# Patent

## Title
Method for operating a power converter circuit and power converter circuit

## Abstract

In accordance with an embodiment, a method includes converting power by a power converter circuit having a plurality of converter cells coupled to a supply circuit. Converting the power includes a plurality of successive activation sequences and, in each activation sequence, activating at least some of the plurality of converter cells at an activation frequency. The activation frequency is dependent on at least one of an output power and an output current of the power converter circuit.

## Background
TECHNICAL FIELD
Embodiments of the present invention relate to a method for converting electrical power and a power converter circuit, in particular a switched-mode power converter circuit with a plurality of converter stages (converter cells).
BACKGROUND
Switched-mode power converters are widely used in automotive, industrial, consumer electronics or information technology (IT) applications for converting an input voltage into an output voltage received by a load. In many applications, such as CPU (Central Processing Unit) powering applications, it is required to generate an output voltage with a relatively low level from an input voltage having a higher voltage level.
One important issue in the design of power converters is the reduction of power losses. One promising converter topology is a multi-phase converter topology with a plurality of converter stages (converter cells) each having a cell input and a cell output, wherein each converter cell receives as a cell input voltage a share of an overall input voltage of the power converter, and wherein the cell outputs are configured to be connected in parallel.
It is desirable to further reduce the power losses in this type of multi-phase power converter.
SUMMARY
One embodiment relates to a method. The method includes, in a charging cycle of a power converter circuit, connecting a plurality of capacitors in series between input nodes of the power converter circuit, wherein the power converter circuit further comprises a plurality of converter cells, with each converter cell being connected to one of the plurality of capacitors, after the charging cycle and before activating at least one of the plurality of converter cells, reducing an electrical potential at a first input node of the at least one of the plurality of converter cells by switching on a ground switch of at least one other of the plurality converter cells, with the ground switch being connected between a second input node of the other one of the plurality of converter cells and a ground node, and activating the at least one of the plurality of converter cells to transfer electrical power from the capacitor connected to the at least one of the plurality of converter cells to an output of the power converter circuit.
One embodiment relates to a power converter circuit. The power converter circuit includes a plurality of capacitors configured to be connected in series between input nodes of the power converter circuit, a plurality of converter cells, wherein each of the plurality of converter cells is connected to one of the plurality of capacitors, and a control circuit. The control circuit is configured, in a charging cycle of the power converter circuit, to connect the plurality of capacitors in series between the input nodes of the power converter circuit, after the charging cycle and before activating at least one of the plurality of converter cells, to reduce an electrical potential at a first input node of at least one of the plurality of converter cells by switching on a ground switch of at least one other of the plurality converter cells, with the ground switch being connected between a second input node of the other one of the plurality of converter cells and a ground node, and to activate the at least one of the plurality of converter cells to transfer electrical power from the capacitor connected thereto to an output of the power converter circuit.
Another embodiment relates to a method. The method includes converting power by a power converter circuit having a plurality of converter circuits coupled to a supply circuit. Converting the power includes a plurality of successive activation sequences and, in each activation sequence, activating at least some of the plurality of converter cells at an activation frequency. The activation frequency is dependent on at least one of an output power and an output current of the power converter circuit.
Another embodiment relates to a power converter circuit. The power converter circuit includes a plurality of converter circuits coupled to a supply circuit and a control circuit. The control circuit is configured to operate the plurality of converter cells in a plurality of successive activation sequences and, in each activation sequence, to activate at least some of the plurality of converter cells at an activation frequency. The activation frequency is dependent on at least one of an output power and an output current of the power converter circuit.



BRIEF DESCRIPTION OF THE DRAWINGS
Examples are explained below with reference to the drawings. The drawings serve to illustrate the basic principle, so that only aspects necessary for understanding the basic principle are illustrated. The drawings are not to scale. In the drawings the same reference characters denote like features.
FIG. 1 illustrates one embodiment of a power converter circuit that includes a power supply circuit, a plurality of converter cells coupled to the power supply circuit, and a control circuit;
FIG. 2 illustrates one embodiment of a power supply circuit;
FIG. 3 illustrates one embodiment of a power supply cell implemented with a MOSFET switch;
FIG. 4 illustrates another embodiment of the power supply circuit;
FIG. 5 illustrates one way of operation of the power supply circuit shown in FIG. 4;
FIG. 6 illustrates in greater detail one embodiment of one of the plurality of converter cells shown in FIG. 1;
FIG. 7 shows timing diagrams that illustrate one way of operation of the converter cell shown in FIG. 6;
FIG. 8 illustrates in greater detail another embodiment of one of the plurality of converter cells shown in FIG. 1;
FIG. 9 shows timing diagrams that illustrate one way of operation of the converter cell shown in FIG. 8;
FIG. 10 illustrates in greater detail another embodiment of one of the plurality of converter cells shown in FIG. 1;
FIG. 11 shows timing diagrams that illustrate one way of operation of the converter cell shown in FIG. 8;
FIG. 12 shows timing diagrams that illustrate one way of operation of a power converter circuit including a supply circuit as shown in one of FIGS. 2 and 4;
FIG. 13 shows timing diagrams that illustrate another way of operation of a power converter circuit including a supply circuit as shown in one of FIGS. 2 and 4;
FIG. 14 shows timing diagrams that illustrate another way of operation of a power converter circuit including a supply circuit as shown in one of FIGS. 2 and 4;
FIG. 15 illustrates one embodiment of a control circuit;
FIG. 16 shows timing diagrams that illustrate one way of operation of the control circuit shown in FIG. 15;
FIG. 17 illustrates another embodiment of a power converter circuit including a supply circuit;
FIG. 18 shows timing diagrams that illustrate one way of operation of the power converter circuit shown in FIG. 17;
FIG. 19 illustrates a further embodiment of a power converter circuit including a supply circuit;
FIG. 20 shows timing diagrams that illustrate one way of operation of the power converter circuit shown in FIG. 19;
FIG. 21 illustrates one embodiment of a first switch in one of the plurality of converter cells;
FIG. 22 illustrates one embodiment of a drive scheme of a power converter circuit with n=6 converter cells;
FIG. 23 shows the supply circuit and parts of the converter cells of a power converter circuit with n=6 converter cells;
FIG. 24 which includes FIGS. 24A and 24B shows another representation of a drive scheme of the type shown in FIG. 22;
FIG. 25 illustrates another embodiment of a drive scheme of a power converter circuit with n=6 converter cells;
FIG. 26 shows one embodiment of an electronic circuit including two power converter circuits connected in parallel;
FIG. 27 illustrates an order in which converter cells in each of the power converter circuits shown in FIG. 26 may be activated;
FIG. 28 which includes FIGS. 28A and 28B shows embodiments of drive schemes for the electronic circuit shown in FIG. 26;
FIG. 29 illustrates an order in which converter cells in each of three parallel connected power converter circuits may be activated;
FIG. 30 shows one embodiment of a drive scheme for an electronic circuit including three power converter circuits connected in parallel;
FIG. 31 which includes FIGS. 31A-31C shows one embodiment of a method for operating a power converter circuits at a varying activation frequency; and
FIG. 32 shows another embodiment of a method for operating a power converter circuits at a varying activation frequency.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and by way of illustration show specific embodiments in which the invention may be practiced. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.
FIG. 1 illustrates one embodiment of a power converter circuit 1. The power converter circuit 1 includes an input 11, 12 for receiving an input voltage Vin and an input current Iin, and an output 17, 18 for outputting an output voltage Vout and an output current Iout. According to one embodiment, the input voltage Vin is a direct voltage (DC voltage). This input voltage Vin may be provided by a conventional power source VS (illustrated in dashed lines in FIG. 1), such as a switched-mode power supply, a battery, or the like. A voltage level of the input voltage Vin is, for example, between 5V and 100V, in particular between 10V and 60V.
The output voltage Vout and the output current Iout can be supplied to a load Z (illustrated in dashed lines in FIG. 1). According to one embodiment, the output voltage Vout is lower than the input voltage Vin. For example, the output voltage Vout is about 1.2V, 1.8V, or about 3V. For example, the load Z is a microprocessor in computer server application or a telecom application.
According to one embodiment, the power converter circuit 1 is configured to control the output voltage Vout to be substantially constant. In this case, the output current Iout may vary dependent on a power consumption of the load Z. When, for example, the input voltage Vin is substantially constant, then the input current Iin may vary dependent on the power consumption of the load Z.
The output 17, 18 includes a first output node 17 and a second output node 18 in the present embodiment. Optionally, an output capacitor 19 is coupled between the first and second output nodes 17, 18. The output voltage Vout is a voltage between the output nodes 17, 18. Equivalently, the input 11, 12 includes a first input node 11, and a second input node 12. The input voltage Vin is a voltage between the first and the second input nodes 11, 12.
According to one embodiment, the input voltage Vin and the output voltage Vout are referenced to the same circuit node, which will be referred to as ground node in the following. In this case, the second input node 12, and the second output node 18 are both connected to the ground node where a ground potential is available.
Referring to FIG. 1, the power converter circuit 1 includes a power supply circuit 2 coupled to the input 11, 12. The power supply circuit 2 is configured to receive the input voltage Vin and the input current Iin and to output a plurality of supply voltages V1, V2, Vn from the input voltage Vin. These supply voltages V1, V2, Vn may also be referred to as DC link voltages. Further, the power converter circuit 1 includes a plurality of converter cells 31, 32, 3n, wherein each of these converter cells 31-3n receives one of the plurality of supply voltages V1-Vn and is configured to supply an output current I1-In to the output 17, 18. The output current Iout of the power converter circuit 1 equals the sum of the output currents I1-In of the individual converter cells 31-3n. The individual converter cells 31-3n control their output currents I1-In such that the output voltage Vout has a predefined set-value. One way of operation of the individual converter cells 31-3n is explained in further detail herein below. In the power converter circuit 1 depicted in FIG. 1, the power supply circuit 2 outputs n=3 supply voltages V1-Vn, and n=3 converter cells 31-3n are coupled to the power supply circuit 2. However, this is only an example. The number n of converter cells implemented in the power converter circuit 1 is arbitrary and not limited to n=3.
Referring to FIG. 1, each of the converter cells 31-3n includes a cell input with a first input node 131-13n and a second input node and receives one of the supply voltages V1-Vn at the respective cell input. Furthermore, each of the plurality of converter cells 31-3n includes a cell output with a first output node 151-15n and a second output node 161-16n. The first output nodes 151-15n of the individual converter cells 31-3n are coupled to the first output node 17 of the power converter circuit 1, and second output nodes 161-16n are coupled to the second output node 18 of the power converter circuit 1.
Referring to FIG. 1, a control circuit 4 controls the operation of the supply circuit 2 and of the individual converter cells 31-3n. In FIG. 1, control signals S2, S31-S3n received by the supply circuit 2 and by the individual converter cells 31-3n, respectively, are only schematically illustrated. Dependent on the specific implementation of the supply circuit 2 and of the individual converter cells 31-3n each of these circuits may receive two or more control signals, so that the control signal S2, S31-S3n illustrated in FIG. 1 may each represent one control signal or two or more control signals.
FIG. 2 illustrates a first embodiment of the power supply circuit 2. This power supply circuit 2 includes a plurality of power supply cells (supply cells) 21-2n connected in series between the first and second input nodes 11, 12. Each of these supply cells 21-2n is configured to output one of the supply voltages V1-Vn. Each of the supply cells 21-2n includes a series circuit with a first switch 211-21n and a first capacitive storage element (capacitor) 221-22n, wherein the supply voltages V1-Vn are available across the first capacitors 221-22n of the supply cells 21-2n.
In the supply circuit 2 shown in FIG. 2, as well as in other embodiments of the supply circuit 2 disclosed below, like features of the individual supply cells 21-2n have like reference characters that are different from each other by subscript indices “1”, “2”, “n”. In the following, if an explanation equivalently applies to each of the supply cells 21-2n and to their individual components, reference characters without indices will be used. Equivalently, the converter cells 31-3n and their components have like reference characters that are different from each other by subscript indices “1”, “2”, “n”. In the following, if an explanation equivalently applies to each of the converter cells 31-3n and to their individual components, reference characters without indices will be used. Each of the individual supply cells 21-2n outputs one of the supply voltages V1-Vn and, therefore, supplies the converter cell (31-3n in FIG. 1) connected thereto.
Referring to FIG. 2, each of the plurality of supply cells 2 is connected to (is associated with) one of the plurality of converter cells 3. Each of the supply cells 2 receives the energy it supplies to the associated converter cell 3 from the input nodes 11, 12. Each of the supply cells 2 can be operated in a charging mode, and in a supply mode. In the charging mode, the supply cell 2 receives energy from the input nodes 11, 12 so that the first capacitor 22 of the supply cell 2 is charged. In the supply mode the supply cell 2 is ready to provide energy to the associated converter cell 3 connected thereto, that is, the supply cell 2 is ready to have the first capacitor 22 discharged. Whether one supply cell 2, in the supply mode, actually provides energy to the associated converter cell 3 is dependent on the operation mode of the converter cell 3. This is explained in further detail below.
In the supply circuit 2 embodiment depicted in FIG. 2, the individual supply cells 21-2n are operated in the charging mode at the same time. In the charging mode, the control circuit 4 (not illustrated in FIG. 2) switches on the first switches 211-21n of the individual supply cells 21-2n. When the first switches 211-21n are switched on, the capacitors 221-22n of the individuals supply cells 21-2n are connected in series between the first and second input nodes 11, 12. The individual capacitors 221-22n are then each charged to a supply voltage V1-Vn. The supply voltage V1-Vn of each supply cell is dependent on the input voltage Vin and the capacitances of the respective capacitor 221-22n. According to one embodiment, the capacitances of the individual capacitors 221-22n are substantially the same. In this case, at the end of the charging phase, the individual capacitors 221-22n have substantially the same voltage level, that is




