Multi-stage buck converter with reverse charging capability

Abstract

Power converter configured to operate in one of the following modes: either in - a step-down or buck mode for transmitting electrical power from a first terminal (61) of the power converter to a second terminal (62) of the power converter, or in - a boost mode for transmitting electrical power from the second terminal (62) of the power converter to the first terminal (61) of the power converter, wherein the power converter comprises a first switching element (631), a second switching element (632), a third switching element (633), a fourth switching element (634), a flying capacitor (642), an inductor (68) and a control unit (65), wherein - the first switching element (631) is coupled between the first terminal (61) of the power converter and a first terminal of the flying capacitor (642), - the second switching element (632) is coupled between the first terminal of the flying capacitor (642) and a first terminal of the inductor (68), - the third switching element (633) is coupled between the first terminal of the inductor (68) and a second terminal of the flying capacitor (642), - the fourth switching element (634) is coupled between the second terminal of the flying capacitor (642) and a reference potential, - the control unit (65) is configured to control the switching elements to temporarily couple the flying capacitor between the first terminal of the power converter and the reference potential; wherein the control unit (65) is configured to, when the power converter operates in the boost mode, - permanently switching on the first switching element (631) and the fourth switching element (634) in order to couple the flying capacitor between the first terminal of the power converter and the reference potential, - to regulate an output voltage or an output current of the power converter by switching the second switching element (632) on and off and by switching the third switching element (633) on and off; - to detect a short-circuit situation based on the current through the first switching element, and when the power converter is operating in the step-down mode, - to regulate an output voltage or an output current by switching the first switching element (631), the second switching element (632), the third switching element (633) and the fourth switching element (634) on and off according to a predetermined schedule.

Description

Technical FieldThe present document relates to power converters. More particularly, the present document relates to power converters capable of transferring electrical power from a first terminal of the power converter to a second terminal of the power converter in a first mode and transferring electrical power from the second terminal to the first terminal in a second mode.BackgroundSystem level specifications for power converters have become more and more stringent in recent years. This is particularly true with respect to power converters used in portable electronic devices. On the one hand, an operating efficiency of a power converter is decisive, since it has a direct influence on the battery life as well as the power density and the associated form factor of the portable electronic device. On the other hand, the power converter must not only provide operating power for the device, but also provide a regulated output for managing battery charging functions.Moreover, modern power converters must also be bi-directional, i.e., such power converters can not only convert power from a wall adapter to a battery of an electronic device in a forward direction. Depending on the circumstances, such power converters may also be required to be capable of converting electrical power also in the opposite direction, i.e., from the battery of the electronic device to an external USB (Universal Serial Bus) device. This functionality is also known as USB On-The-Go (OTG).For example, Trackner et al., "Design and Control of Fault-Tolerant Non-Isolated Polyphase Multilevel DC-DC Converters for Automotive Power Systems", IEEE Transactions on Industry Applications, 52(2), 2016, shows a bidirectional, non-isolated DC-DC converter system having a plurality of interconnected, phased multilevel modules for dual motor vehicle application. More specifically, the above-mentioned system provides a control system by means of which, by means of switching four above-mentioned modules on and off over time, a reduction of the existing harmonic voltage and automatic limiting of the fault current are made possible.DE 112016 004 202 T5 relates to an electric power converter that converts a voltage applied from the outside and then transmits electric power, and a drive device using such an electric power converter.U.S. Pat. No. 6,031,702 A relates to solid state direct current sources, in particular DC / DC switching converters.US 2009 / 0 010 035 A1 describes a freewheeling MOSFET which is connected in parallel with the inductor in a switched DC / DC converter. If the free-wheeling MOSFET is turned on during the switching operation of the converter while the low-side and power transfer MOSFETs are turned off, the induction current circulates or "free-wheeling" through the free-wheeling MOSFET.US 2008 / 0 079 410 A1 relates to a power supply device for supplying different electronic devices with a DC voltage and in particular to a power supply device which contains a step-up converter.SummaryThe multi-stage power converter topology, such as the multi-stage buck converter, is a promising approach to remedy many of the deficiencies of conventional power converter topologies. In a multi-stage buck converter, for example, two high-side switches connected in series replace the single high-side switch of the conventional buck converter. Moreover, two low-side switches connected in series replace the single low-side switch of the conventional buck converter, and a so-called flying capacitor is connected in parallel to the series connection of the lower high-side switch and the upper low-side switch.A major advantage of the multi-stage buck converter is that the root mean square (RMS) voltage at the inductor node is only 50% of the corresponding voltage of a conventional buck converter. In addition to reducing the RMS voltage across and the RMS current through the inductor, the voltages across the switching capacitors are also reduced, thereby reducing switching losses. Moreover, transistors with lower breakdown voltage values typically have lower drain-source resistances Rds, leading to reduced conduction losses.The present document addresses