EP3381118A1

EP3381118A1 - Power converter

Info

Publication number: EP3381118A1
Application number: EP16804880.9A
Authority: EP
Inventors: Emre GURPINAR; Dipankar DE; Alberto Castellazzi
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2015-11-27
Filing date: 2016-11-25
Publication date: 2018-10-03
Also published as: WO2017089821A1; GB201520961D0

Abstract

A power converter for converting direct current (DC) to alternating current (AC), comprising: a first input terminal (1) and a second input terminal (2); a first output terminal (3) and a second (4) output terminal; a first subcircuit (5) connecting the first input terminal (1) and the second output terminal (4) to the first output terminal (3), and a second subcircuit (6) connecting the second input terminal (2) and the second output terminal (4) to the first output terminal (3);a first capacitance (C1) connecting the first input terminal to the second output terminal;a second capacitance (C2) connecting the second input terminal (2) to the second output terminal (4); and a controller (10); in which each of the first (5) and the second (6) subcircuits comprises: a first current path (5a, 5b) joining the first (1) or second (2) input terminal respectively to the first output terminal (3) through a junction (7a, 7b); a second current path (6a, 6b) joining the second output terminal (4) to the first output terminal (4) through a junction (8a, 8b); a floating capacitor (C3, C4) joining the junctions (7a, 7b, 8a, 8b) of the first (5a, 5b) and second (6a, 6b) current paths; the first current path (5a, 5b) comprising first (S1, S2) and second (S5, S6) controllable switching elements, the first (S1, S2) and second (S5, S6) controllable switching elements being provided in series between the respective first (1) or second (2) input terminal to the first output terminal (3), either side of the junction (7a, 7b) of the first current path (5a, 5b); the second current path (6a, 6b) comprising a third controllable switching element (S3, S4) and first (D7, D8) and second (D9, D10) switching elements provided in series between the second output terminal (4) and the first output terminal (3), with the first switching element (D7, D8) being connected to the second output terminal (4), and the second switching element (D9, D10) and the third controllable switching element (S3, S4) being connected to the first output terminal (3) on the other side of the junction (8a, 8b) of the second current path (6a, 6b) to the first switching element (D7, D8); and in which the controller (10) is arranged so as to control the operation of each controllable switching element (S1, S2, S3, S4, S5,S6).

Description

POWER CONVERTER

This invention relates to a power converter, and to a method of using the same . Companies manufacturing power converters for using in photovoltaic (PV) installations, such as SMA, Sunways and Conergy introduced high efficient power converters and each of them offers a different solution. Most popular power converter topologies are H5 (SMA, Germany), HERIC (Sunways, Germany) and Conergy NPC (Conergy, Germany) . Simulation results of these converters show that although these converters have high efficiency, the quality of output electrical waveform is not good enough and they require large filtering systems at the output in order to meet the grid requirements. The H6, HERIC and Conergy NPC topologies can be seen in Figures la to lc of the accompanying drawings, respectively. These converters can produce three-level (+E, 0 and -E) output signal waveform with high efficiency. The number of levels at output signal defines the quality of the alternating signal. Higher number of levels yields to better quality and reduced filter size.

Conventional multilevel converters can achieve better output electrical waveform quality by increasing output levels (5, 7, 9 levels etc.), but these systems are too complex for low power applications in terms of component count and generally used in high power propulsion or electrical drive systems, where the output signal quality is crucial. Even for those systems, companies like ABB, GE and Siemens are doing research in order to reduce complexity and improve efficiency by looking at different power converter topologies. Conventional NPC five-level topology can be seen in Fig 2 (a) and patented five-level ABB and FUJI topology can be seen in Fig 2 (b) and (c) respectively. ABB's topology uses same amount of active switches as conventional NPC but eliminates the use of clamp diodes and provides better loss distribution in 8 switches. It is currently used in ABB's industrial drives. However, number of conducting switches is always three at any voltage stage and although it performs better than conventional five-level converter in Fig 2 (a), the number of conducting switches can be reduced.