  
    
      
        
          
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where n is the number of supply cells 21-2n in the supply circuit 2.

Whether the individual first capacitors 221-22n are completely charged (to the voltage Vin/n in the embodiment explained before) or whether the individual capacitors 221-22n are only charged partially (to a voltage lower than Vin/n) is dependent on the duration of the charging phase. According to one embodiment, the duration of the charging phase is selected such that the first capacitors 221-22n are completely charged during the charging phase. At the end of the charging phase, the control circuit 4 opens the first switches 211-21n and the individual capacitors 221-22n are ready to be discharged by the individual converter cells 31-3n. In this embodiment, the first switches 211-21n can be switched on and off simultaneously, so that one control signal S2 received from the control circuit 4 can be used to control the individual first switches 211-21n.
The first switches 211-21n can be implemented as conventional electronic switches. According to one embodiment, the first switches 211-21n are transistors. According to another embodiment the first switches 211-21n are relays. FIG. 3 illustrates one embodiment of one supply cell Z that includes a first switch 21i implemented as a transistor. In this specific embodiment, the transistor 21i is a MOSFET. This MOSFET can be implemented as an n-type MOSFET or as a p-type MOSFET and can be implemented as an enhancement (normally-off) transistor or as a depletion (normally-on) transistor, such as a depletion MOSFET or a JFET (Junction Field-Effect Transistor). The transistors can be implemented using a conventional semiconductor material, such as silicon (Si), silicon carbide (SiC), gallium arsenide (GaAs), gallium nitride (GaN), or the like. The MOSFET has a control terminal (gate terminal) for receiving the control signal S2 and a load path (drain-source path) connected in series with the capacitor 22i. The control circuit 4 is configured to generate a signal level of the control signal S2 dependent on the specific type of MOSFET such that the MOSFET is switched on when the supply cell Z is in the charging mode and such that the MOSFET is switched off when the supply cell Z is in the supply mode. The MOSFET 21i may include an internal body diode which is also illustrated in FIG. 3. The polarity of this body diode may be selected such that the capacitor 22 of the supply cell 2 cannot be charged via the body diode. For this, an anode of the body diode is connected to the capacitor 22i in the embodiment of FIG. 3. Implementing the switch 21i as an n-type MOSFET is only an example. Any other type of electronic switch, such as another type of MOSFET, or another type of transistor, such as a BJT (Bipolar Junction Transistor), a JFET (Junction Field-Effect Transistor), or a GaN-HEMT (Gallium-Nitride High Electron-Mobility Transistor) may be used as well. According to one embodiment, the switches 211-21n are implemented using Power IC technology supporting lateral power transistors.
FIG. 4 illustrates a further embodiment of the supply circuit 2. In the embodiment of FIG. 4, the supply circuit 2 includes an inductor 23 connected in series with the supply cells 21-2n. In the supply circuit shown in FIG. 2, the input current Iin may have a relatively high current level at the beginning of the charging phase (charging mode). This current level is dependent on how much the individual capacitors 221-22n have been discharged before. In the supply circuit 2 of FIG. 4, the inductor 23 helps to limit the current level of the input current Iin. Further, the inductor 23 makes it possible to switch on and switch off the switches 211-21n of the supply cells 21-2n when a current through the inductor 23 and the switches 211-21n is substantially zero. Referring to FIG. 4, an optional freewheeling element 26, such as a diode, is connected in parallel with the inductor 23. The freewheeling element 26 takes the current through the inductor 23 when the switches 211-21n switch off before the inductor 23 has been completely demagnetized. The inductor 23 is not necessarily a discrete device but may be realized by the total stray inductance in the current path that includes the capacitor series circuit between the first input node 11 and the second input node 12.
One way of operation of the supply circuit of FIG. 4 is explained with reference to FIG. 5, in which timing diagrams of the operation mode of the supply circuit 2 and of the input current Iin are illustrated. The operation mode is represented by the control signal S2. For explanation purposes it is assumed that the control signal S2 has a high level (that switches on the individual first switches 211-21n) when the supply circuit 2 is in the charging mode and a low level when the supply circuit is in the supply mode. For explanation purposes it is further assumed that the individual capacitors 221-22n are not completely charged at the beginning of the charging mode, that is the sum of the individual supply voltages V1-Vn is lower than the input voltage Vin:




  
    
      
        