the above mentioned technical problems and relates to new ways of implementing and controlling a bidirectional power converter capable of transferring, in a first mode, electric power in a forward direction from a first terminal of the power converter to a second terminal of the power converter and transferring, in a second mode, electric power in a rearward direction from the second terminal to the first terminal. In particular, it is an object of the present document to provide an improved bidirectional power converter with an over-current protection mechanism.According to one aspect, a power converter is provided that is configured to operate in either a buck mode for transferring electrical power from a first terminal of the power converter to a second terminal of the power converter or a boost mode for transferring electrical power from the second terminal of the power converter to the first terminal of the power converter.The power converter may include a first switching element, a second switching element, a third switching element, a fourth switching element, a flying capacitor, an inductor, and a control unit. The first switching element may be coupled between the first terminal of the power converter and a first terminal of the flying capacitor. The second switching element may be coupled between the first terminal of the flying capacitor and a first terminal of the inductor. The third switching element may be connected between the first terminal of the inductor and a second terminal of the flying capacitor. The fourth switching element may be coupled between the second terminal of the flying capacitor and a reference potential. Finally, the control unit may be configured to control the switching elements.Each of the four switching elements may be implemented with a suitable device, such as a metal-oxide-semiconductor field effect transistor (MOSFET), an insulated-gate bipolar transistor (IGBT), a MOS-gate thyristor, or other suitable power devices. Each switching element may have a gate to which a respective drive voltage or control signal may be applied to turn on the switching element (i.e., close the switching element) or to turn off the switching element (i.e., open the switching element).In this document, the term "reference potential" is understood in its broadest possible sense. In particular, the reference potential is not limited to ground, i.e. a reference potential with a direct physical connection to ground. Rather, the term "reference potential" may refer to any reference point to and from which electrical currents may flow or from which voltages may be measured. Moreover, it should be noted that the reference potentials mentioned in this document need not necessarily relate to the same physical contact. Instead, the reference potentials mentioned in this document may refer to different physical contacts, although reference is made to "the" reference potential for convenience of illustration.In the buck mode, the power converter may be configured to convert a higher input voltage at the first terminal of the power converter to a lower output voltage at the second terminal of the power converter. That is, the power converter may operate as a regular buck power converter in a forward direction. For example, the power converter may be used in a portable electronic device and receive the higher input voltage at its first terminal, e.g., from an external travel adapter performing AC / DC conversion. The power converter may then provide a (lower) regulated output voltage or current at its second terminal, e.g. for charging an internal battery of the portable electronic device or for providing a stable supply voltage to the portable electronic device.Conversely, in the boost mode, the power converter may be configured to convert a lower input voltage at the second terminal of the power converter to a higher output voltage at the first terminal of the power converter. That is, the power converter may operate as a regular boost power converter in the reverse direction. Integrated with the portable electronic device described above, the same power converter may, for example, receive the lower input voltage at the second terminal of the power converter from an internal battery of the portable electronic device and provide a (higher) regulated output voltage or current to an external device, such as an external Universal Serial Bus (USB) device. The described reverse or chargeback capability is commonly referred to as USB On-The-Go (OTG) boost. An advantage of the presented power converter is thus that both the buck mode and the boost mode can be implemented in a single power converter.As another advantage, a multi-stage power converter architecture for implementing a power converter with reverse charging capability is proposed. Here, the term "multi-stage" refers to the fact that the four switching elements are capable of generating at least two voltage levels different from the voltage at the reference potential at the first terminal of the inductor. This first terminal of the inductor is sometimes also referred to as a switching node of the power converter. In comparison with a conventional buck converter having a single high-side switching element and a single low-side switching element, the proposed multi-stage power converter having four switching elements exhibits the advantages of substantially reduced switching losses and substantially reduced conduction losses.A second terminal of the inductor may be coupled to the second terminal of the power converter. The power converter may include a first capacitor coupled between the first terminal of the power converter and the reference potential. This first capacitor may serve as an output capacitor for stabilizing the output voltage at the first terminal of the power converter in the boost mode. The first capacitor may also serve as an input capacitor for stabilizing the input voltage at the first terminal of the power converter in the buck mode. The power converter may include a second capacitor coupled between the second terminal of the power converter and the reference potential. This second capacitor may serve as an output capacitor for stabilizing the output voltage at the second terminal of the power converter in the buck mode.On the one hand, the power converter may include a buck feedback circuit for coupling the controller to the second terminal of the power converter, the controller being configured to regulate, when the power converter is operating in the buck mode, an output voltage or an output current at the second terminal