Driver circuit and method for controlling a switching element

Abstract

A driver circuit is provided for controlling a switching element. The driver circuit (200) comprises a plurality of bootstrap capacitors (210) and a switching unit (211). Furthermore, the driver circuit (200) comprises a control unit (102) configured to control the switching unit (211) in response to a control signal (ActHS) to provide a parallel arrangement of the plurality of bootstrap capacitors (210) between a charging voltage (VDD) and a reference voltage for charging the plurality of bootstrap capacitors (210) during a charging phase; and to provide a series arrangement of the plurality of bootstrap capacitors (210) between the charging voltage (VDD) and a control terminal of the switching element (MHS) for controlling the switching element (MHS) during a control phase.

Description

Technical FieldThe present document relates to a driver circuit and a corresponding method for controlling a switching element.BackgroundControlling and driving a switching element, particularly a non-ground related transistor, presents several challenges. Power conversion topologies, such as a buck converter and / or a half-bridge converter topology, include one or more non-ground high-side switching elements. A bootstrap driver circuit may be used to generate a voltage sufficient to drive a high-side switching element, where the bootstrap driver circuit typically includes a bootstrap capacitor for generating the voltage for controlling the high-side switching element.The bootstrap capacitor of a bootstrap driver circuit may need to have a relatively high capacitance to provide a sufficiently high voltage for driving a high-side switching element. This can result in relatively high costs and chip area.The present document addresses the technical problem of reducing the capacitance of a bootstrap capacitor of a driver circuit for driving a switching element. The technical problem is solved by the independent claims. Preferred examples are described in the dependent claims.SummaryAccording to one aspect, a driver circuit for controlling a switching element is described. The driver circuit includes a plurality of bootstrap capacitors and a switching unit. Further, the driver circuit comprises a control unit configured to control the switching unit in dependence on a control signal, wherein the switching unit is controlled to provide a parallel arrangement of the plurality of bootstrap capacitors between a charging voltage and a reference voltage for charging the plurality of bootstrap capacitors during a charging phase; and to provide a serial arrangement of the plurality of bootstrap capacitors between the charging voltage and a control terminal of the switching element for controlling the switching element during a control phase.According to another aspect, a method of controlling a switching element using a plurality of bootstrap capacitors is described. The method includes providing a parallel arrangement of the plurality of bootstrap capacitors between a charging voltage and a reference voltage for charging the plurality of bootstrap capacitors during a charging phase, and providing a serial arrangement of the plurality of bootstrap capacitors between the charging voltage and a control terminal of the switching element for controlling the switching element during a control phase.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 out in connection with a system can also be applied to a corresponding method. Further, all aspects of the methods and systems set forth in the present document may be combined in any combination. In particular, the features of the claims can be combined with one another in any desired manner.Brief Description of the FiguresThe invention is explained by way of example below with reference to the attached drawings, in which FIG. 1 a illustrates an example power converter; FIG. 1 bshows an exemplary relationship between the target voltage for driving a switching element and the required capacitance of the bootstrap capacitor; FIG. 2 ashows an exemplary power converter with a driver circuit comprising a cascade of bootstrap capacitors; FIG. 2 b shows an example cascade of bootstrap capacitors; FIG. 2 cshows an exemplary relationship between the target voltage for driving a switching element and the required capacitance of the bootstrap capacitor; and FIG. 3 shows a flow chart of an example method for controlling a switching element using a plurality of bootstrap capacitors.Detailed DescriptionAs stated above, the present document is directed to providing a particularly compact and efficient driver circuit for driving a switching element that is not referenced to ground. In this regard, FIG. 1 ashows an exemplary DC / DC power converter 100. Depending on the converter topology, the connection notes V X, V Y and V Z can correspond to the following voltage values• Buck configuration: V X= V IN, V Y= V OUT and V Z= GND;• Boost configuration: V X= V OUT, V Y= V IN and V Z= GND; and / or• Inverting buck-boost configuration: V X= V IN, V Y= GND, and V Z= V OUT.VIN, VOUT, and GND respectively denote the input voltage, the output voltages, and the ground potential. Converter output voltage regulation may be achieved via a duty cycle operation of the power stage that includes controlling low-side power switching element M LS and high-side power switching element M HS. The switching elements may be field effect transistors (FETs). The switching elements are controlled via respective gate drivers 101, 102. The conversion of the digital control signals Act LS and Act HS into the corresponding supply domain of the respective gate drivers 101, 102 may be performed using a level shifter 'LVL'.The power stage, i.e. the switching elements M LS and M HS, may be implemented using NMOS transistors. An n-type high-side switching element may be used due to its relatively small area (due to increased charge mobility) and its relatively low gate capacitance compared to a p-type switching element. The use of an NMOS transistor as a high-side switching element typically requires the use of a bootstrap gate driver 102 with a bootstrap capacitor C B110, to accommodate the movement of the voltage level (i.e., reference voltage) of the switching node SW.It is typically desirable to charge the bootstrap capacitor C B110 to a relatively high voltage to ensure efficient charge redistribution when the high-side switching element M HS is turned on. Depending on the converter topology and the technical environment in which the switching element is operated, different voltage sources may be available for charging the bootstrap capacitor C B110. As an example, in the case of a buck converter, the input voltage of the buck converter may supply and / or charge bootstrap capacitor C in B110 fashion. The input voltage may be provided by, for example, a lithium ion battery of a portable device.A mathematical expression of the gate-source voltage 111 (i.e., the control voltage) of the high-side switching element M HS, resulting from the charge redistribution between the capacitance C B of the bootstrap capacitor 110 and the gate capacitance C gg of the high-side switching element, may be as follows (if the voltage drop across the diode D 1 is ignored) where Q B represents the charge across the bootstrap capacitor 110, where C total represents the total capacitance after the charge redistribution, which consists of the gate capacitance C gg of the high-side switching element and the capacitance C B of the bootstrap capacitor 110. V DD represents the charging voltage to which bootstrap capacitor 110 is charged prior to the beginning of charge redistribution. The charge redistribution from bootstrap capacitor 110 to the gate capacitance results in the gate-source voltage V gs111 of the high-side switching element.In one example, a switching cycle for operating the power converter 100 may begin activating the low-side switching element M LS using, e.g., a gate-source voltage V GS,LS≈4V. When the switching node SW is pulled to ground by the low-side switching element M LS the bootstrap capacitor 110 is charged using the charging voltage V DD e.g., V CB ≈4.8V. After the low-side switching element M LS is turned off, When the high side switching element M HS is turned on, the switching node SW is brought into a high state (e.g., to the voltage level of the terminal node Vx). The gate-source voltage 111 of the high-side switching element is the result of the charge redistribution between the bootstrap capacitor 110 and the gate capacitance of the high-side switching element M HS. Therefore, when the high-side switching element is turned on, the voltage across the bootstrap capacitor 110 drops from its initial value, e.g., V CB ≈4.8 V, to a reduced value, e.g., V CB= V GS,HS ≈4 V.The size, in particular the capacitance, of the bootstrap capacitor 110 required for providing a certain target gate-source voltage for controlling the high-side switching element is illustrated in FIG. 1 b. It can be seen that the capacitance 120 of the bootstrap capacitor 110 increases significantly with increasing setpoint values 121 for the gate-source voltage 111 (according to the relationship 122). The required capacitance 120 of the bootstrap capacitor 110 typically depends on the gate capacitance C gg of the high-side switching element and / or on the bootstrap capacitor charging voltage V DD.In the present document, a bootstrapped high-side driver circuit is described that is