          
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In this case, the input current Iin increases at the beginning of the charging mode, wherein the beginning of the charging mode is represented by time t1 in FIG. 5. Referring to FIG. 5, the input current Iin increases to a maximum input current Iinmax and then decreases to zero. The maximum input current IinMAX is dependent on the difference between the voltage level of the input voltage Vin and the voltage level of the voltage across the series circuit with the capacitors 221-22n at the beginning of the charging phase, wherein the maximum input current IinMAX increases as the voltage difference increases. A charging period T, which is a time period between the beginning of the charging period at time t1 and a time t2 when the input current Iin decreases to zero, is independent of the voltage difference and is only dependent on the inductance of the inductor 23 and of an overall capacitance of the capacitor series circuit with the plurality of capacitors 221-22n. According to one embodiment, a time period in which the supply circuit 2 is operated in the charging mode corresponds to the charging period T or is even shorter.
In the embodiment shown in FIG. 4, the overall voltage across the capacitor series circuit at the end of the charging period can be higher than the input voltage Vin, that is:




  
    
      
        
          
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Whether the overall voltage is higher than the input voltage Vin is dependent on the  point in time when the switches 211-21n are being turned off. According to one embodiment, the switches 211-21n turn off when the input current Iin substantially has reached the maximum level (which is between times t1 and t2 in FIG. 5). In this case, the overall voltage corresponds to the input voltage. However, when the switches 211-21n switch off later, energy that has been (magnetically) stored in the inductor 23, between the first time t1 and the time when the input current Iin reaches the maximum is transferred to the capacitors 221-22n and causes the overall voltage to increase to above the input voltage Vin. According to one embodiment, in order to prevent the capacitors 221-22n from being discharged, the switches 221-22n are switched off when or before the input current Iin turns zero.
In the charging mode, the supply voltages output by the individual supply cells 21-2n are referenced to different electrical potentials. The supply voltage of the supply cell 2n connected to the second input node 12 is referenced to the electrical potential at the second input node 12. This electrical potential at the second input node 12 will be referred to as first ground potential in the following. The supply cell 2n directly connected to the second input node 12 will be referred to as lowermost supply cell in the following, and the converter cell 3n to the lowermost supply cell 2n will be referred to as lowermost converter cell in the following.
The supply voltage V2 of the supply cell 22 adjacent the lowermost supply cell 2n is referenced to P12+Vn, where P12 denotes the first ground potential and denotes the supply voltage of the lowermost supply cell 2. Equivalently, the supply voltage V1 of the supply cell 21 is referenced to P12+Vn+V2. In general, in the charging mode, the supply voltage Vi of one supply cell 2i (wherein 2i denotes an arbitrary one of the supply cells 21-2n) is referenced to




  
    
      
        
