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  1. 1. High-Frequency Link Inverter Based on Multiple-Carrier PWM Philip T. Krein, Xin Geng, Robert Balog University of Illinois March 2002
  2. 2. Outline <ul><li>The PWM cycloconverter. </li></ul><ul><li>Dual-carrier PWM to build waveforms for HF links. </li></ul><ul><li>Properties of dual-carrier signals for gate drives and other purposes. </li></ul><ul><li>Experimental results. </li></ul><ul><li>Conclusions. </li></ul>
  3. 3. The PWM Cycloconverter <ul><li>High-frequency (HF) link inverters can be constructed as: </li></ul><ul><ul><li>A cascade of a high-frequency dc-dc converter and inverter. </li></ul></ul><ul><ul><li>A “square-wave cycloconverter,” in which a high-frequency square-wave inverter provides the input to a cycloconverter. </li></ul></ul>
  4. 4. The PWM Cycloconverter <ul><li>The dc-dc converter alternative has multiple power conversion stages. </li></ul><ul><li>The cycloconverter would seem to have complicated operation, since it is treated as a nonlinear phase control problem. </li></ul><ul><li>The complexity has been one factor limiting use. </li></ul><ul><li>Now consider a conventional PWM inverter. A two-level inverter has a single input (V in ) and can produce an output of  V in . </li></ul><ul><li>There should be some way to work from an input of  V in and generate exactly the same output waveform. </li></ul>
  5. 5. The PWM Cycloconverter <ul><li>This is the PWM cycloconverter : use as input a simple square wave input at high frequency, then control the switches to produce an output that is exactly a conventional two-level PWM waveform. </li></ul><ul><li>The PWM cycloconverter is not a new concept. </li></ul><ul><li>What is new is that conventional PWM can be extended to cycloconverter operation with a multiple-carrier PWM process. </li></ul>
  6. 6. Dual-Carrier PWM <ul><li>Consider the use of two separate PWM waveforms, modulated with a desired low-frequency waveform m(t). </li></ul><ul><li>This could be separate rising and falling ramps, triangles with phase shifts, or the like. </li></ul><ul><li>Call these Carrier 1 and Carrier 2 . </li></ul><ul><li>Now modulate both Carrier 1 and Carrier 2 with the signal m(t), to give PWM 1 and PWM 2 . </li></ul><ul><li>The sum PWM 1 + PWM 2 has low-frequency content 2m(t). </li></ul><ul><li>The difference PWM 1 – PWM 2 has no low-frequency content. </li></ul>
  7. 7. Dual-Carrier PWM <ul><li>Is the result trivial? Not if we use time multiplexing to make sure the final waveform retains switching behavior. </li></ul><ul><li>Several choices of combinations are available. </li></ul>
  8. 8. Dual-Carrier PWM <ul><li>There are systematic ways to develop specific desirable properties in the final output waveform. </li></ul><ul><li>Example: </li></ul><ul><ul><li>Create two carriers from a single ramp just by blanking every other pulse. </li></ul></ul><ul><ul><li>Modulate both with m(t), then subtract. </li></ul></ul><ul><ul><li>The result is a three-level high-frequency link signal. </li></ul></ul><ul><li>Another example: </li></ul><ul><ul><li>Split a triangle into separate rising and falling ramps. </li></ul></ul><ul><ul><li>Modulate, respectively, with m(t) and –m(t). </li></ul></ul><ul><ul><li>This yields a two-level signal “PWM” signal with no low-frequency content. </li></ul></ul>
  9. 9. Dual-Carrier PWM
  10. 10. Dual-Carrier PWM <ul><li>The PWM sum has 50% duty, but retains the information. </li></ul>
  11. 11. Dual-Carrier PWM <ul><li>What about the desired two-level PWM output? </li></ul><ul><li>Use the square wave clock as the input to a cycloconverter. </li></ul><ul><li>Use the sum waveform PWM(t) as the gate control. </li></ul><ul><li>This “convolution” process of clock and PWM(t) recovers the desired two-level PWM output. </li></ul>
  12. 12. Dual-Carrier PWM <ul><li>The PWM waveform is this example is also always phase-advanced with respect to the original square wave. </li></ul><ul><li>List the combinations for two-carrier PWM. </li></ul>
  13. 13. Dual-Carrier PWM <ul><li>Triangle-based carrier sets with output delay and advance, respectively. </li></ul>
  14. 14. Dual-Carrier PWM <ul><li>Three-level PWM examples for HF links. </li></ul>
  15. 15. Properties in the Two-Carrier Case <ul><li>We can select among several properties: </li></ul><ul><ul><li>By using carriers that alternately control turn-on and turn-off, gate waveforms with 50% duty can be generated. </li></ul></ul><ul><ul><li>The combined signal can have pure advance or delay. </li></ul></ul><ul><ul><li>The resulting PWM output can be generated with an effectively doubled switching frequency. </li></ul></ul>
  16. 16. Properties in the Two-Carrier Case <ul><li>The two-carrier process allows conventional PWM modulators, combined with some simple logic, to generate waveforms for PWM cycloconverters. </li></ul><ul><li>With use of both advanced and delayed gating waveforms, natural commutation can be supported, in a manner equivalent to conventional sine wave SCR cycloconverters. </li></ul><ul><li>The cases that yield 50% duty ratio gating signals are especially valuable for transformer gate drives. </li></ul>
  17. 17. Experimental Results <ul><li>The two-carrier technique has been used to build a simple “naturally commutated PWM cycloconverter.” </li></ul><ul><li>This represents a high-frequency link inverter that makes use of conventional PWM to provide control – with no intermediate dc-dc converter. </li></ul><ul><li>SCRs are used – only the leading edge of the combined two-carrier signal is needed for the gate drives. (IGBTs could have been used instead.) </li></ul>
  18. 18. Experimental Results <ul><li>NCC square-wave cycloconverter (three-phase). </li></ul>
  19. 19. Experimental Results <ul><li>Test circuit, single-phase output. </li></ul>
  20. 20. Experimental Results <ul><li>The modulation process. </li></ul><ul><li>Only the leading edges are needed. </li></ul>
  21. 21. Experimental Results <ul><li>The crossover behavior from delayed gating to advanced gating. </li></ul>
  22. 22. Experimental Results <ul><li>Devices here switch at 3750 Hz. </li></ul><ul><li>Output is two-level PWM at 7500 Hz. </li></ul>
  23. 23. Conclusion <ul><li>A multiple-carrier method can be used to generate PWM gating signals with a variety of properties. </li></ul><ul><li>Two-carrier signals chosen to cancel the baseband signal m(t) support high-frequency link inverters. </li></ul><ul><li>These inverters, PWM cycloconverters, eliminate a power stage but produce conventional PWM output waveforms. </li></ul><ul><li>The control complexity is similar to familiar PWM, but with multiple signal paths. </li></ul>
  24. 24. Conclusion <ul><li>The multiple-signal output can be tailored for useful properties, such as gate drives with 50% duty ratio under all modulating signals, and outputs that provide an effective doubling of the switching frequency. </li></ul>