Multilevel converter and a control method for operating a multilevel converter
View Patent ↗A converter includes an active stage for converting an AC input voltage at an AC input into an intermediate DC voltage, and a DC/DC converter for transforming the intermediate DC voltage into an output DC voltage at a DC output. The DC/DC converter has a resonant transformer formed by a resonant circuit and a transformer. The converter also includes control unit configured to actively operate the active stage only based on an output DC voltage of the DC/DC converter, an input voltage, and an input current of the converter, and to operate the DC/DC converter in an open loop mode. A method for operating such a converter is also provided.
1. A converter comprising:
an active stage for converting an AC input voltage at an AC input into an intermediate DC voltage;
a DC/DC converter for transforming the intermediate DC voltage into an output DC voltage at a DC output, wherein the DC/DC converter includes a resonant transformer formed by a resonant circuit and a transformer; and
a control unit configured to
actively operate the active stage only based on an output DC voltage of the DC/DC converter, an input voltage of the DC/DC converter, and an input current of the active stage, and
operate the DC/DC converter in an open loop mode.
2. The converter according to claim 1 , wherein the control unit is configured to operate the DC/DC converter in the open loop mode by switching the active stage with a switching frequency which corresponds to a resonance frequency of the resonant transformer or below.
3. The converter according to claim 1 , wherein the DC/DC converter includes a first switching unit and a second switching unit each having switching elements each respectively paralleled by freewheeling diodes,
wherein the resonant transformer is arranged between the first switching unit and the second switching unit, and
wherein the control unit is configured to operate one of the switching units with a duty cycle of about 50% and at the switching frequency.
4. The converter according to claim 3 , wherein the control unit is configured to one of (i) not drive the respective other of the switching units so that freewheeling diodes act as a passive rectifier, and (ii) operate the respective other one of the switching units to act as an active rectifier.
5. The converter according to claim 1 , wherein the control unit has a control function which feedback controls the DC output voltage and the input current by taking into account the DC output voltage, the input voltage of the active stage, and the input current of the active stage.
6. The converter according to claim 5 , wherein the control function is a cascaded control function.
7. The converter according to claim 5 , wherein the active stage has an H-bridge circuit, and
wherein the H-bridge circuit is operated according to a modulation index which is a result of the control function.
8. The converter according to claim 1 , wherein the active stage has a serially applied input inductor.
9. The converter according to claim 1 , wherein the resonant transformer is formed by a resonant circuit and a transformer, and
wherein the resonant circuit includes a resonant inductor, a resonant capacitor and a parallel inductor.
10. An arrangement comprising a plurality of converters according to claim 1 ,
wherein respective AC inputs of respective active stages of the plurality of converters are serially connected, and respective DC outputs of the DC/DC converter of the plurality of converters are connected in parallel,
wherein the control unit is configured to actively operate the active stages only based on a common output DC voltage of the paralleled DC/DC converters, the input voltage at an input of the arrangement and the input current through the arrangement .
11. The arrangement according to claim 10 , comprising:
a common input inductor in series with the serially connected active stages.
12. A method for operating a converter, wherein the converter includes:
an active stage for converting an AC input voltage at an AC input into an intermediate DC voltage;
a DC/DC converter for transforming an intermediate DC voltage into an output DC voltage at an DC output, the DC/DC converter having a resonant transformer formed by a resonant circuit and a transformer, and
wherein the method comprises:
actively operating the active stage only based on an output DC voltage of the DC/DC converter, an input voltage of the DC/DC converter, and an input current of the active stage; and
operating the DC/DC converter in an open loop mode.
13. The converter according to claim 2 , wherein the DC/DC converter includes a first switching unit and a second switching unit each having switching elements each respectively paralleled by freewheeling diodes,
wherein the resonant transformer is arranged between the first switching unit and the second switching unit, and
wherein the control unit is configured to operate one of the switching units with a duty cycle of about 50% and at the switching frequency.
14. The converter according to claim 13 , wherein the control unit is configured to one of (i) not drive the respective other of the switching units so that freewheeling diodes act as a passive rectifier, and (ii) operate the respective other one of the switching units to act as an active rectifier.
15. The converter according to claim 14 , wherein the control unit has a control function which feedback controls the DC output voltage and the input current by taking into account the DC output voltage, the input voltage of the active stage, and the input current of the active stage.
16. The converter according to claim 15 , wherein the control function is a cascaded control function.
17. The converter according to claim 16 , wherein the active stage has an H-bridge circuit, and
wherein the H-bridge circuit is operated according to a modulation index which is a result of the control function.
18. The converter according to claim 13 , wherein the active stage has a serially applied input inductor.
19. An arrangement comprising a plurality of converters according to claim 13 ,
wherein respective AC inputs of respective active stages of the plurality of converters are serially connected, and respective DC outputs of the DC/DC converter of the plurality of converters are connected in parallel,
wherein the control unit is configured to actively operate the active stages only based on a common output DC voltage of the paralleled DC/DC converters, the input voltage at an input of the arrangement and the input current through the arrangement .
20. The arrangement according to claim 19 , comprising:
a common input inductor in series with the serially connected active stages.