of the power converter in the buck mode. For example, the buck feedback circuit may include an error amplifier configured to generate an error signal based on a reference value and an output signal indicative of the output voltage or current at the second terminal of the power converter. Furthermore, the step-down feedback circuit may comprise e.g. a pulse width modulation (PWM) unit or a pulse frequency modulation (PFM) unit for translating the generated error signal into a sequence of pulses with corresponding duration and / or frequency for controlling the switching elements.For example, the control unit may be configured to regulate the output voltage or the output current at the second terminal of the power converter by controlling all four switching elements such that each switching element is turned on and off according to a predetermined schedule. Alternatively, the control unit may be configured to regulate the output voltage or the output current by turning on and off the second switching element and turning on and off the third switching element. More specifically, the control unit may be configured to control the output voltage or the output current by turning on and off only the second and third switching elements. At the same time, the control unit may be configured to turn on the first switching element and the fourth switching element when the power converter is operating in the buck mode. In other words, during the buck mode, the control circuit may be configured to permanently turn on the first and fourth switching elements, while the control circuit may be simultaneously configured to turn on and off the second and third switching elements to regulate the output voltage or current. In this way, the flying capacitor is permanently coupled between the first terminal of the power converter and the reference potential. In other words, the flying capacitor is coupled in parallel with the first capacitor, thereby increasing the effective capacitance of the first capacitor and ultimately improving the ability of the power converter to stabilize the input voltage at its first terminal in the buck mode. In other words, the flying capacitor of the multi-stage power converter may be used as an input capacitor in the buck mode.In summary, the control unit may be configured to temporarily turn on both the first and fourth switching elements, so that the flying capacitor becomes available as an (additional) input capacitor and / or as an (additional) output capacitor of the power converter.On the other hand, the power converter may include a boost feedback circuit for coupling the controller to the first terminal of the power converter, the controller being configured to regulate an output voltage or current at the first terminal of the power converter when the power converter is operating in the boost mode. For example, the boost feedback circuit may include an error amplifier configured to generate an error signal based on a reference value and an output signal indicative of the output voltage or current at the first terminal of the power converter in the boost mode. Further, the up-feedback circuit may comprise e.g. a pulse width modulation (PWM) unit or a pulse frequency modulation (PFM) unit for translating the generated error signal into a sequence of pulses of corresponding duration and / or frequency for controlling the switching elements.For example, the control unit may be configured to regulate the output voltage or the output current at the first terminal of the power converter by controlling all four switching elements such that each switching element is turned on and off according to a predefined schedule. Alternatively, the control unit may be configured to regulate the output voltage or the output current by turning on and off the second switching element and turning on and off the third switching element. In particular, the control unit may be configured to regulate the output voltage or the output current by turning on and off only the second and third switching elements. At the same time, the control unit may be configured to turn on the first switching element and the fourth switching element when the power converter is operating in the boost mode.In other words, during the boost mode, the control circuit may be configured to permanently turn on the first and fourth switching elements, while the control circuit may be simultaneously configured to turn on and off the second and third switching elements to regulate the output voltage or current. In this way, the flying capacitor is permanently coupled between the first terminal of the power converter and the reference potential. In other words, the flying capacitor is coupled in parallel with the first capacitor, thereby increasing the effective capacitance of the first capacitor and ultimately improving the ability of the power converter to stabilize the output voltage at its first terminal in the boost mode. In other words, the flying capacitor of the multi-stage power converter may be used as an output capacitor in the step-up mode.The power converter may include a current detector disposed on a current path between the first terminal of the power converter and the second terminal of the power converter. In particular, the current detection device can be arranged on a current path between the first terminal of the power converter and the first terminal of the inductor. As a first example, the current sensing device may include a resistor. For example, the resistor may be a shunt resistor disposed on a current path between the first terminal of the power converter and the first switching element. Using a simple resistor to measure current through the power converter is an efficient and low-loss manner for implementing current measurement to implement over-current protection. The resistor may be constructed from the metal layers of the integrated circuit (IC) and may therefore have a very low resistance value and very low power losses compared to alternative solutions requiring e.g. a transistor to measure a corresponding current. Moreover, the area required for resistance may be reduced compared to solutions based on transistors.As a second example, the current detecting means may be the first switching element. Since the first switching element may be permanently turned on, so that the flying capacitor may be used as an input capacitor or an output capacitor (depending on the current mode of the power converter), the current through the first switching element may be measured with high accuracy, as the amount of noise is