A Compact Bi-Directional Power-Conversion System Scheme with Extended Soft-Switching Range

1. A Compact Bi-Directional Power-Conversion System Scheme with Extended Soft-Switching Range Sudip. K. Mazumder, Liang Jia Laboratory of Energy and Switching-Electronics System Department of Electrical and Computer Engineering University of Illinois, Chicago IEEE Electric Ship Technologies Symposium (ESTS’09) Baltimore, Maryland April, 2009 Funding Agencies: National Science Foundation (NSF)

2. Overview of the Presentation • Introduction and Background • Principles of the Proposed ZCS Scheme • Operation modes of the ZCS scheme • Unique features of the proposed ZCS scheme • General Methodology and Optimization Method for the Proposed ZCS Scheme • General requirements for the ZCS scheme with nonzero pulsating Dc output and the optimal ZCS range • Particular examples for ZCS control condition • Implementation of the proposed ZCS scheme • Simulated and Experimental Validations for the Proposed ZCS Scheme • Key Conclusions

3. Introduction and Background • The need for realizing power-dense power-conversion modules that support bi-directional power flow is an important factor for Navy and Defense from the standpoints of reduced-footprint-space, weight, labor cost, and mobility. • The existing megawatt class converters usually operate at low switching frequency limited to about 1.2 kHz due to the limited turn on/off performances of the high-voltage power devices, resulting in high efficiency but, also yield bulky and costly magnetic materials and filters. • New problems related to the high-frequency conversion schemes occur, such as, low efficiency, high stress and high EMI, because of the high frequency hard switching. One proper solution for these problems is the soft switching technique.

4. Topology Overview for High-frequency-link Converter • Bridge I: Three single-phase full-bridge dc-ac converter • Bridge II: Three-phase active rectifier Output voltage of the rectifier can be expressed as: • Bridge III: Pulsating Dc/three phase Ac converter

5. Principles of the Proposed ZCS Scheme • Implement the pulse-width and pulse-placement modulation (PWM & PPM) scheme to make the switching control more flexible • Take advantage of the features of three-phase rectifier to create the zero current condition for Bridge I and zero voltage condition for the Bridge II • Generate the voltage overlaps between lead and lag phases (considering the leakage inductance of the HFL transformers) • Regulate the average value of the rectifier output voltage equal to the six-pulsed modulated reference defined as: (u(t), v(t) and w(t) are the three-phase sinusoidal reference signals, M represents the modulation index)

6. Modes of Operation of the Proposed ZCS Scheme I • Mode 1: • the top switches U2T and W1T turn on • Vu provides -Vdc and Vw provides +Vdc to the secondary side • the bottom switches V1B and V2B are on, so Vv is equal to zero • Mode 2: • VVsupplies negative voltage to the secondary side rectifier, and VU is also negative • the negative current from the load side flows through the diode DUB • the top switch V2T is ZCS turn on.

7. Modes of Operation of the Proposed ZCS Scheme II • Mode 3: • U2T turns off and the voltage VUequals zero • the diode DVB will handle the negative current • U2T suffers a hard switching off • Vvis negative since last mode, so DVB endures zero voltage, which creates a ZVS condition for DVBon • Mode 4: • V1T turns on • phase W supplies positive voltage and the rest provide zero voltage to the secondary side • V1T goes on flowing the negative current, so the switch V1T achieves ZVZCS turn on.

8. Modes of Operation of the Proposed ZCS Scheme III • Mode 5: • U1T switches on, therefore, the VU will be +Vdc. • DWT occupies the positive current and DVB the negative one. • based on the same principle as Mode 2, U1T is ZCS turn on • Mode 6: • W2T turns on. • U1T starts taking off the positive current and V2T still handles the negative portion of the current • W2T is a ZVZCS turn on. • DUT has a ZVS on

9. Modes of Operation of the Proposed ZCS Scheme IV • Mode 7: • W1T turns off, and W2T begins to handle negative current • anti-parallel diode of W1T may take off the negative current • W1T is made a ZCS turn off • this mode is similar to Mode 1 and DWB obtains a ZVS on • Mode 8: • V2T is off, and DUT (positive) and DWB (negative) take the current on the secondary side • the current flows through V1T is zero, which corresponds to a ZCS turn off for V2T. • this mode is similar to Mode 2.

10. Unique features of the proposed ZCS scheme • The voltage Vrec on the secondary side has only two voltage levels: 2∙N∙Vdc and N∙Vdc but no zero level Advantage: Less requirement for the clamp circuit owing to the reduced parasitic device body capacitance on the secondary side under the nonzero voltage. • Using linear programming to implement the optimal solution for the soft switching range Switching waveforms of the proposed ZCS scheme

11. General Requirements for the ZCS Scheme with Nonzero Pulsating Dc Output and the Optimal ZCS Range I The figure above is shown to denote the definitions for phase voltage PWM widths (αi), phase shift between phase voltages (βi) and the width of gap (γ) for N∙Vdc. So the conditions and constraints can be listed as follows:

12. General Requirements for the ZCS Scheme with Nonzero Pulsating Dc Output and the Optimal ZCS Range II It can be expressed in the linear programming format: where, For the optimal ZCS range, we can define our desired objective function as

13. Implementation of the Proposed ZCS Scheme Optimal value Solve the linear programming problem and get the control coefficients for the ZCS scheme (Plotted and rated at FPGA clock signal frequency) DSP looks up data for optimization from an inside embedded table, then packs, compacts and transmits it to the FPGA with 2 WORDS. To balance the power for each phase, we can rotate the order of the gate signals every 1/6 line period Phase (deg)

14. Particular Examples for ZCS Control Condition I Case 1: Various-width-constant-phase-shift The constraints for this case are: And we can also fix and the simulation waveforms will be shown later. In this particular case, the modulation index M should be .

15. Particular Examples for ZCS Control Condition II Case 2: Various-phase-shift-constant-width The constraint for this case is: Also we can also set to facilitate the implementation on the hardware platform. In this particular situation, the modulation index M should be

16. 1 kVA RHFL Converter Prototype Main Components Used in the Prototype • A 1 kVA RHFL-converter is designed to validate the proposed soft-switching scheme. • The designed input voltage is 36 V dc and rated output voltage is 208 V ac (line to line). • Switching frequencyat Bridge I is 21.6 kHz and at the Bridge III is 43.2 kHz. • Transformer turns ratio is around 1: 8.4. Figure of the 1 kVA RHFL converter prototype

17. Simulated and Experimental Validations of the Proposed ZCS Scheme I Switching sequences Diode current and voltage in Bridge II Simulation result for Various-width-constant-phase-shift case

18. Simulated and Experimental Validations of the Proposed ZCS Scheme II Experimental result for Various-width-constant-phase-shift case

19. Simulated and Experimental Validations of the Proposed ZCS Scheme III Experimental results for the optimal soft switching range

20. Measured Efficiency Comparison Result

21. Key Conclusions • The proposed ZCS scheme achieves 75 % ZCS for all the switching actions on the Bridge I as well as ZVS on for the secondary-side three-phase rectifier. • With a special objective function, a practical optimum solution is given and two practical modulation conditions are also presented. • Through the simulations and hardware experiments, all the proposed ZCS situations are validated. • The overall efficiency of the prototype is measured to be promoted compared with hard-switched Bridge I and II.

22. Thank You!