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LLC Power Conversion Explained, Part 6: Resonant Frequency Operation and Waveforms

This is the sixth in a series on the LLC converter. The LLC converter is very complicated which explains why Reference 1 is a PhD dissertation on this circuit. I attended Virginia Tech and I can tell you that the average PhD took seven years to complete. Therefore, six blogs aren’t really too much coverage while barely introducing the basic operation principals.

This blog focuses on the waveforms that result when operating an LLC circuit. The best explanation of the waveforms was in Reference 5 by Infineon.

The operating frequency for the LLC is based on the leakage inductance Lr and series capacitance element is shown in the following figure.

Source: Figure 2.2 from the Infineon application note, Reference 5.

Source: Figure 2.2 from the Infineon application note, Reference 5.

In LLC Power Conversion Explained, Part 1: Introduction, it was established that resonant converters offer the benefit of zero voltage or zero current switching. Thus, a resonant waveform allows the sinusoidal return to zero instead of a hard-switched, abrupt voltage or current change in the typical Switched Mode Power Converter or SMPS. This sinusoidal return to zero is what keeps the rectifier current switching at zero. Thus diode reverse recovery is not an issue with the LLC converter. However, the controller must keep the switching frequency at the resonant frequency or waveform distortion takes place as shown in the following figure.

Table 1

Table 1 from the Infineon application note, Reference 5

Table 1 from the Infineon application note, Reference 5

This table is an excellent summary of the six blogs to date as it shows the rectifier currents ID as well as the magnetizing current and leakage inductance current. The table also shows how operating at the resonant frequency is critical to Zero Current Switching or ZCS.

A second benefit of the LLC is Zero Voltage Switching or ZVS of the input transistors. This is favorable especially when switching from a Power Factor Correction or PFC typical value of 400 VDC. Although the voltage of the transistor returns to zero, it’s actually the current flow that causes ZVS by reversing through the MOSFET body diode as it goes to the negative side of the sine wave as shown in the following figure.

Figures 2.7 and 2.8 from the Infineon application note, Reference 5

Figures 2.7 and 2.8 from the Infineon application note, Reference 5

The basic principles of operation have been explained in this series of blogs. However, I’m still having startup issues for the combination of the LLC and PFC circuit. If I can resolve them, I may be able to squeeze one more blog out of this advantageous circuit.

References

  • “Design Considerations for an LLC Resonant Converter” Fairchild Semiconductor Power Seminar 2007 Appendix A: White Papers; couldn’t get a website URL; suggest you Google the text in brackets [“Design Considerations for an LLC Resonant Converter” Fairchild Semiconductor Power Seminar 2007 Appendix A: White Papers]
  • Square Wave Signals”, Chapter 7 – Mixed-Frequency AC Signals, All About Circuits website

1 comment on “LLC Power Conversion Explained, Part 6: Resonant Frequency Operation and Waveforms

  1. D Feucht
    October 21, 2017

    Scott, Thanks for bringing this kind of resonant converter to our attention. I have examined in detail and built a somewhat different converter. The differences are 1. Half-bridge instead of full-bridge switching circuit. 2. The other branch of the half-bridge is primary winding coupling C and storage C for the input rectifier. 3. The inductor is moved to the secondary circuit. The consequences of these changes are far too much for a posting, but most are beneficial for many applications. The math required to derive the design equations is nontrivial but not as bad as that for optimal switching-transformer design. One of the disadvantages of this scheme is that the current in the inductor is bidirectional, so its ripple is dominant, as in a transformer. Consequently, ferrite and not the lower-cost iron-powder cores are used to reduce core loss.

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