Patent Application
20150162840
This application claims priority to U.S. patent application Ser. No. 13/469,060 filed on May 10, 2012, which is hereby incorporated by reference in its entirety (the “Base Patent”).
This present invention relates generally to a power converting apparatus, and more specifically to a DC-DC converter using an LLC circuit in the region of voltage gain above unity.
Direct current (DC) architectures are well known, for example for the transmission and distribution of power. DC architectures generally provide efficient (low loss) distribution of electrical power relative to alternating current (AC) architectures.
The importance of DC architectures has increased because of factors including: (1) the reliance of computing and telecommunication equipment on DC input power, (2) the reliance of variable speed AC and DC drives on DC input power, (3) the production of DC power by solar photovoltaic systems, fuel cells, and various wind turbine technologies; (4) propulsion systems in electric and hybrid vehicles, marine applications; (5) aerospace applications; (6) micro-grids and smart grids, including the above, energy storage and electric charging stations; and (7) other systems that require converters with varying input voltage and load.
The widespread use of DC architectures has also expanded the need for DC-DC power converter circuits. Moreover, there is a further need for DC-DC power converter circuits that are efficient and low cost.
Traditionally, cost reduction is achieved in part by (1) reducing the components of DC-DC power converters, and (2) increasing the switching frequency of DC-DC power converters. These cost reduction methods can be achieved by implementing transformerless DC-DC converters that switch at high frequency. High frequency operation allows the circuit designer to reduce the size, and therefore the crust, of expensive components such as transformers, inductors and capacitors. Two of the most common transformerless DC-DC converters are the buck converter 10, as shown in
While both of these circuits are capable of achieving very high conversion efficiency when the input-to-output voltage ratio is near unity and the switching frequency is relatively low, their efficiency Is less than optimal when the voltage ratio becomes high or the switching frequency is increased to reduce the total size of the converter. In addition, in their basic form they do not provide galvanic isolation. Loss of efficiency, along with other operational problem, are caused by circuit parasitics, including such circuit effects as diode forward voltage drop, switch and diode conduction losses, switching losses, switch capacitances, inductor winding capacitance, and lead and trace inductances. Furthermore, it is known in the prior art that boost converters in particular are susceptible to parasitic effects and high efficiency operation requires low step up ration, e.g. 1:2 or 1:3.
B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, is an example of a resonant DC-DC power converter, where a resonant tank is excited at its resonant frequency to achieve high step-up/step-down conversion ratios without the use of transformers. An H-bridge based resonant DC-DC power converter was proposed by D. Jovcic (D. Jovcic, “Step-up MW DC-DC converter for MW size applications,” Institute of Engineering Technology, paper IET-2009-407) and modified for enhanced modularity by A. Abbas and P. Lehn (A. Abbas, P. Lehn, “Power electronic circuits for high voltage DC to DC converters,” University of Toronto, Invention disclosure RIS#10001913, 2009-03-31).
The converter disclosed in B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency” EPE, Dresden, 2005, requires two perfectly, or near to perfectly, matched inductors, each only utilized half of the time, to function properly. Perfect matching is not viable in many applications. Moreover, the fact that the inductor is only utilized half of the time effectively doubles the inductive requirements of the circuit. This is undesirable as the inductor is typically the single most expensive component in the power circuit. Furthermore, the converter in B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, requires both a positive and negative input supply. This is often not available.
The converters disclosed in D. Jovcic. “Step-up MW DC-DC converter for MW size applications,” Institute of Engineering Technology, paper IET-2009-407, and A. Abbas, P. Lehn, “Power electronic circuits for high voltage DC to DC converters,” University of Toronto Invention disclosure RIS#10001913, 2009-03-31, uses four high voltage reverse blocking switching devices. For medium frequency applications (approx. 20 kHz-100 kHz) such devices are not readily available thus they need to be created out of a series combination of an insulated-gate bipolar transistor (“IGBT”) and a diode, or a metal oxide semiconductor field effect transistor (“MOSFET”) MOSFET and a diode. This not only further increases system cost but it also nearly doubles the device conduction losses of the converter.
Galvanic isolation and larger voltage boost and buck ratios are possible with resonant and quasi-resonant DC-DC converters. These converters use inductive and capacitive components to shape the currents and/or voltages so that the switching losses are reduced allowing higher switching frequencies without a large efficiency penalty as explained in N. Mohan, T. Undeland, W. Robbins, “Power electronics; converters, applications, and design,” Wiley, 1995. Resonant and quasi-resonant DC-DC converters can be implemented with or without galvanic isolation.
A resonant converter with galvanic isolation is found in Bor-Ren and Shin-Feng Wu, “ZVS Resonant Converter With Series-Connected Transformers,” Industrial Electronics, IEEE Transactions on, vol. 58, No. 8, pp. 3547-3554, August 2011. In this work, a series resonant converter is implemented with multiple transformers connected in series. The proposed converter is designed to be used as a power factor pre-regulator in consumer electronic applications. The converter operates near the characteristic frequency defined by the resonant capacitor and resonant inductor. ZVS is achieved for all of the input switching component.