As such, it is desirable to develop a power converter for converting direct current to alternating current that is efficient and/or that requires a small filter. Such a power converter would be useful in industrial drives, high power propulsion systems, or any other suitable application. According to a first aspect of the invention, there is provided a power converter for converting direct current (DC) to alternating current (AC), comprising:

• a first input terminal and a second input terminal;

• a first output terminal and a second output terminal;

• a first subcircuit connecting the first input terminal and the second output terminal to the first output terminal, and a second subcircuit connecting the second input terminal and the second output terminal to the first output terminal;

• a first capacitance connecting the first input terminal to the second output terminal;

• a second capacitance connecting the second input terminal to the second output terminal; and

• a controller;

in which each of the first and the second subcircuits comprises:

• a first current path joining the first or second input terminal respectively to the first output terminal through a junction;

• a second current path joining the second output terminal to the first output terminal through a junction;

• a floating capacitor joining the junctions of the first and second current paths;

• the first current path comprising first and second controllable switching elements, the first and second controllable switching elements being provided in series between the respective first or second input terminal to the first output terminal, either side of the junction of the first current path;

• the second current path comprising a third controllable switching element and first and second switching elements provided in series between the second output terminal and the first output terminal, with the first switching element being connected to the second output terminal, and the second switching element and the third controllable switching element being connected to the first output terminal on the other side of the junction of the second current path to the first switching element;

and in which the controller is arranged so as to control the operation of each controllable switching element. As such, this converter allows for multilevel DC to AC conversion, with fewer controllable switching elements used. The inventors have appreciated that such a converter can achieve higher efficiency than similar converters of the prior art or require a smaller inductive filter than such prior art converters.

The first and second switching elements of each of the first and second subcircuits may each comprise a diode and typically no controllable switching element, which will permit current to flow only in a single direction; the direction for the first and second switching elements of the first subcircuit may be from the second output terminal to the first output terminal, whereas the direction for the first and second switching elements of the second subcircuit may be from the first output terminal to the second output terminal. Such a circuit is useful, as it can potentially only comprise six controllable switching elements, the other switching elements being simple diodes; however, it is best suited to applications where the power factor of an AC load with which the converter is used is unity.

Alternatively, the first switching element of each of the first and second subcircuits may comprise a controllable switching element and the second switching element of the first and second subcircuits may comprise a diode and typically no controllable switching element, which will permit current to flow only in a single direction; the direction for the second switching element of the first subcircuit may be from the second output terminal to the first output terminal, whereas the direction for the second switching element of the second subcircuit may be from the first output terminal to the second output terminal. Such a circuit is useful, as it can potentially only comprise eight controllable switching elements, the other switching elements being simple diodes, whilst still being able to cope with limited reactive loads, such as those with a power factor of greater than 0.7.

In another alternative, the first and second switching elements of the first and second subcircuits each comprise a controllable switching element. This can allow the converter to work with a load with any power factor.

The converter may comprise an inductive filter, typically comprising an inductor, at the first or second output terminals. The present invention allows the size of this filter to be reduced with respect to the prior art. The controller may be arranged to as to control the controllable switching elements so that, when a Direct Current (DC) voltage is applied across the first and second input terminals, a multilevel Alternating Current (AC) voltage is produced across the first and second output terminals. Typically, the multilevel AC voltage will have at least five levels, which may comprise first, second, third, fourth and fifth levels in order by voltage.

Typically, each of the controllable switching elements will have an off state in which it blocks current passing through it (in at least one direction) and an on state when it permits the flow of current, with the controller being able to switch each controllable switching element between its on and off states. Each of the levels may correspond to at least one distinct combination of on and off states of the controllable switching elements.

However, for the second and fourth levels, there may be at least two distinct combinations for each level; for each level there may be a state which corresponds to one of the floating capacitors charging and a state which corresponds to that floating capacitor discharging. The controller may alternate between the combinations for the same level in order to keep the voltage at the second or fourth level respectively, typically whilst maintaining a voltage of the floating capacitors around a level. The level may be a quarter of the DC voltage.

Typically, the levels would be evenly spaced; the third level may be zero voltage, and the first and fifth voltages may each be half the DC voltage, but in opposing senses. In the latter case, the first and second capacitances may be equal.

The combinations for each of the levels may be that indicated in the table below:

Subcircuit First Second

Controllable First Second Third First Second Third switch:

First level On On Off Off Off Off

Second Off On Off Off Off Off level

Second On Off On Off Off Off Subcircuit First Second

Controllable First Second Third First Second Third switch:

level

Third level Off Off On Off Off Off

Third level Off Off Off Off Off On

Fourth level Off Off Off On Off On

Fourth level Off Off Off Off On Off

Fifth level Off Off Off On On Off

Each of the capacitances will typically comprise a capacitor.