substantially reduced compared to solutions in which current measurements are obtained from a switching element that is repeatedly turned on and off.As already stated, the control unit may be configured to detect, based on a detection signal generated by the current detection device, a short-circuit situation and, in response to this detection, to switch off the second switching element. In this way, the first terminal of the power converter is isolated from the second terminal of the power converter, e.g., to prevent damage to a battery connected to the second terminal in the event of an over-current at the first terminal.Moreover, the second switching element may include a field-effect transistor (FET) and a back-body switch configured to connect a back-body of the FET to a reference potential when the short-circuit situation is detected. In particular, the control circuit may be configured to control the back body switch to connect the back body of the FET to the reference potential when the short circuit situation is detected. When a short circuit situation is not detected, the back body switch may be configured to connect the back body of the FET to a source terminal of the FET. The back body of the transistor implementing the second switching element may also be referred to as a body, base, ground, or substrate. By coupling the back body of the second switching element to the reference potential (e.g., ground), a current flowing across the body diode of the second switching element may be substantially reduced. As a result, it becomes possible to use the second switching element as a means for interrupting a current flow through the power converter in an overcurrent situation. Consequently, it becomes possible to use the above-described simple resistor as an efficient current detection device, as opposed to back-to-back FETs (see FIGS. 4 S 3 and S 4) that significantly increase the silicon area. The flying capacitor therefore serves as input capacitance in the buck mode or output capacitance in the boost mode without requiring the FETs S3 and S4 and continuing to meet the USB Power Delivery 3.0" standards.According to another aspect, a method for operating a power converter is described. The method may include steps corresponding to the features of the power converter described herein. In particular, the method may include providing a first switching element, a second switching element, a third switching element, a fourth switching element, a flying capacitor, an inductor, and a control unit in the power converter. The method may include operating the power converter in a buck mode to transfer electrical power from a first terminal of the power converter to a second terminal of the power converter, or operating the power converter in a boost mode to transfer electrical power from the second terminal of the power converter to the first terminal of the power converter. Additionally, the method may further include coupling the first switching element between the first terminal of the power converter and a first terminal of the flying capacitor, and coupling the second switching element between the first terminal of the flying capacitor and a first terminal of the inductor. The method may further include coupling the third switching element between the first terminal of the inductor and a second terminal of the flying capacitor, coupling the fourth switching element between the second terminal of the flying capacitor and a reference potential, and controlling the switching elements by the control unit.The method may further include coupling the controller to the second terminal of the power converter using a buck feedback circuit and regulating an output voltage or current at the second terminal of the power when the power converter is operating in the buck mode. The method may further include regulating the output voltage or the output current by switching the second switching element on and off and by switching the third switching element on and off. The method may further include turning on the first switching element and the fourth switching element when the power converter is operating in the buck mode.The method may further include coupling the controller to the first terminal of the power converter using a boost feedback circuit and regulating an output voltage or current at the first terminal of the power converter when the power converter is operating in the boost mode. The method may further include regulating the output voltage or the output current by switching the second switching element on and off and by switching the third switching element on and off. The method may further include turning on the first switching element and the fourth switching element when the power converter is operating in the boost mode.The method may further comprise providing a current sensing device arranged on a current path between the first terminal of the power converter and the second terminal of the power converter. In this case, the current detection device can have a resistor. Alternatively or additionally, the current detection means may be the first switching element. The method may further comprise detecting a short-circuit situation based on a detection signal generated by the current detection device and, in response to the detection, switching off the second switching element. The second switching element may include a field effect transistor (FET) and a back body switch. The method may further include connecting, by the back body switch, a back body of the FET to the reference potential when the short circuit situation is detected.It should be noted that the methods and systems, including their preferred embodiments, as set forth herein may be used alone or in combination with the other methods and systems disclosed herein. In addition, the features set forth in the context of a system are also applicable to a corresponding method. Moreover, all aspects of the methods and systems described in this document can be combined as desired. In particular, the features of the claims can be combined with one another as desired.In the present document, the term "couple" or "coupled" refers to elements that are in electrical communication with each other, either directly connected, for example, via wires, or in some other manner.Brief Description of the DrawingsThe present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like reference numerals refer to similar or identical elements, and in which FIGS. 1A and 1B show a down converter and its graph; FIGS. 2A, 2B, and 2C show a multi-stage buck converter and its graphs; FIGS. 3A, 3B, 3C, and 3D show different switching states of a multi-stage buck converter; FIG. 4 shows a power converter with USB OTG capability in the buck mode; FIG. 5 shows a power converter with USB OTG capability in the boost mode; FIG. 6 shows a multi-stage power converter with USB OTG capability in the buck mode; and FIG. 7 shows a multi-stage power converter with USB OTG capability in the boost mode.DESCRIPTION OF THE INVENTIONFig. 