This converter analyzed by Bor-Ren Lin and Shin-Fang Wu uses a conventional resonant converter design approach. The resonant tank is only able to provide minimal voltage boosting, if necessary, and any voltage boosting or bucking must come entirely from the transformer turns ratio. The small amount of voltage boosting that can be provided is used when the input voltage is low. Furthermore, due to the resonant tank design, this converter would not be suitable to control of the power flow between an input and an output voltage source.
Series resonant converters and parallel resonant converters are known to be very efficient for a small range of operating points. They can be implemented without galvanic isolation or with galvanic isolation. For applications that require a large range of input voltages and loads, they are not ideal. As shown in B. Yang, “Topology Investigation for Front End DC/DC Power Conversion for Distributed Power System”, Ph.D. Dissertation, Virginia Tech, 2003, both series resonant converters and parallel resonant converters suffer from large circulating currents, and large switching cents when the input voltage is high.
In B. Yang. “Topology Investigation for Front Bad DC/DC Power Conversion for Distributed Power System”, Ph.D. Dissertation, Virginia Tech, 2003 the author shows that some of the limitations in traditional series resonant or parallel resonant converters can be overcome by using an LLC resonant converter.
R. L. Lin and C. W. Lin, “Design criteria for resonant tank or LLC DC to DC resonant converter”. IEEE 2010, presents a conventional design approach to obtain an LLC step down converter. The designed converter has a maximum voltage gain from the resonant tank of only 1.44, which is needed when the input voltage is at a minimum. For high input voltage the circuit is operated at, or just below, unity gain. A 9:1 transformer provides the net voltage step down needed for the application.
IL Hu. X. Pang, Q. Zhang, Z. Shen, and I. Batarseh, “Optimal design considerations for a modified LLC converter with wide input voltage range capability suitable for PV applications,” ECCE 2011, is an example of a conventional LLC design methodology applied to a step up converter where the resonant circuit provides close to unity gain. All of the voltage gain is achieved through the output transformer.
In both of the works of R. L. Lin et. al. and H. He et. al., the conventional LLC design methodology used yields a resonant tank with very low voltage boosting properties. Furthermore, both designs require a resistive load at the output for proper functionality. These converters, and all LLC converters designed with the conventional method, are not suitable for applications where the power flow between two voltage sources is regulated.
In U.S. Pat. No. 6,344,979 an LLC converter is claimed where the converter is operated between the two characteristic frequencies of the converter,
to maintain output voltage regulation. However, the authors failed to address the high voltage gain region of operation and the advantages of operating there, as well as bow, by choosing the right components, the designer can always ensure operation in this region. In addition, the zero current switching region of operation, designated as “LHS Operation” in this document, was not utilized nor were the benefits of operating in this region identified. The “LHS Operation” region is also only usable by a careful selection of resonant tank components, as identified in the current invention.
In one aspect of the invention, a method of operating a resonant DC-DC converter is provided, the resonant DC-DC converter comprising a high voltage boost LLC circuit, wherein the method comprises providing variable power flow control to the LLC circuit with externally determined input and output voltages using frequency control.
In a further aspect of the invention, the externally determined output voltage is created by either a single externally determined output voltage, or a series connection of two externally determined output voltages to create a bi-polar output.
In another aspect of the invention, a method is provided wherein frequency control is applied such that it emulates different loading conditions thus operating along horizontal curves on the voltage gain compared to the switching frequency operating plane.
In a still other aspect of the invention, the LLC circuit includes an LLC resonant tank, and wherein the LLC resonant tank operates with a minimum booting having an effective value that is above unity over the entire operating range.
In another aspect, of the method of the invention, the minimum boosting results in controllable transfer of power via change of switching frequency.
In another aspect of the invention, the method further comprises maintaining an externally determined voltage gain and using frequency control to enable movement between the load curves, and to control this movement within a frequency control region where there is horizontal separation amongst the load curves.
In another aspect of the invention, the method further comprises: (A) operating the high voltage boost LLC circuit in a region close to a resonant frequency determined by a resonant inductor, magnetizing inductor and a resonant capacitor, to achieve a high voltage boost and (B) utilizing unipolar or bipolar resonant rank excitation to improve converter efficiency in the high voltage boost circuit.
In yet another aspect of the invention, a balanced bipolar DC output is provided wherein the output capacitor voltages are automatically balanced.
In a sill other aspect of the invention, the DC-DC converter further includes a resonant inductor, a magnetizing inductor and a resonant capacitor, and the method comprises the further step of selecting these components such that the yield over the entire range of operation is an effective voltage gain that is greater than unity.
In another aspect of the invention, the LLC converter is implemented with a transformer to allow decoupling of the resonant circuit pin from the externally determined voltage gain.
In a still other aspect of the invention, the effective voltage gain value and the components am selected so as to minimize the effective voltage gain of the resonant circuit, while being greater than unity, and provide controllability of the DC-DC converter via frequency.
In another aspect of the invention, the method further comprises operating at a range of input stage switching frequencies in an LLC circuit whereby a change in input voltage results in a change in load or transferred power, such that a decoupling between the input voltage and load is not required.