According to a second aspect of the invention, there is provided a method of converting a Direct Current (DC) voltage to an Alternating Current (AC) voltage using the converter of the first aspect of the invention, the method comprising applying the DC voltage across the first and second input, and the controller operating so as to cause the AC voltage to be provided across the first and second output terminals. The AC voltage may be multilevel AC, in that it comprises a plurality of discrete levels. Typically, the AC voltage will have at least five levels, which may comprise first, second, third, fourth and fifth levels in order by voltage. The method may comprise cycling from first to fifth to first levels, sequentially in order. Typically, each of the controllable switching elements will have an off state in which it blocks current passing through it (in at least one direction) and an on state when it permits the flow of current, with the controller being able to switch each controllable switching element between its on and off states. Each of the levels may correspond to at least one distinct combination of on and off states of the controllable switching elements.

However, for the second and fourth levels, there may be at least two distinct combinations for each level; for each level there may be a state which corresponds to one of the floating capacitors charging and a state which corresponds to that floating capacitor discharging. The method may comprise alternating between the combinations for the same level in order to keep the voltage at the second or fourth level respectively.

There now follows, by way of example only, description of embodiments of the invention, described with reference to the accompanying drawings, in which:

Figures la to lc show prior art three level DC to AC converters;

Figures 2a to 2c show prior art five level DC to AC converters;

Figure 3 shows a circuit diagram of a DC to AC converter in accordance with a first embodiment of the invention;

Figure 4 shows the current flow through the circuit for various different switching combinations; Figure 5 shows a circuit diagram of a DC to AC converter in accordance with a second embodiment of the invention; Figure 6 shows a circuit diagram of a DC to AC converter in accordance with a third embodiment of the invention;

Figures 7a to 7h each show graphs of the control signals applied to the switches of the circuit of Figure 6 in order to commutate between states;

Figure 8 shows a circuit diagram of a DC to AC converter in accordance with a further embodiment of the invention;

Figures 9 and 10 show graphs of current and voltage output by the circuit of Figure 8;

Figures 11 and 12 show graphs of the voltage across two of the capacitors of the circuit of Figure 8; Figure 13 shows graphs of simulated power against efficiency for the circuit of Figure 8 as compared to a prior art circuit; and

Figure 14 shows a graph of experimental results of the efficiency of a circuit in accordance with Figure 8.

Figure 3 of the accompanying drawings shows a power converter in accordance with a first embodiment of the invention, arranged to convert a direct current (DC) input to an alternating current (AC output) . The circuit has first 1 and second 2 input terminals across which a DC voltage is applied and first 3 and second 4 output terminals across which an AC voltage is generated.

The converter comprises two DC link capacitors C I , C2, each connected between one of the input terminals 1 , 2 and the second output terminal 4. Furthermore, first 5 and second 6 subcircuits each connect one of the input terminals 1 , 2 and the second output terminal 4 to the first output terminal 3. Each subcircuit comprises a first current path 5a, 5b connecting the respective input terminal 1 , 2 to the first output terminal 3, and a second current path 6a, 6b, connect the second output terminal 4 to the first output terminal 3.

The first current paths 5a, 5b, each comprise a first controllable switching element S I , S2 connecting the respect input terminal 1 , 2 to a junction 7a, 7b. From the junctions 7a, 7b, there is a further, second controllable switching element S5, S6, which connects the junction to the first output terminal 3.

The second current paths 6a, 6b, each comprise a first diode D7, D8 (still a type of switching element, albeit not one that is externally controllable) connecting the second output terminal 4 to a junction 8a, 8b. A second diode D9, D 10 connects the junction 8a, 8b to a third controllable switching element S3, S4, which in turn is connected to the first output terminal 3. It is to be noted that the diodes D7, D9 of the first subcircuit allow current to pass from the second output terminal 4 to the first output terminal 3, whereas the diodes D8, D 10 of the second subcircuit allow current to flow in the opposite direction. A floating capacitor C3, C4, is connected between the junctions of the first current path (7a, 7b) and the second current path (8a, 8b), respectively in each of the first and second subcircuits 5, 6.

An inductive filter L I is provided at the first output terminal 3. The load (shown as V_ac) is connected between the filter L I and the second output terminal 4.

A controller 10 controls the operation of the controllable switches.