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In the supply mode, the supply voltage V1-Vn of each supply cell 21-2n is referenced to the electrical potential at the second output node 18, which will be referred to as second ground potential in the following. For this purpose, each of the converter cells 31-3n includes a second switch 311-31n connected between the capacitor 221-22n of the corresponding supply cell 21-2n and the second output node 18. These second switches, which will also be referred to as ground switches in the following, are schematically illustrated in the converter cells 31-3n of FIGS. 2 and 4. The control circuit 4 controls these second switches 211-21n to be switched off (to be open) when the corresponding supply cell 21-2n is in the charging mode. In the lowermost converter cell 3n, the second switch 31 is optional and may be omitted.
Besides the charging mode and the supply mode each of the supply cells 2 can be in a standby mode, in which the capacitor 22 has been charged and in which the first switch 21 and the second switch 31 are open. In this operation mode, the supply voltage V provided by the respective supply cell is floating.
Optionally, a further switch 24 is connected between the series circuit with the supply cells 21-2n and the second input node 12. This further switch 24 is switched on and off simultaneously with the first switches 211-21n. In this embodiment, the lowermost converter cell 3n also includes a second switch 31n. In this embodiment, the input voltage Vin and the output voltage Vout can be referenced to different ground potentials. That is, the input voltage Vin can be referenced to the first ground potential, that is, the potential at the second input node 12, and the output voltage Vout can be referenced to the second ground potential, that is, namely the potential at the second output node 18.
According to a further embodiment, the second switch of one of the converter cells 31-3n that is not the lowermost converter cell 3n is omitted, while the lowermost converter cell 3n the second switch 31n. If, for example, the second switch 312 of the second converter cell 32 is omitted then the output voltage Vout is referenced to the electrical potential at one of the terminals of the capacitor 222 associated with the second converter cell 32.
Different topologies are possible for implementing the individual converter cells 31-3n. Two possible implementations are explained with reference to FIGS. 6 and 8 below.
FIG. 6 shows one embodiment of a converter cell 3i (wherein 3i represents an arbitrary one of the converter cells 31-3n). This converter cell 3i is implemented with a buck converter topology and includes a series circuit with a third switch 32i and an inductor 33i connected between a first input node 13i and a first output node 15 wherein the first output node 15i is connected to the first output node 17 of the power converter circuit 1. A freewheeling element 34i is connected between a second output node 16i and a circuit node common to the third switch 32 and the inductor 33i. The second output node 16i is the output node that is connected to the second output node 18 of the power converter circuit 1. The freewheeling element 34i can be implemented as a conventional freewheeling element, such as a diode or a synchronous rectifier (SR) MOSFET. The third switch 32i will be referred to as control switch in the following.
One way of operation of the converter cell 3i shown FIG. 6 is explained with reference to FIG. 7 that shows timing diagrams of a control signal S31i controlling the second switch 31i, a control signal S32i controlling the control switch 32 and the output current Ii of the converter cell 3i. The control signal S31i controlling the second switch 31i will be referred to as supply mode control signal in the following, and the control signal S32i controlling the control switch 32i will be referred to as current control signal, because this control signal helps to control the output current Ii. This is explained in greater detail below.
According to one embodiment, the control signal S32i is a pulse-width modulated (PWM) signal that drives the control switch 32i in a pulse-width modulated (PWM) fashion. That is, there is a plurality of subsequent drive periods, wherein in each drive period the control signal S32i switches on the control switch 32i for an on-period TON and switches off the control switch 32i for an off-period TOFF. According to one embodiment, the periods TPWM of the individual drive cycles are identical, wherein a duty cycle, which is a ratio between the duration of the on-period TON and the duration TPWM of the drive cycle, may vary. Just for explanation purposes it is assumed that a high level of the drive signal S32i of FIG. 7 represents an on-state of the control switch 32i, while a low level of the drive signal S32i represents an off-state of the control switch 32i.
Referring to FIG. 7, the output current Ii increases during the on-period TON and decreases during the off-period TOFF. FIG. 7 shows the timing diagram of the output current Ii in the steady state of the converter cell 3i and in a continuous current mode (CCM). CCM is an operation mode in which the output current Ii does not decrease to zero during the off-period TOFF. The average output current Ii can be varied by varying the duty cycle of the control signal S32i. The average output current can be increased by temporarily increasing the duty cycle, and the average output current can be decreased by temporarily decreasing the duty cycle. The control signal S32i controlling the control switch 32i will also be referred to as current control signal in the following. In the steady state the duty cycle is substantially constant, such as, for example, about 0.25 (if, e.g., n=4 and Vout is about 1V). Operating the converter cell 3i in the CCM is only an example. It is also possible to operate the converter cell 3i in a discontinuous current mode (DCM) in which the output current Ii decreases to zero during the off-period TOFF.
During the on-period TON energy is magnetically stored in the inductor 33i. During the off-period, the energy stored in the inductor 33i causes the output current Ii to continue to flow, wherein the freewheeling element 34i provides a freewheeling path that allows the output current Ii to continue to flow.
In the converter cell 3i shown in FIG. 6, the ground switch 31i is connected between the second input node 14i and the freewheeling current path with the rectifier element 34i and the inductor 33i so that the freewheeling current can flow when the second switch 31i has been switched off. Referring to the explanation before, the converter cell 3i is in the supply mode when the second switch 31i is switched on. According to one embodiment (illustrated in solid lines in FIG. 7), the ground switch 31i and the control switch 32i are switched on and off simultaneously by their corresponding control signals S31i, S32i. In this case, the ground switch 31i and the control switch 32i can be controlled by a common control signal S3i, and the supply cells (not shown in FIG. 6) coupled to the converter cell 3i is in the supply mode only when the control switch 32i is switched on. The supply cell may be re-charged between subsequent on-times of the control switch 31i.
According to a further embodiment, the ground switch 31i is switched on before the control switch 32i switches on, so that there is a delay time between switching on the ground switch 31i and the control switch 32i.
According to a further embodiment, there are two or more drive cycles of the control switch 31i before the associated supply cell is recharged. In this case, the ground switch 31i may be kept in the on-state for several drive cycles. This is illustrated in dashed-and-dotted lines in FIG. 7.
According to yet another embodiment, the ground switch 31i is operated as the control switch and the control switch 32i serves to match the electrical potentials. That is, in this embodiment, the ground switch 31i is driven in a PWM fashion as explained in connection with the control switch 32i herein before, and the control switch 32i can be operated like the ground switch 31i explained herein before. This is beneficial when the freewheeling element 34 and the first switch 311 are implemented as transistors, in particular as MOSFETs. In this case, these transistors can be driven using drive voltages that can be referenced to the same reference potential, namely the potential at the circuit node common to the second switch 311 and the freewheeling element 34i, so that these drive voltages can be produced by a common driver.
When the diode 34i shown in FIG. 6 is replaced with a switch that acts as a freewheeling element, the converter cell 3i can be operated in a zero-voltage switching (ZVS) mode. The zero-voltage mode is, in particular, useful when the control switch 32i is an electronic switch with an output capacitance that charges when the control switch 32i switches off. FIG. 8 shows one embodiment of the converter cell 3i that includes a switch 34i as the freewheeling element and that includes a MOSFET with an output capacitance COSS as the control switch 32i. One way of operation of this converter cell 3i in the zero-voltage switching mode is explained with reference to FIG. 9 that shows timing diagrams of the output current Iin, the control signal S32i of the control switch 32 and the control signal S34i of the freewheeling switch 34i.
In the zero-voltage switching mode, the freewheeling switch 34i switches on when the control switch 32i switches off, and the freewheeling switch 34i stays in the on-state until the output current Iin changes the current flow direction (turns negative). This negative current (slightly) magnetizes the inductor 33i. The voltage across the output capacitor COSS of the control switch 32i substantially corresponds to the difference between input voltage Vi and output voltage Vout, when the control switch 32i is in the off-state. When the freewheeling switch 34i switches off, the current induced by the magnetized inductor 33i discharges the output capacitor COSS of the control switch 32 so that the control switch 32i can be switched on when the voltage across the control switch 32i is substantially zero. This helps to reduce switching losses.
Especially when the converter cell 3i is operated in the DCM or in the ZVS mode, the inductor can be implemented with a lower inductance than in the CCM mode. The switching frequency of the switch driven in the PWM fashion (that is the control switch 32i or the first switch 31i) is, for example, several MHz, such as 10 MHz, or even more.
In the converter cell 3i shown in FIG. 6, a maximum voltage level of the output voltage Vout is lower than a voltage level of the supply voltage Vi. FIG. 10 shows an embodiment of a converter cell 3i that can generate an output voltage Vout with a higher voltage level than the supply voltage Vi. This converter cell 3i has a boost converter topology. In this embodiment, a series circuit with an inductor 33i and a control switch 32i is connected between the input nodes 13i, 14i. Further, a rectifier element 34i is connected between a circuit node common to the inductor 33i and the control switch 32i and the first output node 15i. Like in the embodiment of FIG. 6, the control switch 32i is driven in a PWM fashion, wherein each time the switch 32i is switched on, energy is magnetically stored in the inductor 33i. The energy stored in the inductor 33i is at least partially transferred to the output nodes 15i, 16i, when the control switch 32i is switched off. The control switch 32i and the second switch 31i can be switched on an off simultaneously. When the second switch 31i has been switched off, the further rectifier element 35i allows the output current Ii to flow.
Timing diagrams of the control signal S32i of the control switch 32i and the output current Ii are illustrated in FIG. 11. In this embodiment, the output current Ii only flows during the off-period. Like in the embodiment of FIG. 6, an average of the output current Ii can be controlled by adjusting the duty cycle of the control signal S32i.
Each of the converter cells 3, explained before can be operated in a continuous current mode (CCM), a discontinuous current mode (DCM), or in the ZVS mode. Furthermore, the converter cell 3i can be implemented with a topology other than a buck converter topology (as shown in FIGS. 6 and 8) and a boost converter topology (as shown in FIG. 10) as well. Examples of those other converter cell topologies include a buck-boost converter topology, or a boost-buck converter topology, to name only two.
FIG. 12 shows timing diagrams illustrating one way of operation of a power converter circuit 1 of the type shown in FIG. 1 when implemented with a supply circuit 2 as shown in one of FIGS. 2 and 4 and with a plurality of converter cells 3i-3n as shown in one of FIGS. 6 and 8. FIG. 12 shows timing diagrams of the control signals S321-S32n of the control switch 32i of each converter cell 31-3n. The ground switch 31i can be controlled as explained before, that is, the ground switch 31i and the control switch 32i of each one converter cell 3i can be controlled simultaneously, or the ground switch 31i can already be in the on-state before the control switch switches on. FIG. 12 shows timing diagrams of PWM control signals S31, S32, S3n, wherein each of these control signals controls the control switch (32i in FIGS. 6 and 8) and the second switch (31i in FIGS. 6 and 8) of one of the converter cells 31-3n. FIG. 12 further shows a timing diagram of the supply circuit control signal S2 that controls the charging mode of the individual supply cell 21-2n. In this embodiment, the individual supply cells 21-2n are operated in the charging mode simultaneously.
Referring to FIG. 12, each of the control signals S31-S3n can have one of an on-level and an off-level. For the purpose of explanation it is assumed that a high-level of a control signal S31-S3n received by a converter cell 31-3n corresponds to the on-level and switches on the control switch (32i in FIGS. 6 and 8) and the ground switch 311-31n of the converter cell 31-3n, while a low-level corresponds to the off-level and switches off the control switch and the ground switch. Equivalently, a high level of the supply circuit control signal S2 represents a charging mode of the supply cells 21-2n.
In the operation scenario illustrated in FIG. 12, the control circuit 4 operates the individual converter cells 31-3n such that the control switch of only one converter cell 31-3n is switched on at one time. That is, on-periods TON1, TON2, TONn of the control switches in the individual converter cell 31-3n do not timely overlap. Referring to explanation above, the individual converter cells 31-3n can be operated in one of the DCM mode, ZVS mode, and the CCM mode, wherein the ZVS mode exhibits the lowest switching losses. In FIG. 12, the control signal S3n is the control signal of lowermost converter cell 3n coupled to the lowermost supply cell 2n. When the input voltage Vin and the output voltage Vout are referenced to the same ground potential, that is, when the first ground potential and the second ground potential explained above are identical, the lowermost supply cell 2n can be operated in the charging mode and in the supply mode at the same time. That is, the control switch of the lowermost converter cell 3n can be switched on to receive power from the lowermost supply cell 2n while the capacitor 22n is coupled to the input 11, 12. Thus, in the embodiment shown in FIG. 12, the on-period TONn of the control switch of the lowermost converter cell 3n and the charging mode period of the lowermost supply cell 2n may overlap. A “charging mode period” of a supply cell is a time period when the supply cell is in the charging mode.
FIG. 13 illustrates a further embodiment for operating the power converter circuit 1. In this embodiment, the control switches in the individual converter cell 31-3n are switched on and off substantially simultaneously so that there is a common on-period. In this embodiment, the charging mode period and the on-period TONn of the lowermost converter cell 3n do not overlap. The supply circuit 2 is operated in the charging mode after the common on-period TON1, TON2, TONn. This mode enables to increase the output current extremely rapidly. For example, this operation mode is used to re-adjust the output voltage Vout after rapid changes of the load.
According to a further embodiment illustrated in FIG. 14, the individual converter cells are operated in a interleaved fashion such that the on-periods TON1, TON2, TONn of the control switches (and the second switches) in the individual converter cells 31-3n overlap. In case the input voltage Vin and the output voltage Vout are referenced to the same ground potential, the charging mode period and the on-period TONn of the control switch in the lowermost converter cell 3n may overlap. However, the charging mode period may only overlap a part of the on-period TONn in which none of the other control switches is switched on. That is, the charging mode period should not overlap one of the other on-periods TON1, TON2.