In one aspect of the invention, a resonant DC-DC converter is provided for high voltage step-up ratio, where the resonant DC-DC converter for high voltage step-up ratio comprises: (A) a low voltage full-bridge or half-bridge DC-AC converter (B) an LLC resonant tank; (C) a high voltage AC-DC converter or rectifier and (D) a high voltage controllable switch; wherein the high voltage controllable switch is controllable to regulate power flow from an input to an output of the DC-DC converter based on an externally determined voltage gain ratio, wherein the LLC resonant tank operates with a minimum boosting having an effective value above unity over the entire operating range.
In another aspect of the invention, the DC-DC converter is designed to provide variable power flow control using frequency control.
In another aspect of the invention, a DC-DC converter is provided wherein application of frequency control emulates different loading conditions thus enabling operation along horizontal curves on a voltage gain compared to a switching frequency operating plane.
In another aspect of the invention, a DC-DC converter is provided wherein the minimum boosting results in controllable transfer of power based on change of switching frequency.
In yet another aspect of the invention, a DC-DC converter is provided that maintains an externally determined voltage gain, and uses frequency control to enable movement between the load curves, and controls this movement within a frequency control region where there is horizontal separation amongst the load curves.
In another aspect of the invention, a DC-DC converter is provided that is designed for: (A) operation of a high voltage boost LLC circuit in a region close to a resonant frequency determined by a resonant inductor, magnetizing inductor and a resonant capacitor, to achieve a high voltage boost; and (B) use of unipolar or bipolar resonant tank excitation to improve converter efficiency in the high voltage boost circuit.
In another aspect of the present invention, a DC-DC converter is provided that further comprises a balanced bipolar DC output wherein output capacitor voltages are automatically balanced.
In a still other aspect, a DC-DC converter is provided that further includes a resonant inductor, a magnetizing inductor and a resonant capacitor, these components being selected such that the yield over the entire range of operation is an effective voltage gain that is greater than unity.
In yet another aspect of the invention, a DC-DC converter is provided that comprises a transformer to allow decoupling of the resonant circuit gain from the externally determined voltage gain.
In a still other aspect of the invention, a DC-DC converter if provided wherein the components are selected so as to minimize the effective voltage gain of the resonant circuit, while being greater than unity, and provide controllability of the DC-DC converter via frequency.
In one aspect of the invention, a method of designing a resonant DC-DC converter for high voltage boost ratio is provided, the DC-DC converter comprising: (A) a low voltage full-bridge or half-bridge DC-AC converter; (B) an LLC resonant tank; (C) a high voltage AC-DC converter or rectifier and (D) optionally, a high voltage controllable switch; wherein the high voltage controllable switch is controllable to regulate power flow from an input to an output of the DC DC converter based on a externally determined input to output voltage gain ratio maintained by the high voltage controllable switch using frequency control, wherein the DC-DC converter includes (i) a resonant capacitor, (I) a resonant inductor, and (iii) a magnetizing inductor wherein the design method comprises: (i) determining a minimum gain sufficient to enable high-resolution control of frequency using available control hardware; (ii) selecting an Lm/Lr ratio that is suitable for an application for the DC-DC converter (iii) generating voltage gain curves for various values of Q, and plotting these values so as to graph a boundary curve that defines LHS and RHS regions, and selecting the Q values whose voltage gain curve Intersects with boundary curve at the maximum voltage boost ratio, thereby defining a act of normalized frequency values; and (iv) using the Q values and the normalized frequency values found to calculate values for the resonant capacitor, the resonant inductor, and the magnetizing inductor so as to enable selection of suitable components for the application.
In another aspect of the invention, a method of designing a resonant DC-DC converter for high voltage boost ratio, the DC-DC converter comprising: (A) a low voltage full-bridge or half-bridge DC-AC converter (B) an LLC resonant tank; (C) a high voltage AC-DC converter or rectifier; and (D) optionally, a high voltage controllable switch; wherein the high voltage controllable switch is controllable to regulate power flow from an input to an output of the DI-DC converter based on a externally determined input to output voltage gain ratio. Power flow control is maintained using frequency control. The DC-DC converter Includes (i) a resonant capacitor, (ii) a resonant inductor, and (ill) a magnetizing inductor, wherein the design method comprises; (1) determining a minimum gin sufficient to enable high-resolution control of frequency using available control hardware; (2) selecting an Lm/Lr ratio that is suitable for an application for the DC-DC converter; (3) generating voltage gain curves for various values of Q, and plotting these values so as to graph a boundary curve that defies LHS and RHS regions, and selecting the Q values whose voltage gain curve intersects with boundary curve at the maximum voltage boost ratio, thereby defining a set of normalized frequency values; and (4) using the Q values and the normalized frequency values found to calculate values for the resonant capacitor, the resonant inductor, and the magnetizing inductor so as to enable selection of suitable components for the application.
It is understood that the invention is capable of operating with other resonant converter configuration known in previous at and/or used in different applications. It is also understood that the invention is usable in applications with different grounding requirements including floating systems, high impedance grounded systems, and solidly grounded systems and that the use or not of a transformer may be influenced by the grounding requirements.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed here in are for the purpose of description and should not be regarded as limiting.
The invention will be better understood and objects of the invention will become apparent when consideration is given to the following detailed description thereof.