This topology requires 6 active switches (S 1 -S6) and 4 additional diodes (D 1 -D4), two main DC link capacitors (C I and C2) and two floating (C3 and C4) capacitors in order to generate five-level output signal (+2E; +E; 0; -E; -2E).

Input DC link voltage (4E) is shared across C I and C2 (2E each) DC link capacitors and the voltage level of C3 and C4 floating capacitors are kept constant at E by using the redundant (E_c, E_d, -E_c, -E_d) voltage states of the converter (so E_c=E_d) . The relation between active states, conducting active switches and floating capacitor status is as follows:

These states are depicted in Figure 4 of the accompanying drawings.

The converter supplies five different output signal states by turning on and off relevant active switches and redundant output voltage levels are used to keep the charge level of the floating capacitors C4, C5 constant.

In comparison to a conventional neutral point clamped (NPC) circuit as shown in Fig 2, this embodiment uses 6 active switches instead of 8 and uses 4 capacitors in total instead of 4 while maintaining the output signal quality and improving the efficiency of the converter by reducing number of conducting switches at any output state. In comparison to five level-NPC, this topology uses 6 active switches, 4 diodes and 2 floating capacitors instead of 8 active switches and 1 floating capacitor. For low power applications with wide band gap (WBG) power devices, due to cost of WBG active power devices, number of active switches is a crucial parameter that determines the desirability of the idea and it can be seen that this topology achieve same output signal quality in comparison to conventional NPC and ABB 5L-Active NPC (ANPC) while reducing the number of active switches from 8 to 6. WBG devices will provide reduction in input filter size and increase in power density of the converter. Usage of WBG in this topology will eliminate the drawback of using addition floating capacitor. In comparison to H6 and HERIC, addition of 2 floating capacitors (and 4 diodes in comparison to HERIC and 2 in comparison to HERIC) leads to better output signal up to 2 times that reduces the need for filtering while maintaining the number of active switches. Whilst the embodiment of Figure 3 can be used with a load with a power factor of unity, further embodiments can be used where the power factor is not unity. Replacing D7 and D8 diodes with active switches provides handling reactive power in a limited range. The modified topology can be seen in Figure 5 of the accompanying drawings. In order to avoid that limitation, the remaining D8 and D9 diodes can also be replaced with active switches and can be seen in Figure 6 of the accompanying drawings. The topology in Figure 6 has unlimited range of power factor capability and can be suitable for industrial drives and high power conversion systems.

The proposed topology in Fig 6 with full reactive power capability has been experimentally validated. In this setup, the converter is designed for 2.5kW output power with up to 600V DC input and 230Vrms output voltage . For the switches, 650V SiC MOSFETs from ROHM (commercially available) are used in order to evaluate the performance at high switching frequencies.

A field programmable gate array (FPGA) development board is used to generate open loop switch signals and also to control the floating capacitor charge levels. The FPGA control system provides high performance at high switching frequencies (up to lMHz) and a reliable control solution for testing wide band-gap devices. Additional protection and signal conditioning boards are developed for capacitor voltage control and safe operation.

The commutation scheme used in this embodiment is shown in Figures 7a to 7h of the accompanying drawings. In each case, the Figure is labelled with the transition to which it relates, with each level being labelled as it is in the table above referring to the embodiment of Figure 4. In each case, the "control signal" represents a timing signal sent by the FPGA control system, which can be high or low. For each transition, the appropriate figure shows the switches of Figure 6 which are involved, and a control signal that is applied to each switch (with a "high" value causing the switch to conduct, and a "low" value causing the switch not to conduct) . The bottom two traces in each of these figures shows the output of the circuit; in each case, the top trace is for unity power factor when voltage and current flow are of the same sense; the bottom trace is shown for use with reactive loads, when the voltage and current flow are in different senses. As can be seen, there are deadtimes and overlaps between switches in order to prevent short circuit in the DC link due to non-ideal switching of power transistors (power transistors require certain amount of time to switch from on state to off state vice versa) . Taking Figure 7a as an example, in the first instant, the control signal changes from 1 to 0 in the first instant that sends the command to the converter to switch from 2E to Ec state. At this instant, S9 switch turns on while S I and S5 are still on and S3 is off. After certain deadtime, S5 switches off and output current commutates from S I , S5 to S9 and S3. When the commutation is completed, S3 is switched on (after certain deadtime, same duration as the first one) and the converter operates at Ec state.