Although FIGS. 12 to 14 illustrates one way of operation of a power converter circuit with n=3 converter cell the operation explained with reference to these FIGS. 12 to 14 is not restricted to power converter circuits with n=3 converter cells, but applies to power converter circuits with only two (n=2) or with more than three (n>3) converter cells equivalently.
In each of the embodiments explained with reference to FIGS. 12 to 14, the control circuit 4 may control the output currents of the individual converter cells 31-3n such that the (average) level of the output voltage Vout equals a predefined reference voltage, or may control the output currents of the individual converter cells 31-3n such that the (average) level of the output current Iout equals a predefined reference current. In an operation mode in which the converter cells 31-3n control the output current Iout the optional output capacitor (illustrated in dashed lines in FIG. 1) can be omitted.
The control circuit 4 may control the individual converter cells 31-3n such that the control signals S321-S32n have the same duty cycle in one drive cycle TPWM. According to a further embodiment, the control circuit 4 controls one of the converter cells, such as converter cell 31, as a master converter cell such that a duty cycle of the corresponding control signal S31 is dependent on the output voltage Vout (or dependent on the output current Iout), and controls the other converter cells, such as converter cells 32-3n, as slave converter cells such that output currents I2-In of these other converter cells substantially equals to the output current I1 of the master converter cell 31. The output currents of the slave converter cells can be controlled by adjusting the duty cycle of the control signals S311-S32n. In this embodiment, output currents I1-In of the individual converter cells 31-3n are substantially balanced. The converter cell 31 acts as a master converter cell and the other converter cells 32-3n act as slave converter cells in this embodiment.
According to another embodiment, the individual converter cells 31-3n are controlled independently of each other such that each converter cell 31-3n supplies a predefined output current the output 17, 18, wherein the output currents I1-In of the individual converter cells 31-3n can be mutually different. According to yet another embodiment, the individual controller cells 31-3n are controlled independently of each other to generate a same predefined output voltage level at the output 17, 18.
FIG. 15 illustrates one embodiment of the control circuit 4 configured to operate the power converter circuit 1 in accordance with the operation scenario illustrated in FIG. 12 and such that the individual control signals S31-S3n have the same duty cycle. Referring to FIG. 15, the control circuit 4 includes a PWM generator 41 configured to receive a reference signal SREF and an output signal SOUT representing one of the output voltage Vout and the output current Iout. The PWM generator 41 is configured to output a PWM signal S3 dependent on the output signal SOUT and the reference signal SREF. In the embodiment of FIG. 15, the PWM generator 41 includes a controller 411 that is configured to output a regulation signal S411 dependent on a relationship between the output signal SOUT and the reference signal SREF. A comparator 412 receives the regulation signal S411 and a sawtooth signal S413 from a sawtooth generator 413. A flip-flop 414 is set each time a falling edge of the sawtooth signal S413 occurs and is reset each time the sawtooth signal reaches the regulation signal S411. The PWM signal S3 is available at the output of the flip flop 414. The PWM signal S32 generated by the PWM generator 41 is used as the control signal S31 in the converter cell 31. Further, time delayed versions of this PWM signal S3 are generated using a first and a second delay element 421, 422, wherein the first delay element 421 delays the first control signal S31 and outputs the control signal S32 for the converter cell 32, and the second delay element 422 delays the second control signal S322 and outputs the control signal S3n to the lowermost converter cell 3n. The supply circuit control signal S2 corresponds to the control signal S3n of the lowermost converter cell 3n in this embodiment.
One way of operation of the control circuit 4 shown in FIG. 15 is illustrated in FIG. 16, in which timing diagrams of the sawtooth signal S413, the regulation signal S411 and of the control signals S31-S3n are illustrated. The period of one drive cycle of one converter cell is defined by the frequency of the sawtooth signal, wherein TPWM=1/fSW, wherein fSW is the frequency of the sawtooth signal. The delay time introduced by each of the delay elements 421, 422 is TPWM/3 in a power converter circuit with n=3 converter cells. In general, n−1 delay elements are required to generate n control signals for n different converter cells, wherein the delay time introduced by each of the delay elements is TPWM/n. The duty cycles of the individual control signals are identical and are dependent on a relationship between the output signal SOUT and the reference signal SREF. The controller 411 provides the regulation signal S411. The controller can be a conventional P-controller, I-controller, PI-controller, or PID-controller.
Optionally, the duty cycle of the control signals S31-S3n is limited to a predefined maximum duty cycle SMAX. Such limitation may be performed by a duty cycle limiting circuit 43 that includes a comparator 432 corresponding to the comparator 412 of the PWM generator, and a flip flop 434 corresponding to the flip flop 414 of the PWM generator 41. The comparator 432 of the limiting circuit 43 receives a maximum duty cycle signal DCMAX instead of the regulation signal. A PWM signal S3MAX output by the limiting circuit 43 represents a PWM signal with a maximum duty cycle. An optional logic gate 44 receives the maximum PWM signal S3MAX and the PWM signal S3 output by the PWM generator 32. The first control signal S321 in this embodiment is either the PWM signal S3 output by the PWM generator 41 or is the maximum PWM signal S43, whichever has the lower duty cycle. According to one embodiment, the logic gate 44 is an AND gate.
In the supply circuit shown in FIG. 2 the individual supply cells 21-2n are operated in the charging mode simultaneously. FIG. 17 illustrates an embodiment of a supply circuit 2 in which the individual supply cells 21-2n can be operated in the charging mode independently. In this embodiment, the supply circuit 2 includes a plurality of second capacitive storage elements (capacitors) 251-25n connected in series between the input nodes 11, 12. Each of these second capacitors 251-25n is connected in parallel with one of the supply cells 21-2n, wherein the individual supply cells 21-2n are connected in series between the input nodes 11, 12. Like in the embodiment shown in FIG. 2, each of the supply cells 21-2n includes a capacitor 221-22n and a first switch 211-21n connected in series with the capacitor 221-22n. Further, each supply cell 21-2n includes an inductor 231-23n connected in series with the capacitor 221-22n and the first switch 211-21n. Further, except for the lowermost supply cell 2n, each supply cell 21-2n includes a further switch 241-24n connected in series with the capacitor 221-22n, the first switch 211-21n and the inductor 231-23n. The series circuit with the capacitor, the first switch, the inductor and the further switch of one supply cell 21-2n is connected in parallel with the second capacitor 251-25n coupled to the corresponding supply cell 21-2n.
The further switch 24 of the lowermost supply cell 2n is optional. In each of the supply cells 21-2n that include a first switch 211-21n and a further switch 241-24n, the capacitor 221-22n is connected between these switches. The supply voltage V1-Vn provided by the individual supply cells 21-2n is the voltage across the capacitor 221-22n of the respective supply cell 21-2n.
In the supply circuit 2 shown FIG. 17 each of the second capacitors 251-25n supplies an input voltage Vin1-Vinn to one supply cell 21-2n. The voltage levels of the individual input voltages Vin1-Vinn is dependent on the overall input voltage Vin between the input nodes 11, 12 and is dependent on the capacitances of the individual second capacitors 251-25n. According to one embodiment, the capacitances of the individual second capacitors 251-25n are substantially equal. In this case, the individual input voltages Vin1-Vinn are equal and correspond to Vin/n. The inductors 231-23n in the individual supply cells 21-2n are optional Like in the embodiment explained with reference to FIG. 4, these inductors help to prevent high inrush currents into the first capacitors 221-22n of the individual supply cells 21-2n. It is also possible, to implement some of the supply cells 21-2n with an inductor and to implement others of the supply cells 21-2n without the inductor.
The individual supply cells 21-2n may be operated in the same way. One way of operation of one supply cells 2 (wherein reference character 2 denotes one of the supply cells 21-2n) is explained in the following. The supply cell 2 is operated in the charging mode when the first switch 21 and the second switch 24 are switched on. In this case, the first capacitor 22 is connected in parallel with the second capacitor 25, so that the second capacitor 22 is charged to the supply voltage provided by the second capacitor 25 (or, by virtue of the inductors 231-23n, to a voltage higher than this supply voltage). At the end of the charging mode the first switch 21 and the further switch 24 are switched off. After the charging phase the first capacitor 22 can be operated in the supply mode. For this purpose, the second capacitor 22 is coupled to the second output node 18 via the second switch 31 of the converter cell 3 coupled to the supply cell 2.
The further switches 241-24n of the supply cells 21-2n protect the second capacitors 251-25n from being discharged when the supply cells 21-2n are operated in the supply mode. The lowermost supply cell 2n can be operated in the charging mode and in the supply mode at the same time if the second input node 12 and the second output node 18 are referenced to the same ground potential. In case the second input node 12 and the second output node 18 are referenced to different ground potentials, the lowermost supply cell 21 includes the further switch 24n, and the converter cell 3n coupled to the lowermost supply cell 2n includes the ground switch 31n.
One way of operation of a power converter circuit including a supply circuit 2 of the type shown in FIG. 17 is explained with reference to FIG. 18 below. For purposes of explanation it is assumed that the supply circuit 2 includes n=6 supply cells, wherein each of these supply cells supplies one of n=6 converter cells. FIG. 18 shows timing diagrams of control signals S321-S32n wherein each of these control signals controls the control switch in one of the converter cell. Further, timing diagrams of supply circuit control signals S21-S2n are illustrated. Each of these supply circuit control signals S21-S2n controls the operation of one of the supply cells 21-2n, wherein supply circuit control signal S21 controls the operation of the supply cells 21 coupled to converter cell 31 that receives control signal S31, supply circuit control signal S22 controls the operation of the supply cell 22 coupled to converter cell 32 that receives control signal S32, and so on.
In the embodiment shown in FIG. 18, a high level of one supply circuit control signal S21-S2n represents a charging mode of the corresponding supply cell 21-2n, and a high level of a converter control signal S31-S3n represents a time period in which a converter cell 31-3n receives energy from the corresponding supply cell 21-2n. That is, a high level of one converter control signal S31-S3n represents the supply mode of the corresponding supply cell 21-2n.
In the embodiment shown in FIG. 18, the individual converter cells are operated in an interleaved fashion wherein each supply cell is recharged after the control switch of the associated converter cell has been switched off. That is, a supply circuit control signal S21-S2n changes to a signal level (a high level in the present embodiment) that operates the corresponding supply cell 21-2n in the charging mode after the control signal S321-S32n of the converter cell 31-3n coupled to the supply cell changes to an off-level. The duration of the charging cycle is at most the duration in which the control switch 32i of the corresponding converter cell 3i is in the off-state.
FIG. 19 illustrates a further embodiment of a power converter circuit. This power converter circuit is a combination of the power converter circuits explained with reference to FIGS. 2 and 17. In the embodiment of FIG. 19, m second capacitors 251, 25m are connected between the input nodes 11, 12, wherein each of these second capacitors 251, 25m is connected in parallel with a series circuit including some the pluralities of supply cells 21-2n. In the embodiment of FIG. 19 there are two second capacitors 251-25m connected between the input nodes 11, 12, and a series circuit with three supply cells is connected in parallel with each second capacitor 251, 25m. Each of the supply cells includes a first capacitor 251-25n and a first switch 211, 21n. Optionally, an inductor 231, 23n is connected in series with each of the supply cell series circuits. Each of the supply cell series circuits connected in parallel with one second capacitor 251, 25m has a lowermost supply cell, which is supply cell 23 in the series circuit connected in parallel with the second capacitor 251, and which is supply cell 2n in the series circuit connected in parallel with the second capacitor 25m. Supply cell 2n is the lowermost supply cell of the overall series circuit. A further switch 241, 24m is connected between the lowermost supply cell 23, 2n of each series circuit connected in parallel with one second capacitor 251-25m, wherein the further switch 24m connected to the lowermost supply cell 2n of the overall supply cell series circuit 21-2n is optional.
In the supply circuit 2 shown in FIG. 19, the supply cells coupled to one second capacitor 251, 25m are operated in the charging mode simultaneously and can be operated in the supply mode independently. FIG. 20 shows timing diagrams that illustrate one way of operation of the power converter circuit shown in FIG. 19. In FIG. 20, S21 denotes the supply circuit control signal that controls the charging mode of the supply cell 21-23 of the first series circuit, and S2m denotes the supply circuit control signal that controls the charging mode of the supply cell 23-2n of the first series circuit, and S2m denotes the supply circuit control signal that controls the charging mode of the supply cells 23-2n of the second series circuit. S31-S3n denotes the converter control signal of the individual converter cells connected to the supply cells. Referring to FIG. 20, the individual converter cells may be operated in an interleaved fashion. A duty cycle is substantially D=0.5 in the present embodiment. The supply cells 21-2n of the first series circuit are re-charged after an activation sequence in which each of the converter cells 31-3n coupled to the supply cells 21-23 of the first series circuit have been activated, and the supply cells 24-2n of the second series circuit are re-charged after an activation sequence in which each of the converter cells 34-3n connected to the supply cells 24-2n of the second series circuit have been activated.
According to one embodiment, the power converter circuit 1 includes m=2 second capacitors 251-25m and a series circuit with three supply cells connected in parallel with each of these m=2 second capacitor 251-25m. This results in an overall number of 6 (=m×3) supply cells. However, this is only an example. The number of second capacitors 251-25m is not restricted to m=2. More than m=2 second capacitors 251-25m can be used as well, and a series circuit with two or more supply cells may be connected in parallel with each second capacitor 251-25m.
In the embodiments explained before, each of the supply cells 3 is operated in the supply mode at least once between two subsequent charging cycles. However, this is only an example. It is also possible, e.g. when a power consumption of the load Z is very low (light load operation) to deactivate one or more converter cells between two charging cycles. To “deactivate” means that the corresponding converter cell 3i is not operated in the supply mode in the time between two subsequent charging cycles, although the corresponding capacitor 22i has been charged. The at least one converter cells 3i that is deactivated may cyclically change from charging cycle to charging cycle.
The power converter circuit explained before can be implemented with electronic switches that have different voltage blocking capabilities. The second switch 311 of the converter cell 31 connected to the uppermost supply cell 21 requires the highest voltage blocking capability, which is V2+Vn in the embodiment of FIG. 1. In general, the voltage blocking capability V31MAXi of a second switch 31i is at least:




  
    
      
        
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          ⁢
          
            
          
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            MAXi
          
        
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        (
        4
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In the supply mode, the maximum voltage across the control switch 32i of one converter cell is substantially the difference between the input voltage Vi of the converter cell 3i and the output voltage Vout. However, in the charging mode the maximum voltage across the control switches 32i is higher and is dependent on the converter cell 3i in which the control switch 32i is implemented. For example, the maximum voltage V32MAX1 across the control switch 321 of the first converter cell 3i is (V1+V2+Vn)−Vout=Vin−Vout. In general, the maximum (static) voltage V32MAX, across one control switch 32i is substantially




  
    
      
        
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          ⁢
          
            
          
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            MAXi
          
        
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The maximum voltages across the freewheeling elements 34i correspond to the output voltage Vout.
According to one embodiment, the supply circuit 2, the individual converter cells 31-3n, and, optionally the inductors 33I, are integrated in a first semiconductor chip, and the control circuit 4 is integrated in a second semiconductor chip. According to one embodiment, the control circuit 4 is implemented in a CMOS technology.
According to one embodiment, the first switches 311-31n of the individual converter cells 31-3n are implemented using several switches (transistors) having a lower voltage blocking capability. FIG. 21 shows one embodiment of a second switch 32i. This second switch 32i includes a first transistor 32i1 and at least one second transistor 32i2, 32i3, namely two second transistors 32i2, 32i3 in this embodiment. The first transistor 32i1 and the at least one second transistor 32i2, 32i3 are connected in series. The first transistor 32i1 receives the control signal S3i and switches on and off dependent on the control signal S3i. The second transistors 32i2, 32i3 are connected such that each of these transistors receives as a drive voltage the load path voltage of the first transistor 32i1 or the load path voltage of another second transistor. In the present embodiment, the load path voltage of one of the first and second transistors is the drain-source voltage V32i1, V32i2, V32i3 of the transistor, and the drive voltage is the gate-source voltage (the voltage between the gate and source terminals). In the present embodiment, a first one 32i2 of the second transistors receives as a drive voltage the load path voltage V32i1 of the first transistor, and a second one 32i3 of the second transistors receives as a drive voltage the load path voltage V32i2 of the second transistor 32i2.
The operation state of the first transistor 32i1 governs the operation state of the first switch 32i. That is, the first switch 32i is switched on when the first transistor 32i1 is switched on, and the first switch 32i is switched off, when the first transistor 32i1 switched off. The second transistors are configured such that transistor 32i2 switches off when the first transistor 32i1 switches off and a load path voltage of the first transistor 32i1 increases. When the transistor 32i2 switches off, the load path voltage of this transistor increases, so that transistor 32i3 is switched off. According to one embodiment, the first transistor 32i1 is implemented as an enhancement (normally-on) MOSFET and the second transistors 32i2, 32i3 are implemented as depletion (normally-on) MOSFETs or as Junction FETs (JFETs).
The overall voltage blocking capability of the second switch 32i illustrated in FIG. 21 is dependent on the voltage blocking capabilities of the individual transistors 32i1-32i3 connected in series and on the number of second transistors 32i2-32i3 connected in series with the first transistor 32i1, wherein the voltage blocking capability increases as the number of transistors 32i2-32i3 connected in series increases. Thus, by simply varying the number of second transistors 32i1-32i3 connected in series with the first transistor 32i1 first switches 32i with different voltage blocking capabilities can be realized.
Referring to the explanation above, in a power converter circuit as shown in FIG. 2, after the charging cycle the first input nodes 131-13n of the individual converter cells 31-3n have different electrical potentials. For example, an electrical potential P131 at the first input node 131 of the upper most converter cell 31 is Vin relative to the first ground potential (the electrical potential at the second input node 12). The electrical potential at the first input node 132 of the first converter cell 32 is Vin-V1, and so on. In general, in a power converter circuit with n converter stages in which the capacitances of the first capacitors 221-22n are selected such that the input voltages of the individual converter cells 31-3n are substantially equal (Vin/n), the electrical potential P13i at the first input node 13i of an arbitrary converter cell 3i is given as:




  
    
      
        
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If, for example, the converter cells 31-3n are implemented with a buck converter topology as shown in FIG. 6 and the converter cells 31-3n, after one charging cycle, are activated in an order as shown in FIG. 12, voltages across the control switches (32i in FIG. 6) of the individual converter cells 31-3n are different right before switching on the respective control switch. If, for example, the first ground potential (the electrical potential at the second input node 12) equals the second ground potential (the electrical potential at the second output node 18), the voltage across the control switch in the first converter cell 31 is

P131−Vout=Vin−Vout  (7a),

the voltage across the control switch of the second converter cell 32 is





  
    
      
        
          
            
              
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and so on. The voltage across the control switch 32i right before switching on the control switch 32i affects the switching losses, whereas the switching losses are the higher, the higher the voltage across the control switch 32i is before switching on the control switch. Furthermore, the higher the voltage across the control switch 32i is before switching on, the more energy is required to obtain the zero voltage switching (ZVS) explained above.