Such description makes reference to the annexed drawings wherein:
a), 3(b) and 3(c) illustrate three representative implementations of the half-bridge resonant DC-DC converter, having a single high voltage switch.
a) and 4(b) illustrate an implementation of a full-bridge resonant DC-DC converter.
a), 5(h), 5(c) and 5(d) illustrate four representative implementations of the full-bridge resonant DC-DC converter of the present invention, having a single high voltage switch and a common ground on the Input and the output.
a), 6(b) and 6(c) illustrate the three representative circuits of an alternate implementation of the circuit design of the present invention that include transformer.
a) and 13(b) are circuit diagrams illustrating alternate implementations of the full-bridge resonant DC-DC converter of the present invention, with a common ground for the input and the output, but without a high voltage switch.
a), 14(b) and 14(c) are a circuit diagrams illustrating a possible LLC converter circuit designs, that am (A) operated in a novel and innovative way based on the methods of the present invention, and (B) redesigned also as described in this disclosure.
a), 24(b). 24(c) and 24(d) illustrate four representative implementations of the resonant DC-DC converter of the present invention with externally determined input and output voltages and no interrupt switch, where
a) and 26(b) show the general form of the current invention with and without an interrupt switch.
In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended at a definition of the limits of the invention.
The present invention describes a number of innovations related to the subject matter of the Base Application. The present invention includes (A) a novel and innovative resonant DC-DC converter that employs a high boost resonant tank to enable power flow control between externally determined input and output voltages using frequency control, with or without use of an interrupt switch (the “Improved DC-DC Converter”), (B) a method of operating a resonant DC-DC converter to achieve high boost resonant tank operation, which is suitable for improving the performance of resonant converters based on different topologies (“method of operation”), including but not limited to the Improved DC-DC Converter; and (C) a method for designing DC-DC converters (having different topologies) for improved performance using the method of operation (“design method”). The design method includes identification of circuit design parameters that enable use of the method of operation. Performance improvements include improved resolution of power flow control between externally determined input and output voltages and maximization of range of allowable voltage conversion ratio, while meeting a specified power flow. Operation of the converter ova a reduced range of frequencies may also allow circuit components to be better optimized for efficiency. In full-bridge embodiments, as exemplified in
For example, use of the embodiment of
There are related patent applications to the Base Patent including PCT/CA2011/000185, filed Feb. 18, 2011, claiming priority to United States patent application No. Mar. 18, 2010 (the “Related Patents”). Certain details of the Related Patents are restated here to aid in understanding of the invention. More particularly the disclosure discusses DC-DC converters that include an interrupt switch and also that do not include an interrupt switch, because aspects (B) and (C) of the invention are relevant to both types of circuits.
One aspect of the invention is a resonant converter circuit design operable to achieve high input-to-output voltage conversion. In particular the invention may include a series of converter circuit topologies that provide high resonant tank boot ratio and achieve high efficiency operation. The converter circuit topologies may include a resonant tank and (in one aspect) a means for interrupting the tank current to produce a near zero-loss “hold” state wherein zero current and/or zero voltage switching is provided, while providing control over the amount of power transfer. Specifically the converter circuit topologies may control energy transfer by controlling the duration of the near zero-loss “hold”. This may be referred to as the “Interrupt control mode” (again, shown for example in
The present invention may avoid unnecessary circulating current during low power operation, thereby reducing losses within the tank components and the low voltage DC/AC converter, and also reducing switching losses based on the zero voltage switching of the low voltage DC/AC converter and zero current switching of the low voltage DC/AC converter. Also, are current switching of the high voltage controllable switch within the tank may be achieved and thereby keep its own switching losses low.
As described herein, the present invention may have several embodiments that present converter circuit topologies that provide high input-to-output voltage conversion and achieve high efficiency operation. Examples of these embodiments are disclosed herein; however a skilled reader will recognize that these examples do not limit the scope of the present invention and that other embodiments of the preset invention may also be possible.
For clarity, the term “low voltage” is used in this disclosure to refer to components with voltage ratings comparable to that of the input, and the term “high voltage” is used in this disclosure to refer to components with voltage rating comparable to, or above, the peak voltage level seen across the resonant tank capacitor.
In embodiments of the peasant invention, appropriate implementation of the near zero-loss hold state, may cause zero voltage switching or zero current switching to be achieved for all controllable switches within the circuit.
Embodiments of the present invention may provide a lower loss converter circuit for high input-to-output voltage convention ratio converters.
A skilled reader will understand that the circuit design of the present invention may include a variety of elements. In one embodiment these elements may include: (1) an input DC/AC converter; (2) a resonant tank; (3) a tank interruption means (such as a switch as described herein); and (4) an output rectifier. The output rectifier may, for example, include a filter inductor that limits the rate of rise of current in the output diode. Regarding the input DC/AC, a skilled reader will recognize that a number of different types of inverters may be suitable, for example, such as a half-bridge or full-bridge type inverter. A skilled reader Ill further recognize that the output rectifier may include any output rectifier stage, for example, such as a half-bridge or full-bridge rectifier. In some embodiments of the present invention, a transformer may be included in the circuit, prior to the output rectification stage.