configured to generate relatively high gate-source voltages 111 for controlling a high-side switching element in an area efficient manner.FIG. 2 ashows an exemplary power converter 100 with a bootstrapped high-side driver circuit 200 for driving a high-side switching element. The driver circuit 200 is configured to provide a relatively large gate-source voltage 111 for controlling the high-side switching element using a relatively small bootstrap capacitor 110. For this purpose, the driver circuit 200 comprises a plurality of bootstrap capacitors 110, 210, 210, in particular a base bootstrap capacitor 110 and two or more (additional) bootstrap capacitors C B1, C B2210. The base bootstrap capacitor 110 is not used for driving the gate of the high-side switching element, thereby enabling the size of the base bootstrap capacitor 110 to be substantially reduced. The two or more (additional) bootstrap capacitors C B1, C B2210 are used to drive the gate of the high-side switching element.Recharging of all bootstrap capacitors 110, 210 is done by coupling the bottom plate of each bootstrap capacitor 110, 210 to the switching node SW and by charging the top plate of each bootstrap capacitor 110, 210 up to the charging voltage V DD. During the turn-on phase of the high-side switching element, the driver circuit 200 performs capacitor stacking by connecting the bootstrap capacitors C B1, C B2210 in series using a switching unit 211 (as opposed to the parallel arrangement used for charging). This results in a voltage "build-up" ("pile-up") effect for providing an increased gate-source voltage 111 at the high-side switching element.FIG. 2 ashows an example power converter 100 in a boost converter topology. A switching cycle may begin with activating the low-side switching element (e.g., using V GS,LS ≈4V). When the switching node SW is pulled to ground by the low-side switching element, all bootstrap capacitors C B, C B1, C B2110, 210 are arranged in parallel and can be charged to a given voltage, respectively, e.g., V CB ≈ 4.8 V. After the low-side switching element is turned off, the high-side switching element can be turned on, thereby bringing the switching node SW to the high state. The gate-source voltage 111 of the high-side switching element is the result of the charge redistribution between the bootstrap capacitor stack formed by the (additional) bootstrap capacitors C B1, C B2210 and the gate capacitance C gg of the high-side switching element. Thus, during high side turn-on, the voltage across each individual (additional) bootstrap capacitor 210 drops from its initial value of V CB ≈ 4.8 V to its new value, which may be half the gate-source voltage 111 of the high side switching element, e.g., V CB1= V CB2= 0,5 × V GS,HS ≈ 2 V, while the gate-source voltage 111 itself is the sum of the voltages across each (additional) bootstrap capacitor 210, For example, V GS,HS= V CB1+ V CB2 ≈4 V. The base bootstrap capacitor C B110 is not involved in driving the high-side switching element, but may be used to control the driving circuit 102. The voltage of the base bootstrap capacitor 110 may remain relatively constant throughout the low-side (i.e., charge) and high-side (i.e., control) phases.A mathematical expression of the gate-source voltage V gs111 of the high-side switching element, which results from the charge redistribution from the bootstrap capacitor stack 240 formed by the (additional) bootstrap capacitors C B1, C B2210 to the gate capacitance C gg of the high-side switching element, can be given by (if the voltage drop at the diodes D 1, D 2 is ignored): where C B,1 / 2 is the total bootstrap capacitance of the gate driver and consists of the two (generally the K) individual (additional) bootstrap capacitors C B1, C B2210.The size 120 of an overall bootstrap capacitor required to achieve a specific setpoint 121 of the gate-source voltage 111 of the high-side switching element 111 is illustrated in FIG. 2 c. It will be appreciated that when using the stacked capacitor implementation (relationship 222), the size 120 of the overall bootstrap capacitor may be significantly reduced, particularly for relatively high target values 121 of the gate-source voltage 111. In particular, it can be seen that the relationship 222 between the bootstrap capacitor value 120 and the resulting value 121 of the gate-source voltage 111 of the high-side switching element has a more gradual curvature compared to the relationship 122. Since the set point 121 for the gate-source voltage 111 of the high-side switching element approaches the nominal value of the charging voltage V DD of the bootstrap capacitors 210, a significantly smaller bootstrap capacitor size may be required when using stacked bootstrap capacitors 210.As can