Thus, it may be desirable to operate the power converter circuit such that the voltages across the control switches in the individual converter cells 31-3n are as low as possible right before switching on the respective control switch.
FIG. 22 illustrates one embodiment of a drive scheme that provides for lower voltages across the control switches of at least some of the converter cells 31-3n than the drive scheme explained with reference to FIG. 12. The drive scheme shown in FIG. 22 is based on a power converter circuit of the type shown in FIG. 2, with n=6 converter cells. FIG. 22 shows timing diagrams of the supply circuit control signal S2, the ground switch control signals S311-S31n, and the electrical potentials P311-P31n at the first input nodes 131-13n of the individual converter cells 31-3n. The drive scheme shown in FIG. 22 has the effect that the voltage across the control switch of each converter cell but the lowermost converter cell 3n is 2Vin/n−Vout right before switching on the respective control switch, while the voltage across the control switch of the lowermost converter cell 3n right before switching on the control switch is Vin/n−Vout.
The timing diagrams shown in FIG. 22 begin with the charging cycle. During the charging cycle, the first capacitors (221-22n in FIG. 2) in the supply circuit 2 are charged so that the voltages across these capacitors 221-22n increase. Consequently, the electrical potentials P131-P13n at the first input nodes 131-13n of the individual converter cells 31-3n increase. Such increase is schematically shown in FIG. 22. At the end of the charging cycle, each of the electrical potentials P131-P13n at the first input nodes 131-13n is given by equation (6).
The drive scheme shown in FIG. 22 is such that before switching on the control switch of some of the converter cells 31-3n, the electrical potential P311-P31n at the first input node is reduced by switching on the ground switch of at least one other converter cell. In this specific embodiment shown in FIG. 22, the electrical potentials P311-P314 at the first input nodes of converter cells 31-34 are reduced by switching on the ground switch of at least one converter cell that is located between the respective converter cell and the second input node 12 and the second output node 18, respectively. This is explained in the following.
In the embodiment shown in FIG. 22, after the charging cycle, the converter cells are activated in the following order: 35-34-33-32-31-3n. According to one embodiment, “activating one converter cell” means switching on the ground switch (311-31n) and the control switch (32i in FIG. 6) of the respective converter cell 31-3n. Each time one of the converter cells 32-35 is activated, the electrical potential P311 at the first input node 131 of the first converter cell 31 decreases so that this electrical potential P131 has decreased to 2Vin/n right before this converter cell 31 is activated. Equivalently, the electrical potential P132 at the first input node 132 of the second converter cell 32 decreases each time one of the converter cells 33-35 is activated so that this electrical potential P132 has decreased to 2Vin/n right before this converter cell 32 is activated, and so on. The electrical potential P135 at the first input node of the fifth converter cell (the converter cell adjacent the lowermost converter cell 3n) is 2Vin/n right from after the charging cycle, and the electrical potential P13n at the first input node of the lower most converter cell 3n is Vin/n right from after the charging cycle. In each case, after a converter cell has been activated, the electrical potential at the respective first input node 13i remains at Vin/n (or lower as the respective capacitor 22i may be discharged during the activation phase) until the next charging cycle.
The decrease of the electrical potential P13i at the first input node 13i of an arbitrary converter cell 3i by switching on the ground switch of another converter cell located between the converter cell 3i and the lower most converter cell 3n is explained with reference to FIG. 23 below. FIG. 23 only shows the supply circuit 2 in a power converter circuit with n=6 converter cells, the first and second input nodes 131-13n, 141-14n and the ground switches 31i-31n of the respective converter cells. Referring to FIG. 23, each of the first switches 211-21n of the supply circuit 2 has a parasitic capacitance in parallel with the actual switch. If, for example, the first switches 211-21n are implemented as MOSFETs, these parasitic capacitances are drain-source capacitances of these MOSFETs. During the charging cycle, when the first switches are switched on, the first capacitors 221-22n form a capacitive voltage divider between the first input node 11 and the second input node 12. During the charging cycle, the parasitic capacitances of the first switches 211-21n are discharged.
After the charging cycle, the first capacitors 221-22n and the parasitic capacitances of the first switches 211-21n form a capacitive voltage divider between the first input node 11 and the second input node 12, wherein the converter cells 31-3n may take energy from the individual capacitors 221-22n and transfer the energy to the output.
Each time, the electrical potential of one circuit node in this capacitive voltage divider is pulled to ground by switching on one of the ground switches 311-315, the electrical potentials P13i of first input nodes located between this circuit node and the first input node 11 are pulled down equivalently. For example, if, after the charging cycle, the electrical potential at the second input node 145 of the converter cell 35 is pulled down to ground by switching on the ground switch 315 of this converter cell 35, the electrical potential at this circuit node 145 decreases from to zero. Consequently, the electrical potentials at the first input nodes 131, 132, 133, 134 are reduced for Vn, with Vn being Vin/n if the input voltages after the charging cycle are substantially equal. After the ground switch 315 is switched off (opened), the electrical potentials at the first input nodes 131-134 are kept on these reduced levels. When the electrical potentials at these circuit nodes 131-134 are pulled down, the voltage across the upper most first switch 211 increases.
If the first switches 211-21n are implemented as MOSFETs they include an internal diode (body diode) in parallel with the actual switch. These diodes are also shown in FIG. 23. According to one embodiment the uppermost first switch 211 is connected such that the body diode is oriented as shown in FIG. 23. In this case, the cathode faces the first input node 11 so that, after the charging cycle, the electrical potential at the first input node 131 of the uppermost (first) converter 31 can fall below the electrical potential at the first input node 11 of the power converter circuit. The other first switches 212-21n may be connected such that their body diodes are oriented opposite (back-to-back) to the orientation of the body diode in the first switch 211. That is, the cathodes of the body diodes in these switches 212-21n face the second input node 12 of the power converter circuit. Thus, these diodes prevent the capacitors 221-22n from being discharged when ground switch other than the ground switch of the associated converter cell is closed. For example, if the ground switch 311 of the first converter cell 311 switches on, the polarity of the diodes in the first switches 212-21n is such that the capacitors 222-22n cannot be discharged via these diodes and the ground switch 311.
In the embodiment shown in FIG. 22, the electrical potential at the first input node of each converter cell 31-34 other than the lowermost converter cell 3n and the converter cell directly adjacent the lowermost converter cell 3n is reduced (pulled down) before activating the respective converter cell 31-34. The converter cell directly adjacent the lowermost converter cell 3n is converter cell 35 in the embodiment shown in FIG. 23, and converter cell 3n-1 in general. Reducing the electrical potential at the first input node of one converter cell includes switching on the ground switch of another converter cell located between the respective converter cell and the lowermost converter cell 3n. In general, reducing the electrical potential at the first input node 13i of one converter cell, with i being one of 1, 2, 3, 4 in the embodiment shown in FIG. 23, includes switching on the ground switch of at least one other converter cell 3k, with k being selected from i−1 to 5 in the embodiment shown in FIG. 23.
In the embodiment shown in FIG. 22, the lowermost converter cell 3n is activated after the uppermost (first) converter cell 31 and before the charging cycle. However, this is only an example. Activation of the lower most converter cell 3n can be inserted at any position in the activation sequence. According to one embodiment (not shown) the lowermost converter cell 3n is activated directly after the charging cycle. In this case, the charging cycle of a new activation sequence directly follows the activation period of the uppermost (first) converter cell 31.
According to one embodiment, the activation periods of the individual converter cells 31-3n do not overlap. That is, only one of the plurality of converter cells 31-3n is activated at the same time. According to another embodiment, the lowermost converter cell 3n is activated together with one of the other converter cells, that is, together with one of converter cells 31-3n-1.
FIG. 24A is another representation of the drive scheme shown in FIG. 22. FIG. 24A illustrates the order in which the individual converter cells 31-3n are activated after the charging cycle. This order is 35-34-33-32-31-3n in the embodiment shown in FIG. 24A. FIG. 24B illustrates a modification of the drive scheme shown in FIGS. 22 and 24A. In this embodiment, the lowermost converter cell 3n is activated directly after the charging cycle so that a charging cycle of a successive drive sequence directly follows the activation period of the uppermost converter cell 31. In FIGS. 24A and 24B, “CH” denotes the charging cycle.
Referring to the above, activating one 3i of the plurality of converter cell may include operating the ground switch 31i and the control switch 32i of the respective converter cell 3i in a PWM fashion, whereas the duty cycle may be controlled in the way explained above in order to control one of the output voltage Vout and the output current Iout. During one activation period, there may be one PWM drive cycle (one on-period) of the control switch 32i or there may be several PWM drive cycles.
FIG. 25 illustrates another embodiment of a drive scheme. In this embodiment, after a charging cycle, there is a grounding sequence in which each of the ground switches is switched on once in order to ground the second input nodes 141-14n-1 so as to pull down the electrical potentials at the first input nodes 131-13n-1 to 2Vin/n. After this grounding sequence, there is an activation sequence in which each of the converter cells 31-3n-1 is activated once, wherein activating a converter cell includes switching on the ground switch and the control switch of the respective converter cell. In the grounding sequence, the ground switches 311-31n-1 can be switched on in an arbitrary order, that is, the order of switching on these ground switches is not restricted to the order shown in FIG. 25. Furthermore, after the grounding sequence, the converter cells 31-3n can be activated in an arbitrary order, that is, the order of activating the converter cells 31-3n is not restricted to the order shown in FIG. 25. the converter cells The lowermost converter cell 3n can be activated at any time in this sequence. In this embodiment, the electrical potential at the first input node 131-13n of each converter cell 31-3n is pulled to Vin/n in the grounding sequence.
The activation scheme shown in FIGS. 22 and 24 is such that the electrical potential at the first input node of each of the converter cells 31-34 (in general, 31-34) is pulled down to Vin/n before the respective converter cell is activated. However, this is only an example. Any reduction of the electrical potential at the first input node of one converter cell reduces the switching losses, although it may not result in the minimum losses. For example, if the driving scheme after the charging cycle is 35-31-32-33-34-3n the electrical potentials at the first input nodes of the converter cells 31, 32, 33, 34 are reduced only once, namely for Vin/n.
FIG. 26 shows one embodiment of an electronic circuit that includes two power converter circuits 11, 12. These power converter circuits 11, 12 are connected in parallel, that is, each of these power converter circuits 11, 12 receives the input voltage Vin at a respective input, and outputs of these power converter circuits 11, 12 are connected. In particular, first output nodes 171, 172 of these power converter circuits 11, 12 are connected, and second output nodes 181, 182 of these power converter circuits 11, 12 are connected. The optional output capacitor 19 is connected between the common first output node 171, 172 and the common second output node 181, 182. In the embodiment shown in FIG. 26, only a first power converter circuit 11 is shown in detail. The second power converter circuit 12 is implemented in the same way. According to one embodiment, the first power converter circuit 11 and the second power converter circuit 12 are implemented with the same number of converter cells.
FIG. 26 shows n1=3 converter cells in the first power converter circuit 11. This, however, is only an example and for the purpose of explanation. In the following, 311-3n1 denote the converter cells of the first power converter circuit 11, and 312-3n2 denote the converter cells of the second power converter circuit 12, whereas 3n1 and 3n2 are the lowermost converter cells and 311, 312 are the uppermost (first) converter cells in each of these power converter circuits 11, 12. In each of the first power converter circuit 11 and the second power converter circuit 12, the individual converter cells can be activated, after the charging cycle, in one of the ways explained with reference to FIGS. 23-25 hereinbefore.
FIG. 27 shows an activation scheme for the first power converter circuit 11 that is based on the embodiment shown in FIG. 24, and an activating scheme for the second power converter circuit 12 that is also based on the embodiment shown in FIG. 24. These activation schemes are based on a first power converter circuit 11 with n1=6 converter cells and a second power converter circuit 12 with n2=6 converter cells. These activation schemes can be performed successively. That is, first the first power converter circuit 11 may be operated in accordance with the activation scheme shown in FIG. 27 and then the second power converter circuit 12 may be operated in accordance with the activation scheme shown in FIG. 27. According to another embodiment, the activation schemes shown in FIG. 27 are interleaved.
FIGS. 28A and 28B show two embodiments of how these activation schemes may be interleaved. At each time only one converter cell of either the first power converter circuit 11 or the second power converter circuit 12 is active, wherein the order in which the converter cells in each of the power converter circuits 11, 12 are activated is in accordance with FIG. 27. In the embodiment shown in FIGS. 28A and 28B converter cells in the first power converter circuit 11 and converter cells in the second power converter circuit 12 are activated alternatingly. However, this is only an example. It is also possible two activate two or more converter cells of one power converter circuit successively and then to activate two or more converter cells of the other power converter circuit successively.
The embodiment shown in FIG. 26 is not restricted to have only two power converter circuits 11, 12 connected in parallel. According to another embodiment, three or more power converter circuits are connected in parallel. FIG. 29 shows activation schemes of three power converter circuits 11, 12, 13, each including n=3 converter cells. In the following, 311-3n1 denote the converter cells of the first power converter circuit 11, 312-3n2 denotes the converter cells of the second power converter circuit 12 and 313-3n3 denotes the converter cells of a third power converter circuit 13. The activation schemes shown in FIG. 29 can be performed successively. However, it is also possible, to interleave the individual activation schemes. FIG. 30 shows one embodiment of how these activation schemes can be interleaved.
Methods of how switching losses in one power converter circuit 1 or in several parallel connected power converter circuits 11, 12 can be reduced are explained with reference to FIGS. 22-30 herein above. FIG. 31 shows another embodiment of a method of how losses may be reduced. This method includes reducing an effective frequency at which the individual converter cells are activated after a charging cycle.
FIG. 31A shows the activation scheme under normal load (full load) conditions. In this embodiment, after a charging cycle, the individual converter cells are activated at an activation frequency fact. During each time period, T=1/fact one of the converter cells is active, that is, transfers electrical power from the respective supply cell to the output. In the active state the control switch (32i in FIGS. 6 and 9) in the respective converter cell 3i is operated in a PWM fashion, wherein there is at least one drive cycle in each activation state. As explained before, the duty cycle is dependent on the desired (average) output power of the converter cell. In the embodiment shown in FIG. 31A, only one converter cell is active at one time. The duration of the charging cycle, which is labeled as CH in FIG. 31A, may correspond to the duration of one activation period T or may be different therefrom.
The order in which the individual converter cells are activated, can be one of the orders explained hereinbefore. The specific order shown in FIG. 31A, which corresponds to the order explained with reference to FIG. 24, is only an example. The power converter circuit, in particular the converter cells, can be implemented with any of the topologies explained herein before.
According to one embodiment, the activation frequency fact is dependent on at least one of the output power and the output current Iout of the power converter circuit, wherein the activation frequency fact decreases as the at least one of the output power and the output current Iout decreases. In the embodiment shown in FIG. 31, the activation frequency fact is given be fact=1/T, wherein T is one activation period. According to one embodiment, decreasing the activation frequency involves inserting time periods T in which none of the converter cells is active. The duration of these time periods may correspond to the duration of activation periods (in which a converter cell is active). These time periods will be referred to as pause periods in the following.
FIG. 31B shows one embodiment in which one pause period is inserted after every activation period so as to reduce the activation frequency fact to fact=1/2T, which 50% as compared to the normal mode shown in FIG. 31A. Another embodiment, shown in FIG. 31C, includes inserting two pause periods after each activation period so as to reduce the activation frequency to fact=1/3T, which is 33% as compared to the normal mode.
FIG. 32 shows yet another embodiment of a drive scheme. In this embodiment, one pause period is inserted after two directly successive activation period. In this case, the effective activation frequency is fact=2/3T, which is 66% of the activation frequency in the normal mode shown in FIG. 31A.
In the description hereinbefore, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing” etc., is used with reference to the orientation of the figures being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. It should be mentioned that features explained with reference to a specific figure may be combined with features of other figures, even in those cases in which this has not explicitly been mentioned. Further, the methods of the invention may be achieved in either all software implementations, using the appropriate processor instructions, or in hybrid implementations that utilize a combination of hardware logic and software logic to achieve the same results. Such modifications to the inventive concept are intended to be covered by the appended claims.
Spatially relative terms such as “under,” “below,” “lower,” “over,” “upper” and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first,” “second” and the like, are also used to describe various elements, regions, sections, etc. and are also not intended to be limiting Like terms refer to like elements throughout the description.
As used herein, the terms “having,” “containing,” “including,” “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a,” “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.
It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