In one embodiment of the present invention, the circuit design may be a circuit that includes: (1) a full-bridge DC/AC converter, (2) a resonant tank consisting of two L components and one C component; (3) a tank interruption switch; and (i) an output rectifier stage (full-bridge or half-bridge), wherein a common ground may be provided for both the input voltage and the output voltage. Possible embodiments of the present invention that include such a circuit design are shown in
The circuit may, or may not, include a transformer. In an embodiment of the present invention wherein a full-bridge output rectifier is utilized a transformer may also be required. In an embodiment of the present invention that includes a transformer, the resonant L components may be integrated into the transformer design. The choice to Include a transformer in an embodiment of the present invention may be based on specifications of the circuit of the embodiment of the present invention, or other preferences or considerations. This document discloses and describes some examples of both: embodiments of the present invention that include a transformer element; and embodiments of the present invention that do not include a transformer element, and therefore are a transformerless.
a), 6(b) and 6(c) show embodiments of the present invention that the circuits 42, 44 and 46 respectively, that include an alternate implementation, wherein additional windings were added to the main inductor's magnetic core dins decreasing the voltage stress on switch Sx. The addition of windings may convert the inductor L into a transformer with isolation, which provides additional circuit implementation options. The embodiment of the present invention shown in
As shown in
As shown in
Transfer of power from the resonant tank to the output may occur twice jar period, once to the positive DC output, once to the negative DC output. Power transfer to the positive output may take place immediately after the turn on of switches S1 and S2p. Power transfer to the negative output may take place immediately after the turn on of switches S2 and Stp.
In one embodiment of the present invention, a circuit may be provided consisting of a DC-AC converter followed by a (parallel) resonant tank with single controllable high voltage switch, followed by an AC-DC converter.
Embodiment of the present invention that includes the proposed “half-bridge floating tank” resonant DC-DC converter configuration are shown in
As shown in
Embodiments of the present invention, as shown in
It should be understood that the DC-DC converter of the present invention as shown in
A skilled reader will recognize that other variants and embodiment of the present invention are possible. For example an embodiment of the present invention may use emerging reverse block IGBT devices, in which case Sx may be eliminated, but S1 and S2 may each need to consist of a high voltage reverse blocking IGBT. Such an embodiment of the present invention may yield precisely the same voltage and current waveforms within the tank and output circuitry. Numerous other variations are possible.
In an embodiment of the present invention, the circuit design may be such that the high voltage switch needs not be reverse blocking, and thus MOSFET or IGBTs may be used instead of, for example, thrysitors (which limit switching frequencies to excessively low values), or MOSFET-series-diode/IGBT-series-diode combinations.
Also, in embodiments of the present invention, the circuit designs may use an electrically floating tank, as further explained below.
Certain aspects of the invention are explained in greater detail below, however theme details should not be read as limiting the scope of the invention in anyway, but as examples of embodiments of the present invention.
The half-bridge floating tank converter may be included in embodiments of the present invention. In such an embodiment of the present invention, the switching process may very slightly based on the type of switches used and the location/orientation of the high voltage switch (Sx) within the tank circuit. A description of a possible switching process to be used in an embodiment of the present invention is provided herein with reference to a topology 30 wherein S1 and S2 are implemented using MOSFETs and Sx is implemented using a high voltage IGBT, as shown in
In one embodiment of the present invention, as shown in
An example of the operation of the circuit may be as follows:
Embodiments of the present invention may include a full-bridge floating tank converter with common pound, as shown in
In one embodiment of the present invention, as shown in
An example of the operation of the circuit may be as follows:
Embodiments of the present invention may include a full-bridge floating tank converter with common ground that is operable to transfer energy during both positive and negative half cycles of the tank current, without use of a transformer, while maintaining a common round on input and output, as required for many applications. The purpose of Sx in this circuit may be to achieve zero current/zero voltage switching while still offering control over the amount of power transfer. Thus near zero switching loan may be achieved while simultaneously maintaining control over the amount of power transfer.
As silicon carbide switching devices, or other devices with low reverse recovery loss, become more cost effective it may become worthwhile to eliminate Sx.
Nonetheless, a common ground arrangement capable of transferring energy during both positive and negative half cycles of the tank current may still be desired. The circuit topologies 38 and 40 of
The full-bridge converter with common ground may offers important benefits compared to the conventional resonant converters as outlined in R. Erickson, D. Maksimovic. “Fundamentals of Power Electronics,” Kluwer Academic Publishers, 2001. Specifically the topology of an embodiment of the present invention that includes a full-bridge converter with common ground may offer common ground on input and output along with a high step-up ratio and may offer power transfer into the tank during both positive and negative half cycles of the tank current.
As examples of embodiments of the present invention and the benefits that these offer over the prior art, benefits of particular features of two principal circuit arrangements (a half-bridge floating tank converter, and a full-bridge floating tank converter with common ground) over the prior art are described below. A skilled reader will recognize that the features and benefits discussed below are merely provided as examples, and other embodiments and benefits are also possible.
Embodiments of the present invention that include a half-bridge floating tank converter may offer particular benefits over the prior art. Some of these benefits include the following.
