SLVAG14 May   2025

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Cycloconverter Fundamentals
  6. 3Design Considerations and Results
  7. 4Cost Optimization
  8. 5Conclusion
  9. 6References

2 Cycloconverter Fundamentals

A cycloconverter or a cyclo inverter converts a constant amplitude and frequency AC waveform to another AC waveform of a lower frequency by synthesizing the output waveform from segments of the AC supply without an intermediate DC link. For the use case of a micro inverter or portable power station, the input waveform is pure DC. The output is the AC-grid connection. Figure 2-1 visualizes a possible implementation.

Cycloconverter With Full Bride on DC and Half Bridge on AC sideFigure 2-1 Cycloconverter With Full Bride on DC and Half Bridge on AC side

In this example, a full bridge is implemented on the DC side to create the input signal VP on the primary side of the transformer T1. A half bridge configuration is implemented on the AC side (with capacitive divider) to resemble the segments VS on the secondary side for AC output VGRID.

For positive output signals, the switches S1B and S2B are permanently turned on. The converter can be seen as a dual active bridge operated in phase shift. Applying PWM to S1A and complementary to S2A, the output voltage and current are resembled. The amount of power transferred is defined by phase shift between VP and VS. For negative output voltages, S1A and S2A are permanently turned on. Again, the switches S1B and S2B are forming a phase shifted dual active bridge for negative output voltage and current.

In the reference design TIDA-010954, GaN devices from TI are used to operate the converter at fast switching frequencies to make all magnetic components as small as possible, while not sacrificing efficiency.

Why GaN?

  • A cycloconverter is a soft switching topology, which means turn-on losses can be neglected.
  • GaN FETs have significant lower turn-off losses compared to SiC or SiFET.
  • A GaN device's output capacitance COSS is lower than SiFET. This helps to achieve a wider zero voltage switching range.
  • The conduction losses are caused by the device's RDSON. This defines how much loss the converter will finally have.

The devices used on the primary side are the 100V GaN half bridge LMG2100R026 (with RDSON 2.6mΩ). For the secondary side of the 650V GaN device with integrated gate driver: the LMG3650R035 (with RDSON 35mΩ).