be seen from FIG. 2 c, it may be more advantageous to use a single bootstrap capacitor 110 if a relatively small gate-source voltage 111 of the high-side switching element is sought. However, since the target gate-source voltage goes above a certain threshold, e.g., ~3.5 V, the stacked capacitor implementation yields a smaller overall bootstrap capacitor size. For example:▪ at V GS,HS= 3,6 V, the stacked capacitor implementation may result in a 15% reduction;GS,HS= 4 V, the stacked capacitor implementation may result in a 50% reduction.The charging voltage V DD of the bootstrap capacitor may have a range with a minimum limit and a maximum limit. The maximum limit may be defined by the oxide thickness of the bootstrap capacitor 110, 210, wherein an increased oxide thickness typically results in an increased maximum limit. The minimum limit may be defined by the source from which the charging voltage V DD is generated (for example, when the charging voltage V DD is generated by a lithium ion battery, the minimum limit may be 2...3V). The stacked capacitor implementation described herein enables generation of gate-source voltages 111 at target voltage levels 121 that are above the charge voltage V DD of the bootstrap capacitor 110, 210 itself. This may be advantageous if the charging voltage V DD of the bootstrap capacitors 110, 210 is relatively close to its minimum limit.FIG. 2 b illustrates an example capacitor circuit 240 comprising a plurality of bootstrap capacitors C B1, C B1,..., C Bn210 and a switching unit 211 configured to,• providing a parallel arrangement of the bootstrap capacitors C B1, C B1,..., C Bn210 between the charging voltage V DD and the reference voltage (corresponding to the voltage of the switching node SW) during a charging phase for charging the individual bootstrap capacitors C B1, C B1,..., C Bn210 to a capacitor voltage (which is typically substantially equal to the charging voltage V DD ) and• To provide a serial arrangement of the bootstrap capacitors C B1, C B1,..., C Bn210 between the charging voltage V DD and the control terminal, in particular the gate, of the switching element during a control phase for generating the control voltage 111, in particular the gate-source voltage, for controlling the switching element.Controlling the lower and / or upper plates of the bootstrap capacitors C B1, C B1,..., C Bn210 may be implemented via inverters as shown in FIG. 2 aand / or via switches as shown in FIG. 2 b.FIG. 3 shows a flow diagram of a method 300 (possibly computer-implemented) for controlling a switching element M HS( not related to ground GND) using a plurality of bootstrap capacitors 210. The method 300 comprises providing 301 a parallel arrangement of the plurality of bootstrap capacitors 210 between a charging voltage V DD and a reference voltage for charging the plurality of bootstrap capacitors 210 during a charging phase. In particular, the parallel arrangement of the plurality of bootstrap capacitors 210 may be arranged between the charging voltage V DD and the switching node SW of the power converter 100 including the switching element M HS. Thus, the voltage across each of the bootstrap capacitors 210 of the plurality of bootstrap capacitors 210 may be the same and may substantially correspond to the charging voltage V DD (as the reference voltage may correspond to the ground GND during the charging phase).The method 300 further comprises providing 302 a series arrangement of the plurality of bootstrap capacitors 210 between the charging voltage V DD and the control terminal (in particular the gate) of the switching element M HS for controlling the switching element M HS during a control phase. As a result, a relatively high control voltage 111 for controlling, in particular closing, the switching element M HS may be provided in an efficient manner (using a relatively small total capacitance of the plurality of bootstrap capacitors 210).Thus, a driver circuit 200 for controlling a switching element M HS( which is not grounded) will be described. The switching element M HS may be a high-side switching element of the power converter 100. Alternatively or additionally, the switching element M HS may be an n-type metal oxide semiconductor (MOS) field effect transistor (FET).The driver circuit 200 includes a plurality of bootstrap capacitors 210 and a switching unit 211. The plurality of bootstrap capacitors 210 may comprise two or more, in particular three or more bootstrap capacitors 210. The capacitance of the individual bootstrap capacitors 210 of the plurality of bootstrap capacitors 210 may be substantially the same. The switching unit 211 may include one or more inverters and / or one or more switches. In particular, the switching unit 211 may be