Embodiments of the present invention that include a full-bridge floating tank converter with common ground may offer particular benefits over the prior art. Some of these benefits include the following:
A skilled reader will recognize that numerous implementations of the technology of the present invention are possible. The circuit designs of embodiments of the present invention may present a modular structure and therefore components may be added or removed, while providing the functionality of the design, as described above. For example, particular embodiments at the DC-DC converter of the present invention may be transformerless. In other embodiments of the present invention it may be desirable to include a transformer in the circuit, such as the circuit shown in
A skilled reader will recognize that in embodiments of the present invention specific aspects of the topologies described and shown herein may be modified, without departing from the essence, essential elements and essential function of the topologies. Pr example, in the circuit design 42 shown in
In one embodiment of the present invention the switching elements, for example as shown in the
The inventors have realized DC-DC converters may be provided that include improved performance characteristics of the DC-DC converters disclosed above, however, without the interrupt switch disclosed in the Base Patent.
More particularly, in another aspect of the present invention, it has been realized by the inventors that it is also possible to build a desired resonant DC-DC converter for providing a high voltage step-up ratio without employing a tank interruption switch Sx as exemplified by the circuit topologies 38 and 40 of
More specifically, it is possible to achieve a high boost ratio from the resonant tank through the careful selection of resonant components. An illustrative example Is shown in
The “Classic” LLC Circuit DC-DC converter topologies shown in
An LLC converter as illustrated in
It can be shown that the voltage gain of the circuit is defined by:
The voltage gain can be then calculated for different loadings and frequencies to produce the plots shown in
The different lines are plots at different loading conditions (constant R), or stated alternatively, at different Q values as determined by Equation (4) below. As seen by the equation, as the load decreases (R decreases), the Q value is reduced in an inverse proportional relationship. In
The resonant frequencies of the circuit are defined by fr1 and fr0, defined below
In conventional applications, such as power supplies, a Classic LLC Circuit is generally operated near fr1 as indicated in
It has not been obvious to a person skilled in the art that the Classic LLC Circuit topology can be operated over a frequency range well below fr1 (fr1 is not within the operating range) by selecting the components such that the value of Q is well below 1 for the full load range specified. Furthermore, the circuit has not been used in applications that require control of the power transfer between two regulated or unregulated DC sources.
In one embodiment of the present invention, the LLC topology is designed to operate with switching frequencies well below fr1, close to the second resonant frequency of the circuit, fr0. Operation in the area near to can be divided into two distinct operating regions as shown in
For a specific application, the range of input voltage and the range of load (power transfer) is known. The output voltage is also known based on components to be powered by the converter or the externally regulated voltage bus that is to receive power. In one aspect of the invention what follows is a possible method for designing circuits based on said LLC topology, but providing relatively high boost ratios:
1) Choose an Lm/Lr ratio that is suitable for the application. Typical values range from, but are not limited to, 3-10. Large values will result in higher peak currents in the tank, while small values will result in larger switching losses at low loads.
2) Generate voltage gain curves for various values of Q. On that plot, the boundary curve separating LHS and RHS regions may be graphed in a similar manner to that shown in
3) From the plot, select the Q value whose voltage gain curve intersects with the boundary curve at the desired voltage boost ratio. Note the Q value and normalized frequency (fn) of this intersection point.
4) Using the Q and normalized frequency values found in step 3, calculate the Lr and Cr values.
5) Using the Lr value calculated above and the desired Lm/Lr ratio, calculate Im
In a aspect, the first method discovered to achieve controllability of the above design was the introduction of an interrupt switch in the LLC Resonant Tank (the “Interrupt Switch LLC Circuit”). The interrupt switch allows the Q valve to be solely dependent on the input voltage and not the load. As the input voltage increases, the Q value decreases. The Input Stage switching frequency of the circuit is used to compensate for changes in the input voltage and the off time of the interrupt switch is used to adjust to the changes in load. The decoupling of the load (using the interrupt switch in the LLC Resonant Tank) from the input voltage (using the Input Stage switching frequency) allows for a simple implementation of a controller and stable control.
As disclosed in earlier described embodiments, the introduction of an interrupt switch into the LLC Resonant Tank also enables the use of the Interrupt Switch LLC Circuit in new applications where the LLC Resonant Tank is operated in the conventional region of operation close to fr1. The use of the Classic LLC Resonant Circuit in this operating region is not easily realizable with the classic frequency control method. In other words, the Interrupt Switch LLC Circuit is suited to new applications where the objective of the LLC circuit is not to regulate the output voltage but instead to regulate the power delivered to an output voltage regulated externally.
Those skilled in the at will understand that prior to the present invention, DC-DC converters of the type described in this disclosure would be operated in the “Conventional Region of Operation” shown in
The inventors discovered that when operating the LLC resonant tank with a minimum boosting having an effective value above unity over the entire operating range, it was unnecessary to decouple the load from the input voltage using the interrupt switch referred to above.
The inventors discovered that if the LLC resonant tank is operated so as to be given sufficient boosting gain, a change in either the input voltage or the switching frequency results in a corresponding change in load (power transfer). This is illustrated in
In particular,
This provides the reduced frequency range of operation required to control the load, and chopping of much smaller currents than conventional non-boosting LLC circuits.
A skilled reader will appreciate that components of a PC-DC converter designed to embody the mode of operation described may be selected so as to improve performance within the frequency range described.
