stage101, the feedback controller105controls operation of the MOSFET switches Q1-Q4. Generally, switches Q1and Q3are operated in unison, and switches Q2and Q4are operated in unison. Further, switches Q1and Q3are operated complementarily to switches Q2and Q4such that switches Q1and Q3are generally not open/conducting at the same time as switches Q2and Q4.

In this manner, in one example, switches Q1and Q3are closed during a first phase of operation while switches Q2and Q4are open. In turn, during a second phase of operation, switches Q1and Q3are open while switches Q2and Q4are closed. In the example, the power converter may alternate between the first and second phases of operation, and the feedback controller105may vary the relative lengths of the first and second phases (e.g., control the pulse width of signals controlling the switches Q1-Q4) in order to regulate the output voltage level VOUTto reach (and maintain) a particular voltage. During the first phase of operation, the capacitors CFand CMare coupled in series between VINand ground by closed switches Q1and Q3, and the capacitors are charged by the voltage VIN. In this phase of operation, the switching node SW2common to capacitors CFand CMmay thus be charged to a voltage of approximately VIN/2. In turn, during the second phase of operation, the capacitors CFand CMare coupled in parallel with each other, and the switching node SW2is brought to a ground voltage by switch Q4.

Overall, as shown inFIG. 1C, the 48 V input signal VINis converted into an output voltage VOUTof close to 12 V using a duty cycle close to 50%. The simulation also shows that the converter has another advantage. Specifically, the switched-capacitor converter current and the integrated PWM converter current are on the opposite direction through FET Q3and Q4in each switching cycle. Therefore, the FET Q3and Q4net current stress is reduced, resulting in reduced power losses and potentially high efficiency and power density.

As shown in the example ofFIG. 1A, the primary side103aof the transformer-based output circuit103is coupled to the switching node SW2of the switched-capacitor converter101. Alternatively, the primary side103aof the transformer-based output circuit103can be coupled to the switching node SW1and the node MID of the switched-capacitor converter101.

FIG. 1Aadditionally shows an illustrative secondary side103bof the transformer-based output circuit103stage in a single power converter. The secondary side103bcircuitry shown inFIG. 1Ais one example of a secondary side circuit that can be used in the hybrid power converter100. Other examples of transformer secondary side circuits are shown inFIGS. 2A-2Damong others.

In particular,FIGS. 2A-2Dshow possible configurations of the coupled transformer secondary side103bwinding rectifier circuits.FIGS. 2A and 2Dare examples of center-tapped winding structures with synchronous rectifier output in which a center-tap on the secondary coil winding of the transformer provides two secondary side coil windings Ls1and Ls2that are both magnetically coupled to the input coil winding NP, similarly to the transformer secondary side103bshown inFIG. 1A.FIG. 2Bis a full-bridge rectifier output, andFIG. 2Cis a current doubler synchronous rectifier output. The synchronous rectifiers ofFIGS. 2A and 2Ccan be used for high efficiency, especially for applications with lower VOUT. A non-synchronous rectifier output stage, such as those shown inFIGS. 2B and 2D, are simpler but less efficient and can be used for applications with higher VOUTvoltages.

The primary side103acircuitry shown inFIG. 1Ais one illustrative example of a primary side circuit that can be used a hybrid power converter, and various other primary side circuits can alternatively be used. For example,FIG. 2Eshows that the integrated hybrid converter103can be a resonant converter with a resonant inductor Lr and a resonant capacitor Cr coupled in series between switching node SW2of the switched-capacitor converter101and the transformer's primary winding NP. Alternatively, the resonant inductor Lr and resonant capacitor Cr can be coupled in series between switching node SW1and the transformer's primary winding NP, while the other terminal of NP is connected to node MID. In both cases, the secondary winding(s) Ns of the transformer can be coupled to any of the secondary side rectifier circuits shown inFIGS. 2F and 2G. Additionally,FIGS. 2F and 2Gshow integrated resonant LLC converter structures which do not need an output inductor in the secondary side rectifier circuit. In the circuits ofFIGS. 2F and 2G, the primary sides103aare the same as that shown inFIG. 2E, while the secondary sides103binclude modified circuits that do not include any inductor.