configured to provide different arrangements, such as a parallel arrangement and a serial arrangement, of the plurality of bootstrap capacitors 210.The driver circuit 200 comprises a control unit 102 configured to control the switching unit 211 in dependence on a (possibly digital) control signal Act HS. In particular, the switching unit 211 may be controlled to provide the parallel arrangement of the plurality of bootstrap capacitors 210 between the charging voltage V DD and the reference voltage for charging the plurality of bootstrap capacitors 210 during a charging phase. The K bootstrap capacitors 210 of the plurality of bootstrap capacitors 210 may be arranged in parallel with each other (where K≥2 or K≥3) such that each of the K bootstrap capacitors 210 is arranged between the charging voltage V DD and the reference voltage. Thereby, each of the K bootstrap capacitors may be charged to a capacitor voltage level (which may substantially correspond to the charging voltage V DD ).Further, the switching unit 211 may be controlled to provide a serial arrangement of the plurality of bootstrap capacitors 210 between the charging voltage V DD and the control terminal, in particular the gate, of the switching element M HS for controlling the switching element M HS during a control phase. As a result, a control voltage 111, in particular a gate-source voltage, can be applied to the control terminal of the switching element M HS. The serial arrangement of the plurality of bootstrap capacitors 210 results in a situation where the voltage across the plurality of bootstrap capacitors 210 is K times the capacitor voltage (before charge redistribution). This increased voltage across the plurality of bootstrap capacitors 210 may be used to generate a relatively high control voltage 111 in an efficient manner, in particular with a relatively low total capacitance of the plurality of bootstrap capacitors 210.The reference voltage may be different, in particular lower, during the charging phase with the parallel arrangement of the plurality of bootstrap capacitors 210 than during the control phase with the serial arrangement of the plurality of bootstrap capacitors 210. In particular, the reference voltage may correspond to a first potential of a first connection node Vx (during the control phase) and a second potential, in particular ground, of a second connection node Vz (during the charging phase).The control unit 102 may be configured to control the switching unit 211 to provide the parallel arrangement of the plurality of bootstrap capacitors 210 when the control signal Act HS indicates that the switching element M HS is to be opened. Further, the control unit 102 may be configured to control the switching unit 211 to provide the series arrangement of the plurality of bootstrap capacitors 210 when the control signal Act HS indicates that the switching element M HS is to be closed. Thus, the switching element M HS may be opened during the charging phase and closed during the control phase.The control unit 102 may be configured to control the switching unit 211 to operate the plurality of bootstrap capacitors 210 in a sequence of switching cycles. One, in particular each, switching cycle can comprise a charging phase and a subsequent control phase, in particular exactly one charging phase and exactly one subsequent control phase. Thereby, efficient continuous operation of the driver circuit 200 can be provided.The driver circuit 200 may be configured such that the individual bootstrap capacitors 210 of the plurality of bootstrap capacitors 210 are each charged to the capacitor voltage level during the charging phase with the parallel arrangement of the plurality of bootstrap capacitors 210. The capacitor voltage level is typically less than the charging voltage V DD. Further, the driver circuit 200 may be configured such that the control voltage 111 for controlling the switching element M HS, which is provided during the control phase by the series arrangement of the plurality of bootstrap capacitors 210, is higher than the charging voltage V DD. As a result, a particularly reliable control of the switching element M HS can be provided.The driver circuit 200 may further comprise a base bootstrap capacitor 110 which is arranged between the charging voltage V DD and the reference voltage during the charging phase and during the control phase, in particular is arranged permanently between the charging voltage V DD and the reference voltage. The base bootstrap capacitor 110 may be configured to provide the controller 102 with electrical energy for operating the controller 102. By providing a base bootstrap capacitor 110, the reliability of the driver circuit 200 may be increased.Further, a power converter 100 including