1A shows a down converter 11 known from the prior art. In the past, buck converter topology has been widely used for battery powered devices where the input power source is provided by a USB (universal serial bus) type adapter. A high-side switch 111 is disposed between an inductor input node of the inductor 113 and an input voltage. A low side switch 112 is disposed between the inductor input node and ground. Finally, an output capacitor 114 is arranged between an inductor output node of the inductor 113 and ground. In addition, the plot 12 in FIG. 1B shows the voltage V SW at the inductor input node of the inductor 113 over time. During a first time interval, the high-side switch 111 is turned on and during a second time interval, the low-side switch 112 is turned on. The two switches are turned ON in alternating cycles and the duty cycle of each cycle provides the required output voltage regulation.In order to increase the operation efficiency of the buck converter, much effort has been made to reduce the ON-time resistance of the switches (i.e., drain-source resistance Rds-on) to reduce losses. However, reducing the drain-source resistance Rds-on results in greater parasitic capacitance once the output field effect transistor (FET) gate overdrive is maximized, resulting in a trade-off between conduction losses and switching losses.Another problem with standard buck converters, as emphasized in FIG. 1, is that the voltage V SW at the inductor input node of the inductor 113 varies between the input voltage Vin (when the high-side switch is ON) and ground (when the low-side switch is ON). This results in significant switching losses of the power converter and a core loss dissipated by the inductor due to the high root mean square (RMS) voltage across the inductor.One topology promising to overcome many of the deficiencies of the buck converter is the multi-stage buck converter 2, as shown in FIG. 2A. The illustrated exemplary multi-stage buck converter 2 includes a first switching element 21, a second switching element 22, a third switching element 23, a fourth switching element 24, an inductor 26, a flying capacitor 25, and an optional output capacitor 27. A first terminal of the inductor 26 may be connected to a switching terminal connecting the second switching element 22 and the third switching element 23. A first terminal of the flying capacitor 25 may be connected to a terminal connecting the first switching element 21 and the second switching element 22, and a second terminal of the flying capacitor 25 may be connected to a terminal connecting the third switching element 23 and the fourth switching element 24.The voltage V SW at the first terminal of the inductor 26 may switch between V_IN and V_IN / 2 when V_IN>V_OUT>V_IN / 2, and may switch between V_IN / 2 and ground when V_IN / 2>V_OUT>ground, where V_IN denotes the input voltage and V_OUT denotes the output voltage. The plots 28 in FIG. 2B and 29 in FIG. 2C show the voltage V SW at the first terminal of the inductor 26 over time for the two different duty cycles.A major aspect of the multi-stage buck topology is that the RMS voltage at the first terminal of the inductor 26 is 50% of the corresponding voltage of a conventional buck converter. In addition to reducing the RMS voltage and current across and through the inductor, the voltage across the switching elements is also reduced, thereby reducing switching losses. Moreover, transistors with lower breakdown voltage values typically have lower Rds-On characteristics, thereby reducing conduction losses.FIGS. 3A, 3B, 3C, and 3D show four different switching states of a multi-stage buck converter. In the switching state D 1 in FIG. 3A, the first and third switching elements are turned on, while the second and fourth switching elements are turned off. In the switching state DV in FIG. 3B, the third and fourth switching elements are turned on, while the first and second switching elements are turned off. In the switching state D 2 in FIG. 3C, the second and fourth switching elements are turned on, while the first and third switching elements are turned off. Finally, in the switching state DP in FIG. 3D, the first and second switching elements are turned on, while the third and fourth switching elements are turned off.The arrow labeled I SW indicates the current flow provided to the output of the multi-stage buck converter. The voltage across the flying capacitor is referred to as V CF.Some portable devices have the capability of using the internal battery to provide a regulated output to an external device. This is commonly referred to as USB On-The-Go (OTG) boost. The battery management feature in a mobile device requires a buck converter for charging the internal battery and a boost converter for providing a regulated output to an external device. Prior art systems use a single power converter that can be used in multiple modes.FIGS. 4 and 5 show a power converter with USB OTG capability. As shown in FIG. 4, when an adapter is connected to the terminal A (input power source), the power converter operates in the buck topology mode. Output voltage regulation is provided by the controller U 1 operating in the buck topology mode that defines the ON and OFF states of the power switches S 1 and S 2. The control device U 1 monitors the terminal B (output voltage) via a voltage feedback signal V_FB.When disconnected from an input power source, the internal battery of the portable device can be used as a power source for an external load. This is illustrated in FIG. 5. In this configuration, the terminal B coupled to the internal battery is converted into an input source, and the terminal A is converted into an output voltage terminal for the external load. The output voltage regulation is provided by the control device U 1 operating in the boost topology mode defining the ON and OFF states of the power switches S 1 and S 2. The control device U 1 monitors the terminal A (output voltage) via a voltage feedback signal V_FB.The input voltage to the battery charger of a mobile device may be provided by a USB host. The power delivery (PD) standards (such as USB PD 3.0) for a USB host limit the maximum "power-on" current that can be provided when a load is first connected. This standard essentially limits the rated value of the input capacitance for the charger to no more than 10 μF. Therefore, the capacitor C 3 shown in FIGS. 4 and 5 may be only 10 μF or less because it is located on the upstream side of the source-coupled switches S 3 and S 4. The capacitor C 3 may be much larger than 10 μF as long as S 3 and S 4 are used to limit the inrush current. The drawback of S 3 and S 4 is that the total FET on-resistance may be relatively high, which deteriorates the efficiency of the charger.When operating in the boost mode (FIG. 5 ), a number of factors must be considered, including overcurrent sensing, short circuit protection, and output voltage regulation according to USB standards (+ / - 5% of 5V). A boost regulator often requires a large output capacitor to handle load transients. The total output capacitance may be the sum of the capacitance values of the capacitors C 2 and C 3. Specifically, the capacitance connected to terminal A in an operation in the up mode may be the sum of C 2 and C 3 when S 3 and S 4 are in the fully on state.During the reverse charging mode, the output current to the external device must be sensed to ensure safe operating conditions. A common technique using the protection FETs S 3 and S 4 is shown in FIGS. 4 and 5.As noted above, the multi-stage buck converter has many advantages over conventional buck converters for many applications, including battery charging functionality in portable devices. As noted above, many portable devices require the ability to provide a regulated source of output power to an external load via the USB port.FIG. 6 illustrates an exemplary multi-stage power converter according to the present invention in the buck mode. FIG. 7 shows the same multi-stage power converter according to the present invention in the boost mode. The illustrated example power converter is configured for operation in either a buck mode for transferring electrical power from a first terminal 61 of the power converter to a second terminal 62 of the power converter or a boost mode for transferring electrical power from the second terminal 62 of the power converter to the first terminal 61 of the power converter. The power converter includes a first switching element 631 (power switch A), a second switching element 632 (power switch B), a third switching element 633 (power switch C), a fourth switching element 634 (power switch D), a flying capacitor 642, an inductor 68, and a control unit 65. The first switching element 631 is coupled between the first terminal of the power converter and a first terminal of the flying capacitor. The second switching element 632 is coupled between the first terminal of the flying capacitor and a first terminal of the inductor. The third switching element 633 is coupled between the first terminal of the inductor and a second terminal of the flying capacitor. The fourth switching element 634 is coupled between the second terminal of the flying capacitor and a reference potential. The control unit 65 is configured to control the switching elements.As seen in FIGS. 6 and 7, a second terminal of the inductor 68 is coupled to the second terminal 62 of the power converter. The power converter includes a first capacitor 641 coupled between the first terminal 61 of the power converter and the reference potential. The power converter includes a second capacitor 643 coupled between the second terminal of the power converter and the reference potential.FIG. 6 shows a buck feedback loop 66 for coupling the controller 65 to the second terminal 62 of the power converter, wherein the controller 65 is configured to regulate, when the power converter is operating in the buck mode, an output voltage or an output current at the second terminal of the power converter in the buck mode. FIG. 7 shows a boost feedback circuit 69 for coupling the controller 65 to the first terminal 61 of the power converter, wherein the controller 65 is configured to regulate an output voltage or current at the first terminal 61 of the power converter when the power converter is operating in the boost mode.Moreover, the power converter includes a resistor 67 as a current detection device. As illustrated in FIG. 7, a back body switch 691 is configured to connect a back body 692 of the second switching element 632 to a reference potential when a short circuit situation is detected by means of the resistor 67. In particular, the control circuit 65 is configured to control the back body switch to connect the back body 692 to the reference potential when the short circuit situation is detected. When a short circuit situation is not detected, the back body switch 692 is configured to connect the back body 692 to a source terminal of the second switching element 632.Figures 6 and 7, in contrast to Figures 4 and 5, show how the present invention can overcome the limitations of the standard buck converter in boost reverse applications. As shown in FIG. 4, the power converter operates in the multistage buck topology mode when connected to terminal A (input voltage source). Output voltage regulation is provided by the controller U 1 operating in the multi-stage buck topology mode defining the ON and OFF states of the power switches A, B, C, and D. The control device U 1 monitors the terminal B (output voltage) via a voltage feedback signal.When disconnected from an input power source, the internal battery of the portable device can be used as a power source for an external load device. This is illustrated in FIG. 7. In this configuration, the terminal B coupled to the internal battery is converted into an input source, and the terminal A is converted into an output voltage terminal for the external load device. The power switches A and D are set to the on state. Regulation of the output voltage is provided by the control device U 1 operating in the reverse boost topology mode defining the ON and OFF states of the power switches B and C. The control device U 1 monitors terminal A (output voltage) via a voltage feedback signal.Further, FIG. 7 shows how the reconfigured multi-stage buck converter uniquely addresses the deficiencies of prior art reverse boost solutions.As stated above, the required capacity at port A in the up mode is large to adequately handle load transients and meet the USB standard specifications. This required additional capacity connected to port A even during operation in the battery charging mode. As shown in FIG. 5, when