FIG. 3shows another illustrative implementation of a switched-capacitor converter stage101and primary side103acircuit. In the illustrative implementation ofFIG. 3, the circuit integrates the switched-capacitor converter101with an isolated half-bridge converter. The switched-capacitor converter stage101ofFIG. 3can be used in conjunction with any of the primary side103aand/or secondary side103bcircuitry shown and described in relation toFIGS. 1A and 2A-2G.

FIGS. 4A-4Dshow various alternative implementations of switched-capacitor converter stage101circuits that can be used in combination with any of the transformer-based output circuits103(including primary side103aand secondary side103bcircuitry) described above. InFIGS. 4A-4D, each of the switched-capacitor converter stages101includes combinations of capacitors and switches coupled between the input node VINand the node VSW2that is coupled to the primary side103aof the transformer-based output circuit103. In detail,FIG. 4Ashows an illustrative capacitor ladder circuit including six switches S1-S6and three capacitors C1-C3;FIG. 4Bshows an illustrative Dickson circuit including eight switches S1-S8and three capacitors C1-C3;FIG. 4Cshows an illustrative Fibonacci circuit including ten switches S1-S10and three capacitors C1-C3; andFIG. 4Dshows an illustrative Series-Parallel circuit including ten switches S1-S10and three capacitors C1-C3.

In operation, in the circuit ofFIG. 4A, the feedback controller105controls operation of the MOSFET switches S1-S4. Generally, switches S2, S4, and S6are operated in unison, and switches S1, S3, and S5are operated in unison. Further, switches S2, S4, and S6are operated complementarily or with 180 degree phase shift to switches S1, S3, and S5such that switches S2, S4, and S6are generally not open/conducting at the same time as switches S1, S3, and S5.

In the circuit ofFIG. 4B, the feedback controller105controls operation of the MOSFET switches S1-S8. Generally, switches S2, S4, S6, and S8are operated in unison, and switches S1, S3, S5, and S7are operated in unison. Further, switches S2, S4, S6, and S8are operated complementarily or with 180 degree phase shift to switches S1, S3, S5, and S7such that switches S2, S4, S6, and S8are generally not open/conducting at the same time as switches S1, S3, S5, and S7.

In the circuit ofFIG. 4C, the feedback controller105controls operation of the MOSFET switches S1-S10. Generally, switches S2, S4, S6, S8, and S10are operated in unison, and switches S1, S3, S5, S7, and S9are operated in unison. Further, switches S2, S4, S6, S8, and S10are operated complementarily or with 180 degree phase shift to switches S1, S3, S5, S7, and S9such that switches S2, S4, S6, S8, and S10are generally not open/conducting at the same time as switches S1, S3, S5, S7, and S9.

In the circuit ofFIG. 4D, the feedback controller105controls operation of the MOSFET switches S1-S10. Generally, switches S1-S6are operated in unison, and switches S7-S10are operated in unison. Further, switches S1-S6are operated complementarily or with 180 degree phase shift to switches S7-S10such that switches S1-S6are generally not open/conducting at the same time as switches S7-S10.

The converters described herein can be bi-directional converters. Additionally, the switched-capacitor converters101are not limited to 2:1 step down voltage dividers. More generally, the switched-capacitor converter101used in the hybrid converter100can be a n:1 switched-capacitor converter.

The proposed transformer-based hybrid power converters provide for reduced voltage stress and reduced power loss on the integrated FETs of the transformer-based output circuit stage as compared to FETs in a conventional PWM or resonant converter. As a result, the hybrid converters advantageously allow for lower power losses and higher switching frequencies, smaller magnetic component sizes, and higher power density.

It will be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. Relational terms such as first and second and the like may be used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a” or “an” does not, without further constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. Additionally, any element that is described as being “connected to” or “coupled to” another element can be directly “connected to” or “coupled to” the other element, or one or more other elements can be connected or coupled in between. In contrast, when an element is referred to as being “directly connected to” or “directly coupled to” the other element, there may be no intervening elements between them.