a switching element M HS and a driving circuit 200 for controlling the switching element M HS will be described. The driver circuit 200 may be configured as described in the present document.The switching element M HS may be disposed between a first terminal node Vx and a switching node SW of the power converter 100. The switching node SW may have a reference voltage that varies between a first potential of the first connection node Vx and a second potential, in particular ground, of a second connection node Vz of the power converter 100.The driver circuit 200 may be configured to control the switching unit 211 to arrange the parallel arrangement of the plurality of bootstrap capacitors 210 between the charging voltage V DD and the switching node SW during the charging phase, thereby providing reliable charging of the K bootstrap capacitors 210.The driver circuit 200 described in the present document comprises bootstrap capacitors C B1, C B2210, which are used for providing the gate-source voltage 111 of the high-side switch by charge redistribution. The switching unit 211 may include a CMOS inverter cell I 1 for connecting the bottom plate of the bootstrap capacitor C B2 either to the top plate of the bootstrap capacitor C B1( to provide the series arrangement) or to the switching node SW (to provide the parallel arrangement). Further, the switching unit 211 may include a CMOS inverter cell I 2 for connecting the gate of the high-side switch V G,HS to either the top plate of the bootstrap capacitor C B2( for the control phase) or the switching node SW (for the charging phase).Further, a plurality of diodes D 1, D 2 may be used for the corresponding plurality of bootstrap capacitors 210 to provide a recharging path (from the charging voltage V DD) for recharging the bootstrap capacitors C B1, C B2.A more generic core circuit structure 240 of the cascaded switched capacitive boost stage is shown in FIG. 2 b.The sequential operation method may be as follows: 1. during low side turn-on, the switching node SW may be pulled down to ground; 2. the switching unit 211, in particular the inverter I 1, may connect the bottom plate of the bootstrap capacitor C B2 to the switching node SW, and the switching unit 211, in particular the inverter I 2, may connect the gate terminal of the high side switch to the switching node SW, thereby keeping the high side switch off; 3. all bootstrap capacitors C B, C B1, C B2210 can be charged by the charging voltage V DD through the respective diodes D 0, D 1, D 2 ; 4. during the subsequent control phase, the low-side switch can be turned off; 5. the switching unit 211, in particular the inverter I 1, may connect the top plate of the bootstrap capacitor C B1 to the bottom plate of the bootstrap capacitor C B2 and the switching unit 211, in particular the inverter I 2, may connect the gate terminal of the high-side switch to the top plate of the bootstrap capacitor C B2 thereby turning on the high-side switch; 6. turning on the high-side switch raises the switching node SW.The proposed stacked bootstrap capacitor scheme is particularly advantageous when the target gate-source voltage of the high-side switch is relatively close to the charging voltage V DD of the bootstrap capacitors. Furthermore, the described scheme is advantageous if the target gate-source voltage of the high-side switch is higher than the charging voltage V DD of the bootstrap capacitors.The structure of the proposed stacked bootstrap capacitor scheme is modular and can be scaled to include multiple capacitor stages depending on the technical environment and application requirements. The various capacitive boost stages may be switched via inverters and / or switches. The driver circuit 200 may be tied to the switching node SW and / or may be related to a different potential, thereby supporting various DC / DC converter and output stage topologies. The proposed stacked bootstrap capacitor scheme may enable efficient power stage operation in the presence of relatively low charging voltages, thereby efficiently expanding the supported duty cycle range of a DC / DC converter 100.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 in the spirit and scope thereof. Furthermore, all examples and embodiments set forth herein are intended to be expressly for illustrative purposes only in the nature of aid the reader in understanding the principles of the proposed methods and systems. Furthermore, all statements herein providing principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

Claims