Therefore, the objective of the design method of the present invention is to provide a DC-DC converter that is designed so that the boosting gain is above unity. Theoretically, the boosting gain can be designed as close to unity as desired provided a frequency controller with an infinite frequency resolution. Practical implementations of the converter which use frequency controllers with a finite resolution will require a minimum boosting gain above unity which achieves the desired controllability, i.e., the desired power flow resolution. For example, using currently available microcontroller hardware and a resonant frequency in the range of 50 kHz to 100 kHz, a boosting gain of 1.25 may be practical to maintain power flow controllability with practical power flow resolution over the entire operating range. A skilled reader will appreciate that this “minimum boosting gain above unity” will vary depending for example on the particular components selected, or that are available on an economic basis. Also, this will vary with further technical or manufacturing developments in regards to such components. Through use of an appropriate design methodology, as will be described later, it is possible to transfer any desired amount load power via frequency control by appropriate selection of circuit parameters.
The operation of the HVBC will now be described more detail. As discussed, the HVBC is operated in a unique mode of operation.
Conventionally, LLC power supplies are designed to operate near the resonant frequency defined by the resonant inductor and resonant capacitor, fr1. This region of operation can be seen in
In the present HVBC embodiment, the LLC is designed such that it is operating very close to the resonant frequency determined by the resonant inductor, magnetizing inductor and the resonant capacitor, which will be referred to as Ir0. In
It will also be appreciated that the regions of operation as defined by in
Now referring to
The following is a description of a possible switching cycle method for an embodiment of the present invention utilizing a full-bridge DC-AC inverter and a split output circuit, operating in the “RHS Operation” region. The circuit is shown in
Although the above control descriptions are based on the circuit using a full bridge DC-AC converter and the split output circuit, a person skilled in the art could be able to identify that the general operation, is similar in other embodiments. Differences in the number of pulses transferred per period, the type of load receiving the power pulses, or the location of the components used to produce the resonance amongst other do not change the operation principles for the circuit.
The benefits of the circuit over the classical LLC converter control are (a significantly longer switching period (approximately 2 times) for a given set of components; (ii) a reduction in switching losses; (ii) a reduction in losses within the resonant tank (comprised of Cr, Lr and Lm); and (iv) the ability to regulate power transfer between two externally determined DC sources.
As described earlier, switching of the DC/AC converter may be carried out such that the DC/AC converter output is either an AC waveform of +V1 and −V1, or an AC waveform of either V1 and 0 or −V1 and 0. The ability to switch between these modes of operation will be called “Unipolar/Bipolar Resonant Tank Excitation Control”. Unipolar/Bipolar Resonant Tank Excitation Control changes how the resonant tank is excited in order to operate the converter in its most efficient control mode for a given input power.
As shown in
As shown in
In summary, the focus of the present embodiment is on a unique mode of operation that yields a large voltage boost in the resonant tank. This voltage boost allows the present HVBC embodiment to achieve very high efficiencies at high conversion ratios. With the present HVBC design, the resonant tank of an LC converter can be designed to yield high voltage gain, useful for step up converters. As well, the converter can be operated with a low Q over the entire load range. This is achieved by knowing the load, and designing the resonant components around it. Furthermore, the resonant tank can be stimulated near the resonant frequency fr0, and operation of the converter in this region yields to ZVS, and low current switching (LCS), to yield a highly efficient, step up converter. This mode of operation makes is viable for the converter to transfer power between two externally determined voltage sources.
Comparative Analysis of Interrupt Switch Control Vs. Frequency Control for Boosting LLC Tank Circuits
As noted above, both interrupt switch control and frequency control may be used for boosting LLC Tank Circuits. This analysis focuses on the application of the interrupt switch concept to LLC converter applications and compares it to frequency control of the LLC converter.
Resonant converters are designed to transfer power from an input source to an output load. The output voltage divide by the input voltage is referred to as the gain of the converter. The theoretical pin of the LLC converter can be approximated using first harmonic approximation (FHA) techniques, it is then analyzed using the simplified approximate circuit shown in =8R/π2 [See H. Huang. “LLC Resonant Half Bridge Converter”, Texas Instruments Presentation from Asia Tech-day, Aug. 27, 2009.] and R is the DC load resistance across a conventional full bridge output rectifier.
In many applications we wish to supply a constant output voltage, Vo, from a given input voltage source, approximated by V. Based on the simplified model, the amount of current, Im, flowing in Lm will be constant for a given Vo. In contrast the amount of current flowing in the load, Ic, will depend on the load resistance R
.
The current, Ir, seen by the input ac source, the capacitor Cr and the inductor Lr therefore has two components:
The current Im itself transfers no power to the load, it is merely required to enable the process of energy transfer.
At higher load Ic comprises a large percentage of Ir, leading to highly efficient operation.
Using frequency control, lighter loading conditions result in Im comprising a larger percentage of Ir. Since numerous loses are related the amplitude of Ir, efficiency will suffer at light load conditions. Particularly at power levels below 15% of rated power, the efficiency typically becomes very poor.
The interrupt switch enables a high Ie to Ir ratio to be employed under all loading conditions. At full load the Ie to Ir ratio is high by its very nature, posing no challenge. To operate at reduced load the interrupt switch introduces a near zero loss hold state. This yields an efficiency that is roughly independent of loading conditions. It should also be noted that each time the convert leaves the hold state one pulse of energy is transferred to the output. For a given input and output voltage the sine of this energy pulse Is constant. Power transfer is controlled by merely regulating the number of energy pulses that are released by the interrupt switch.