## Claims

1. A method comprising:
in a charging cycle of a power converter circuit, connecting a plurality of capacitors in series between input nodes of the power converter circuit, wherein the power converter circuit further comprises a plurality of converter cells, with each converter cell being connected to one of the plurality of capacitors;
after the charging cycle and before activating at least one of the plurality of converter cells, reducing an electrical potential at a first input node of the at least one of the plurality of converter cells by switching on a ground switch of at least one other of the plurality converter cells, with the ground switch being connected between a second input node of the other one of the plurality of converter cells and a ground node; and
activating the at least one of the plurality of converter cells to transfer electrical power from the capacitor connected to the at least one of the plurality of converter cells to an output of the power converter circuit.

2. The method of claim 1, wherein reducing an electrical potential at a first input node of the at least one of the plurality of converter cells comprises reducing the electrical potential at the first input node of each but two of the plurality of converter cells.

3. The method of claim 1, further comprising:
between two successive charging cycles, activating each of the plurality of converter cells once.

4. The method of claim 1, wherein activating the at least one of the plurality of converter cells comprises:
operating a control switch in the at least one of the plurality of converter cells in a pulse-width modulated fashion.

5. The method of claim 4, wherein the at least one of the plurality of converter cells comprises a buck-converter topology.

6. The method of claim 5, wherein the at least one converter cell comprises:
a series circuit with the control switch and an inductor connected between the first input node and a first output node of the at least one converter cell;
a ground switch connected between a second input node and a second output node of the at least one converter cell; and
a freewheeling element connected between the second output node and a circuit node common to the control switch and the inductor.

7. The method of claim 1, wherein the power converter circuit is one of at least two power converter circuits connected in parallel.

8. The method of claim 7, further comprising:
activating converter cells in the at least two power converter circuits in an interleaved fashion.

9. A power converter circuit comprising:
a plurality of capacitors configured to be connected in series between input nodes of the power converter circuit;
a plurality of converter cells, wherein each of the plurality of converter cells is connected to one of the plurality of capacitors;
a control circuit configured
in a charging cycle of the power converter circuit, to connect the plurality of capacitors in series between the input nodes of the power converter circuit,
after the charging cycle and before activating at least one of the plurality of converter cells, to reduce an electrical potential at a first input node of at least one of the plurality of converter cells by switching on a ground switch of at least one other of the plurality converter cells, with the ground switch being connected between a second input node of the other one of the plurality of converter cells and a ground node, and
to activate the at least one of the plurality of converter cells to transfer electrical power from the capacitor connected thereto to an output of the power converter circuit.

10. The power converter circuit of claim 9, wherein the control circuit is configured to reduce the electrical potential at the first input node of each but two of the plurality of converter cells.

11. The power converter circuit of claim 9, wherein the control circuit is configured, between two successive charging cycles, to activate each of the plurality of converter cells once.

12. The power converter circuit of claim 9, wherein the control circuit is configured to activate the at least one of the plurality of converter cells by operating a control switch in the at least one of the plurality of converter cells in a pulse-width modulated fashion.

13. The power converter circuit of claim 12, wherein the at least one of the plurality of converter cells comprises a buck-converter topology.

14. The power converter circuit of claim 13, wherein the at least one converter cell comprises:
a series circuit with the control switch and an inductor connected between the first input node and a first output node of the at least one converter cell;
a ground switch connected between a second input node and a second output node of the at least one converter cell; and
a freewheeling element connected between the second output node and a circuit node common to the control switch and the inductor.