A driver circuit for controlling a switching element, the driver circuit comprising: a plurality of bootstrap capacitors; a switching unit; and a control unit configured to control the switching unit in dependence on a control signal to provide a parallel arrangement of the plurality of bootstrap capacitors between a charging voltage and a reference voltage for charging the plurality of bootstrap capacitors during a charging phase; and to provide a serial arrangement of the plurality of bootstrap capacitors between the charging voltage and a control terminal of the switching element for controlling the switching element during a control phase.The driver circuit of claim 1, wherein the controller is configured to control the switching unit to provide the parallel arrangement of the plurality of bootstrap capacitors when the control signal indicates that the switching element is to be opened; and to provide the serial arrangement of the plurality of bootstrap capacitors when the control signal indicates that the switching element is to be closed.The driver circuit of claim 1, wherein: the driver circuit further comprises a base bootstrap capacitor arranged between the charging voltage and the reference voltage during the charging phase and during the control phase; in particular permanently arranged between the charging voltage and the reference voltage; and the base bootstrap capacitor is configured to supply the control unit with electrical energy for operating the control unit.The driver circuit of claim 1, wherein the driver circuit is configured such that the individual bootstrap capacitors of the plurality of bootstrap capacitors are each charged to a capacitor voltage level during the charging phase with the parallel arrangement of the plurality of bootstrap capacitors; wherein the capacitor voltage level is less than the charging voltage; and a control voltage for controlling the switching element provided during the control phase by the serial arrangement of the plurality of bootstrap capacitors is higher than the charging voltage.The driver circuit of claim 1, wherein: the driver circuit is configured to provide a control voltage to the control terminal of the switching element during the control phase using the series arrangement of the plurality of bootstrap capacitors; the control terminal is a gate of the switching element; and the control voltage is a gate-source voltage.Driver circuit according to claim 1, wherein the plurality of bootstrap capacitors comprises two or more, in particular three or more bootstrap capacitors.Driver circuit according to claim 1, wherein the reference voltage is different, in particular lower, during the charging phase with the parallel arrangement of the plurality of bootstrap capacitors than during the control phase with the serial arrangement of the plurality of bootstrap capacitors.The driver circuit of claim 1, wherein the switching unit comprises: one or more inverters; and / or one or more switches.The driver circuit of claim 1, wherein a capacitance of the individual bootstrap capacitors of the plurality of bootstrap capacitors is substantially equal.The driver circuit of claim 1, wherein: the control unit is configured to control the switching unit to operate the plurality of bootstrap capacitors in a sequence of switching cycles; and one, in particular each, switching cycle comprises a charging phase and a subsequent control phase, in particular exactly one charging phase and exactly one subsequent control phase.A power converter, comprising: a switching element; and a driving circuit according to claim 1 for controlling the switching element.The power converter of claim 11, wherein: the switching element is disposed between a first terminal node and a switching node of the power converter; the switching node has a reference voltage varying between a first potential of the first terminal node and a second potential of a second terminal node of the power converter; and the driver circuit is configured to control the switching unit to arrange the parallel arrangement of the plurality of bootstrap capacitors between the charging voltage and the switching node during the charging phase.The power converter of claim 11, wherein: the switching element is a high-side switching element of the power converter; and / or the switching element is an n-type metal oxide semiconductor field effect transistor.A method of controlling a switching element using a plurality of bootstrap capacitors, the method comprising: providing a parallel arrangement of the plurality of bootstrap capacitors between a charging voltage and a reference voltage for charging the plurality of bootstrap capacitors during a charging phase; and providing a serial arrangement of the plurality of bootstrap capacitors between the charging voltage and a control terminal of the switching element for controlling the switching element during a control phase.

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