Under frequency control we operate along a horizontal line, moving to higher frequencies to decrease power. The amount of power transfer varies nonlinearly with the operating frequency.
A clear negative impact of employing the interrupt switch is that this device adds additional conduction losses to the resonant tank circuit.
This leads to a trade-off between low power and high power efficiency ma follows:
The following is a list of benefits of the interrupt switch:
The following is a list of drawbacks of the interrupt switch:
The following are benefits of tow using frequency control in place of interrupt control in an LLC converter:
The following we drawbacks of the using frequency control in place of interrupt control in an LLC converter.
The interrupt switch topology Is used in two main applications:
1. In applications where a high efficiency is desired and the converter operates at low power for long periods of times, such as standby power applications.
2. In low boosting applications where the power flow between two voltage sources needs to be controlled, including but not limited to, i) residential application of solar photovoltaic systems (including module level optimizers and micro-inverter), fuel cells, permanent magnet wind turbines, micro-grids and energy storage ii) small power marine and aerospace applications (low voltage); and iii) and other systems that require converters with varying input voltage and load at low input and output voltages.
This design example illustrates how the selection of appropriate components in an LLC converter can yield the desired low Q operation. A brief overview of the theory will be presented followed by a step-by-step design example. The document concludes with a discussion section about the component selection.
The theoretical gain of the LLC converter can be approximated using first harmonic approximation (FHA) techniques. Assuming the circuit is stimulated by a perfect sinusoid, one can use conventional circuit analysis to determine the voltage gain of the circuit. The LLC converter under study can be simplified to the circuit shown in
It can be shown that the voltage gain of the circuit is defined by:
where
Furthermore, one can find a transfer function between the input voltage and the resonant current, Ir. The phase of the resonant current determines the region of operation of the converter. For example, if the resonant current is leading the input voltage, the LC converter is in the “LHS Operation” region. Conversely, when the resonant current is lagging the input voltage, the converter is in the “RHS Operation” region. The border between the two regions is where the resonant tank behaves like a perfect resistor. The dashed line in
The values that make up the dashed line can be determined by setting the imaginary part of the input voltage to resonant current transfer function to zero. The result is to solve for the roots or the following quadratic equation in ω2 (ω*2πf);
For voltage boosting applications, the circuit must be designed such that it can operate with voltage gains greater than 1. In
This section will present an iterative design procedure to design the components for an LLC circuit based on a low Q operation.
Consider the following design constraints:
Therefore, we can determine:
R=V
out
2
/P
max=80 Ω
R
c=8R/π2=64.8 Ω
M
maximum
=V
out maximum
/V
in maximum=200V/50V=4
M
minimum
=V
out minimum
/V
in minimum=180V/90V=2
Using these design constraints, the Cr, Lr, and Lm need to be determined.
As calculated above, this particular example of a converter requires a maximum gain of 4 based on the voltage that converter will be exposed to. Therefore, the method enables the determination of the resonant components that will yield the required maximum voltage gain, while operating in the LHS region. A skilled reader will appreciate that maximum gain drives the circuit design.
The design process can be easily automated through software and can be applied to any general form of the LLC circuit as shown in
This section will implement the design step presented in the previous section to the converter constraints listed above.
The final converter design can then have LLC components with the following values:
The converter design described in the previous sec don 12 unique for the given constraints and the selected Lm/Lr ratio. However, each time the designer selects new constraints, a new sat of components must be calculated. As a consequence, there are in Infinite number of different LLC converters that operate with high boosting and low Q. Table A shows a small sample of possible resonant Link component values for converters designed to operate at 300 kHz and various Q and voltage boosting values. All of these converters may be successfully operated using frequency control to regulate load power.
indicates data missing or illegible when filed
This design methodology is used to design resonant LLC converters with high voltage gain. Traditionally, resonant LLC converters are designed with unity voltage gain, for voltage step down conversion. As a result, traditional designs will have larger Q values, and will operate near the resonant frequency fr1.
It will be appreciated by those skilled in the art that other variations of the embodiments described herein may also be practiced without departing from the scope of the invention. Other modifications are therefore possible. A skilled reader will recognize that the are numerous applications for the DC-DC converter technology described. The DC-DC converters of the present invention may provide an efficient, low cost alternative to numerous components providing high input-to-output voltage conversion. Moreover, DC-DC converters with high amplification ratios that are embodiments of the present invention may be used to create a fixed voltage DC bus in renewable/alternative energy applications.
A skilled reader will understand that the (A) method of operating a resonant DC-DC converter of the present invention; (B) the DC-DC converter disclosed herein; and (C) the method of designing a resonant DC-DC converter for high voltage boost ratio, may be used in connection with a range of different applications, including in connection with photovoltaic systems; a fuel cells; permanent magnet wind turbines; electric and hybrid vehicles; electric charge stations; aerospace systems; marine systems; power grids or smart grids including micro grids; and energy storage systems.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2012/001021 | 11/6/2012 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61305590 | Feb 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13469060 | May 2012 | US |
| Child | 14399563 | US | |
| Parent | 13384294 | Apr 2012 | US |
| Child | 13469060 | US |
