High-Frequency Link Inverter Based on Multiple-Carrier PWM

1. High-Frequency Link Inverter Based on Multiple-Carrier PWM Philip T. Krein, Xin Geng,Robert Balog University of Illinois March 2002

2. Outline • The PWM cycloconverter. • Dual-carrier PWM to build waveforms for HF links. • Properties of dual-carrier signals for gate drives and other purposes. • Experimental results. • Conclusions.

3. The PWM Cycloconverter • High-frequency (HF) link inverters can be constructed as: • A cascade of a high-frequency dc-dc converter and inverter. • A “square-wave cycloconverter,” in which a high-frequency square-wave inverter provides the input to a cycloconverter.

4. The PWM Cycloconverter • The dc-dc converter alternative has multiple power conversion stages. • The cycloconverter would seem to have complicated operation, since it is treated as a nonlinear phase control problem. • The complexity has been one factor limiting use. • Now consider a conventional PWM inverter. A two-level inverter has a single input (Vin) and can produce an output of Vin. • There should be some way to work from an input of Vin and generate exactly the same output waveform.

5. The PWM Cycloconverter • 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. • The PWM cycloconverter is not a new concept. • What is new is that conventional PWM can be extended to cycloconverter operation with a multiple-carrier PWM process.

6. Dual-Carrier PWM • Consider the use of two separate PWM waveforms, modulated with a desired low-frequency waveform m(t). • This could be separate rising and falling ramps, triangles with phase shifts, or the like. • Call these Carrier 1 and Carrier 2. • Now modulate both Carrier 1 and Carrier 2 with the signal m(t), to give PWM 1 and PWM 2. • The sum PWM 1 + PWM 2 has low-frequency content 2m(t). • The difference PWM 1 – PWM 2 has no low-frequency content.

7. Dual-Carrier PWM • Is the result trivial? Not if we use time multiplexing to make sure the final waveform retains switching behavior. • Several choices of combinations are available.

8. Dual-Carrier PWM • There are systematic ways to develop specific desirable properties in the final output waveform. • Example: • Create two carriers from a single ramp just by blanking every other pulse. • Modulate both with m(t), then subtract. • The result is a three-level high-frequency link signal. • Another example: • Split a triangle into separate rising and falling ramps. • Modulate, respectively, with m(t) and –m(t). • This yields a two-level signal “PWM” signal with no low-frequency content.

9. Dual-Carrier PWM

10. Dual-Carrier PWM • The PWM sum has 50% duty, but retains the information.

11. Dual-Carrier PWM • What about the desired two-level PWM output? • Use the square wave clock as the input to a cycloconverter. • Use the sum waveform PWM(t) as the gate control. • This “convolution” process of clock and PWM(t) recovers the desired two-level PWM output.

12. Dual-Carrier PWM • The PWM waveform is this example is also always phase-advanced with respect to the original square wave. • List the combinations for two-carrier PWM.

13. Dual-Carrier PWM • Triangle-based carrier sets with output delay and advance, respectively.

14. Dual-Carrier PWM • Three-level PWM examples for HF links.

15. Properties in the Two-Carrier Case • We can select among several properties: • By using carriers that alternately control turn-on and turn-off, gate waveforms with 50% duty can be generated. • The combined signal can have pure advance or delay. • The resulting PWM output can be generated with an effectively doubled switching frequency.

16. Properties in the Two-Carrier Case • The two-carrier process allows conventional PWM modulators, combined with some simple logic, to generate waveforms for PWM cycloconverters. • With use of both advanced and delayed gating waveforms, natural commutation can be supported, in a manner equivalent to conventional sine wave SCR cycloconverters. • The cases that yield 50% duty ratio gating signals are especially valuable for transformer gate drives.

17. Experimental Results • The two-carrier technique has been used to build a simple “naturally commutated PWM cycloconverter.” • This represents a high-frequency link inverter that makes use of conventional PWM to provide control – with no intermediate dc-dc converter. • 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.)

18. Experimental Results • NCC square-wave cycloconverter (three-phase).

19. Experimental Results • Test circuit, single-phase output.

20. Experimental Results • The modulation process. • Only the leading edges are needed.

21. Experimental Results • The crossover behavior from delayed gating to advanced gating.

22. Experimental Results • Devices here switch at 3750 Hz. • Output is two-level PWM at 7500 Hz.

23. Conclusion • A multiple-carrier method can be used to generate PWM gating signals with a variety of properties. • Two-carrier signals chosen to cancel the baseband signal m(t) support high-frequency link inverters. • These inverters, PWM cycloconverters, eliminate a power stage but produce conventional PWM output waveforms. • The control complexity is similar to familiar PWM, but with multiple signal paths.

24. Conclusion • 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.