power switches A and D are set to the ON state, flying capacitor CF provides additional capacitance from terminal A to GND (via Rds-on of switches A and D). Since the typical value of CF is sufficient to provide output capacitance in the boost mode, additional capacitance may not be required. A boost start is made with CF(as Cout) / bootstrap capacitor for charging switch A, which is required to turn switch A on, followed by switch B bootstrap capacitor charging. This is done by using an internal charge pump circuit. The back body switched switch B enables boost start control without feedback to the OTG output when the switches S 3 and S 4 are not present.As noted above, in the boost configuration, the output current to the external load device must be monitored to avoid over-current conditions. As shown in FIG. 4, the power switches A and D are set to the ON state, and the power switches B and C are used to generate a boost converter in the reverse direction.In this configuration, a sense resistor R1 67 is used to monitor the output current of the boost converter. When the detection resistor 67 is constructed of the metal layers of the IC, a very low value can be obtained. A sense resistor of this type results in a smaller area and power loss than the circuit breakers S 3 and S 4 shown in FIG. 4 in both the charger (buck) and OTG (boost) modes, resulting in a better thermal result at load. However, using a sense resistor implies that another means is required to disconnect the battery from a short to ground at terminal A. This means is achieved by controlling the back body terminal of switch B, which is a symmetric n-channel FET. For example, if the OTG output (terminal A) is inadvertently shorted to ground, switch B must be fully turned off with a modified gate driver scheme, the residual inductor current must be discharged via the body diode of switch B, after which the back body of switch B is moved to ground (from the drain of switch C to ground) to completely prevent feedback between the battery and the OTG output (terminal A).The additional challenge of OTG boost with the sense resistor is the ability to launch into a load when the output voltage reaches a certain level, as opposed to launching without load if the circuit breakers S 3 and S 4 were present as in the prior art.With the above approach of sensing resistance at the input of the buck converter for a charging application, the input capacitor (C3) must be at least 10 μF, which is dictated by the power-on USB requirement. Now, the CF is advantageous and helps the higher output capacity required for the up mode. Thus, the efficiency in the forward and reverse direction of the charger and the OTG is significantly improved.Finally, a new way is presented to increase the up-output capacity without the need for additional components. In particular, the flying capacitor 642 of the multi-stage buck converter may be used as the boost output capacitor. Further, when operating a multi-stage buck converter in the boost mode, a low-loss manner for implementing output current sensing and providing over-current protection is presented without requiring additional circuit breakers S 3 and S 4 while not compromising boost transient performance due to limited output capacitance and still meeting the USB inrush current specifications.It should be noted that the description and drawings merely illustrate the principles of the proposed methods and systems. Those skilled in the art will be able to implement various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Moreover, all examples and embodiments described herein are expressly intended for purposes of explanation only to aid the reader in understanding the principles of the proposed methods and systems. Moreover, all statements herein providing principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to include equivalents thereof.

Claims

A power converter configured to operate in a mode of either - a buck mode for transferring electrical power from a first terminal (61) of the power converter to a second terminal (62) of the power converter, or - a boost mode for transferring electrical power from the second terminal (62) of the power converter to the first terminal (61) of the power converter, the power converter comprising a first switching element (631), a second switching element (632), a third switching element (633), a fourth switching element (634), a flying capacitor (642), an inductor (68), and a control unit (65), wherein - the first switching element (631) is coupled between the first terminal (61) of the power converter and a first terminal of the flying capacitor (642), - the second switching element (632) is coupled between the first terminal of the flying capacitor (642) and a first terminal of the inductor (68), - the third switching element (633) is coupled between the first terminal of the inductor (68) and a second terminal of the flying capacitor (642), - the fourth switching element (634) is coupled between the second terminal of the flying capacitor (642) and a reference potential, - the control unit (65) for controlling the switching elements is configured to temporarily couple the flying capacitor between the first terminal of the power converter and the reference potential; wherein the control unit (65) is configured to, when the power converter operates in the step-up mode, - permanently turn on the first switching element (631) and the fourth switching element (634) to couple the flying capacitor between the first terminal of the power converter and the reference potential, - regulate an output voltage or an output current of the power converter by turning on and off the second switching element (632) and by turning on and off the third switching element (633); - detect a short-circuit situation based on the current through the first switching element, and when the power converter operates in the step-down mode, - regulate an output voltage or an output current by turning on and off the first switching element (631), the second switching element (632), the third switching element (633) and the fourth switching element (634) according to a predetermined schedule.The power converter of claim 1, wherein a second terminal of the inductor (68) is coupled to the second terminal (62) of the power converter.The power converter of claim 1, wherein the power converter comprises a first capacitor (641) coupled between the first terminal (61) of the power converter and the reference potential.The power converter of claim 1, wherein the power converter comprises a second