The prototype is tested up to 500V DC link voltage, 1.6kW output power at 16, 32, 55 and 65kHz switching frequencies successfully. Thus, it can be seen that such a power converter can be used to provide a high switching frequency, and require less filtering, for the same efficiency as a prior art converter, or can be used to provide improved efficiency at the same switching frequency (or some combination thereof).

The topology in Figures 3 and 5 can be easily applied to photovoltaic (PV) applications due to its simplicity, low component count, high efficiency and reduced output filter need. This solution with WBG power devices offers reduced filter size, higher efficiency and higher power density without increasing the system complexity. It also ensures maximum user safety without the need of electrical isolation from the grid. A further embodiment of the circuit which has been experimentally tested is shown in Figure 8 of the accompanying drawings. The embodiment has been tested up to lkV DC link voltage and 14kW input power

Series connected devices are used for S5 , S6, S9, S4 in order to achieve same voltage rating for all devices and turn-off snubbers are implemented in order to provide voltage balancing and sharing among all devices. DC link film capacitors for C I and C2, and floating capacitors C3 and C4 are directly mounted on the power plane PCB .

C I and C2 capacitors are formed of film and electrolytic capacitors (0.9mF in total for C 1/C2) and C3 and C4 are formed of parallel connected film capacitors for adequate high frequency performance (200uF each). Output filter inductor L I is 1.5mH. Converter switching frequency is 10kHz and tested up to 14kW continuous power up to IkV DC link voltage. 6 Layer 4oz printed circuit board (PCB) is used for power plane construction in order to provide low-inductance commutation paths between planes and integration of 16 discrete insulated gate bipolar transistors (IGBTs). Converter is controlled with a digital signal processing/field programmable gate array (DSP+FPGA) platform for open loop and closed loop tests.

The output voltage and current waveforms of the converter at 2kW and 12kW output power are presented in Figures 9 and 10 of the accompanying drawings respectively. This embodiment can successfully operate up to 50A (root mean square/rms) (75Arms) output current with 240V (rms) output voltage . The traces are as follows:

Trace 20 - output voltage at 2kW output

Trace 21 - output current at 2kW output

Trace 22 - output voltage at 12 kW output

Trace 23 - output voltage at 12 kW output.

The floating capacitors C3 and C4 are controlled by using a hysteresis controller in the DSP. For this controller, voltage across DC link capacitors C I and C2, and floating capacitors C3 and C4 are measured by using high voltage DC sensors. According to measured floating capacitor voltage, upper and lower limits of the hysteresis band and direction of output current, adequate output voltage state is selected in order to charge or discharge the capacitors. The voltage across C I and C3 capacitors at IkV input voltage, 2kW and 8kW output power are presented in Figures 1 1 and 12 respectively, with the voltage across C I being the upper trace on each graph and the voltage across C3 being the bottom trace. The results show that proposed control topology for balancing floating capacitors work successfully and at higher loads, the voltage variation across the floating capacitor increases, understood to be due to processing time of DSP and the accuracy of voltage sensors.

Figure 13 of the accompanying drawings shows the simulated efficiency of the circuit of Figure 8 (EDA5) against a prior art circuit (ABB 5L-ANPC) in accordance with Figure 2b of the accompanying drawings, at various power levels. As can be seen, the proposed circuit is more efficient than the prior art. Figure 14 of the accompanying drawings shows some experimental results using the circuit of Figure 8, which shows that the circuit can operate at a substantial efficiency. As such, the power converter proposed in the above embodiments can be useful in the fields of photovoltaics, industrial drives, high power propulsion systems, or any other suitable application.

Claims

1. A power converter for converting direct current (DC) to alternating current (AC), comprising:

• a first input terminal and a second input terminal;

• a first output terminal and a second output terminal;

• a controller;

in which each of the first and the second subcircuits comprises:

and in which the controller is arranged so as to control the operation of each controllable switching element.

2. The power converter of claim, 1 , in which the first and second switching elements of each of the first and second subcircuits each comprise a diode which will permit current to flow only in a single direction

3. The power converter of claim 2 in which the direction for the first and second switching elements of the first subcircuit is from the second output terminal to the first output terminal, whereas the direction for the first and second switching elements of the second subcircuit is from the first output terminal to the second output terminal.

4. The power converter of claim 2 or claim 3, in which the first and second switching elements of each of the first and second subcircuits comprise no controllable switching element.

5. The power converter of claim 1 , in which the first switching element of each of the first and second subcircuits comprises a controllable switching element and the second switching element of the first and second subcircuits comprises a diode which will permit current to flow only in a single direction.

6. The power converter of claim 5, in which the direction for the second switching element of the first subcircuit is from the second output terminal to the first output terminal, whereas the direction for the second switching element of the second subcircuit is from the first output terminal to the second output terminal.

7. The power converter of claim 5 or claim 6, in which the second switching element of the first and second subcircuits comprises no controllable switching element.

8. The power converter of claim 1 , in which the first and second switching elements of the first and second subcircuits each comprise a controllable switching element.

9. The power converter of any preceding claim, comprising an inductive filter at the first or second output terminals.

10. The power converter of any preceding claim, in which the controller is arranged to as to control the controllable switching elements so that, when a Direct Current (DC) voltage is applied across the first and second input terminals, a multilevel Alternating Current (AC) voltage is produced across the first and second output terminals.

1 1. The power converter of claim 10, in which the multilevel AC voltage has at least five levels, comprising first, second, third, fourth and fifth levels in order by voltage.

12. The power converter of claim 1 1 , in which each of the controllable switching elements has an off state in which it blocks current passing through it in at least one direction and an on state when it permits the flow of current, with the controller being able to switch each controllable switching element between its on and off states, in which each of the levels may correspond to at least one distinct combination of on and off states of the controllable switching elements and in which for the second and fourth levels, there may be at least two distinct combinations for each level; for each level there may be a state which corresponds to one of the floating capacitors charging and a state which corresponds to that floating capacitor discharging.

13. The power converter of claim 12, in which the controller alternates between the combinations for the same level in order to keep the voltage at the second or fourth level respectively, typically whilst maintaining a voltage of the floating capacitors around a level.

14. The power converter of any of claims 1 1 to 13, in which the levels are evenly spaced.

15. The power converter of claim 14, in which the third level is zero voltage, and the first and fifth voltages are each half the DC voltage, but in opposing senses.

16. The power converter of any of claims 1 1 to 15, in which the combinations for each of the levels are indicated in the table below: Subcircuit First Second

Controllable First Second Third First Second Third switch:

First level On On Off Off Off Off

Second Off On Off Off Off Off level

Second On Off On Off Off Off level

Third level Off Off On Off Off Off

Third level Off Off Off Off Off On

Fourth level Off Off Off On Off On

Fourth level Off Off Off Off On Off

Fifth level Off Off Off On On Off

17. A method of converting a Direct Current (DC) voltage to an Alternating Current (AC) voltage using a converter in accordance with any preceding claim, the method comprising applying the DC voltage across the first and second input, and the controller operating so as to cause the AC voltage to be provided across the first and second output terminals.

18. The method of claim 17, in which the AC voltage is multilevel AC, in that it comprises a plurality of discrete levels.

19. The method of claim 18, in which the AC voltage has at least five levels, comprising first, second, third, fourth and fifth levels in order by voltage .

20. The method of claim 19, comprising cycling from first to fifth to first levels, sequentially in order.

21. The method of claim 19 or claim 20, in which each of the controllable switching elements has an off state in which it blocks current passing through it in at least one direction and an on state when it permits the flow of current, with the controller being able to switch each controllable switching element between its on and off states and each of the levels corresponds to at least one distinct combination of on and off states of the controllable switching elements, in which for the second and fourth levels, there are at least two distinct combinations for each level, including a state which corresponds to one of the floating capacitors charging and a state which corresponds to that floating capacitor discharging.

22. The method of claim 21 , comprising alternating between the combinations for the same level in order to keep the voltage at the second or fourth level respectively.

23. The method of any of claims 19 to 22, in which the levels are evenly spaced, with the third level being zero voltage, and first and fifth voltages each being half the DC voltage, but in opposing senses.

24. The method of claim 21 , 22 or claim 23 as dependent from claim 21 , in which the combinations for each of the levels are as indicated in the table below:

Subcircuit First Second

Controllable First Second Third First Second Third switch:

First level On On Off Off Off Off

Second Off On Off Off Off Off level

Second On Off On Off Off Off level

Third level Off Off On Off Off Off

Third level Off Off Off Off Off On

Fourth level Off Off Off On Off On

Fourth level Off Off Off Off On Off

Fifth level Off Off Off On On Off