capacitor (643) coupled between the second terminal (62) of the power converter and the reference potential.The power converter of claim 1, comprising a buck feedback circuit for coupling the controller (65) to the second terminal (62) of the power converter, wherein the controller (65) is configured to, when the power converter is operating in the buck mode, regulate the output voltage or current at the second terminal (62) of the power converter.The power converter according to claim 5, wherein the control unit (65) is configured to control the output voltage or the output current by turning on and off the second switching element (632) and turning on and off the third switching element (633).The power converter of claim 1, wherein the controller (65) is configured to turn on the first switching element (631) and the fourth switching element (634) when the power converter operates in the buck mode.The power converter of claim 1, comprising a boost feedback circuit for coupling the controller (65) to the first terminal (61) of the power converter, wherein the controller (65) is configured to regulate an output voltage or current at the first terminal (61) of the power converter when the power converter is operating in the boost mode.The power converter of claim 1, wherein the power converter comprises a current detector (67) disposed on a current path between the first terminal (61) of the power converter and the second terminal (62) of the power converter.The power converter of claim 9, wherein the current sensing device (67) comprises a resistor.The power converter according to claim 9, wherein the current detection means (67) is the first switching element (631).The power converter according to claim 9, wherein the control unit (65) is configured to - detect a short-circuit situation based on a detection signal generated by the current detection means (67), and - turn off the second switching element (632) in response to this detection.The power converter of claim 12, wherein the second switching element (632) comprises a field-effect transistor (FET) and a back body switch configured to connect a back body of the FET to the reference potential when the short circuit situation is detected.A method for operating a power converter, the power converter comprising a first switching element (631), a second switching element (632), a third switching element (633), a fourth switching element (634), a flying capacitor (642), an inductor (68), and a control unit (65) for controlling the switching elements, wherein the first switching element (631) is coupled between the first terminal (61) of the power converter and a first terminal of the flying capacitor (642), the second switching element (632) is coupled between the first terminal of the flying capacitor (642) and a first terminal of the inductor (68), the third switching element (633) is coupled between the first terminal of the inductor (68) and a second terminal of the flying capacitor (642), the fourth switching element (634) coupled between the second terminal of the flying capacitor (642) and a reference potential, the control unit is configured to temporarily couple the flying capacitor between the first terminal of the power converter and the reference potential, the method comprising: - operating the power converter in a buck mode for transferring electrical power from a first terminal (61) of the power converter to a second terminal (62) of the power converter, or - operating the power converter in a boost mode for transferring electrical power from the second terminal (62) of the power converter to the first terminal (61) of the power converter, the method comprising, when the power converter operates in the boost mode: - permanently turning on the first switching element (631) and the fourth switching element (634), In order to couple the flying capacitor between the first terminal of the power converter and the reference potential, - regulating an output voltage or an output current at the second terminal (62) of the power converter by turning on and off the second switching element (632) and by turning on and off the third switching element (633), - detecting a short-circuit situation based on the current through the first switching element, and when the power converter operates in the buck mode, - regulating an output voltage or an output current by turning on and off the first switching element (631), the second switching element (632), the third switching element (633) and the fourth switching element (634) according to a predetermined schedule.The method of claim 14, wherein the controller is coupled to the second terminal (62) of the power converter using a buck feedback circuit, the method further comprising: - controlling, when the power converter is operating in the buck mode, the output voltage or current at the second terminal (62) of the power converter.The method of claim 15, further comprising: - regulating the output voltage or the output current by turning on and off the second switching element (632) and by turning on and off the third switching element (633).The method of claim 14, further comprising: - turning on the first switching element (631) and the fourth switching element (634) when the power converter is operating in the boost mode.The method of claim 14, wherein the control unit is coupled to the first terminal (61) of the power converter using a boost feedback circuit, the method further comprising: - regulating an output voltage or an output current at the first terminal (61) of the power converter when the power converter is operating in the boost mode.The method of claim 14, further comprising: - arranging a current detector (67) on a current path between the first terminal (61) of the power converter and the second terminal (62) of the power converter.The method of claim 19, wherein the current sensing means (67) comprises a resistor or wherein the current sensing means is the first switching element (631).The method according to claim 19, further comprising: - detecting, based on a detection signal generated by the current detection means (67), a short-circuit situation, and - in response to this detection, turning off the second switching element (632).The method according to claim 19, wherein the second switching element (632) comprises a field effect transistor (FET) and a back body switch, the method further comprising: - connecting, by the back body switch, a back body of the FET to the reference potential when the short circuit situation is detected.

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