STDA011 September   2025 UCC25661

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Benefits of GaN In LLC Resonant Converter
    1. 2.1 Higher Efficiency
    2. 2.2 Faster Switching Speeds
    3. 2.3 Reduced Parasitic Capacitances
    4. 2.4 Improved Power Density
    5. 2.5 High Thermal Conductivity
    6. 2.6 Lower Junction Temperatures
  6. 3LLC Resonant Converter
    1. 3.1 The Relationship Between Output Voltage (VOUT) and Switching Frequency (fS) in an LLC Resonant Controller
      1. 3.1.1 The LLC Charging Challenge
      2. 3.1.2 A Wide VIN/VOUT Capable LLC
  7. 4Practical Application of LLC Converters for a Battery Charger Leveraging GaN Switches
    1. 4.1 Requirements and Scope
    2. 4.2 Charging Curve for Lithium-Ion Battery
    3. 4.3 How to Support Wide VOUT Range in an LLC Design for Battery Chargers
    4. 4.4 The Prototype Hardware
  8. 5Summary

3.1.1 The LLC Charging Challenge

To support the wide output voltage required for battery charging, the LLC resonant tank and burst mode threshold must be tuned perfectly as a low battery voltage charging in the constant current phase could go in and out of burst mode which could potentially damage the cells.

One way the LLC has been used to support the wider output voltage requirements of a battery was to use it in the LCC configuration. The LCC is a modification to the resonant tank which has two capacitive elements and one inductive element.

LCC Converter Circuit Figure 3-4 LCC Converter Circuit

The LCC is setup to operate at a minimum resonant frequency and has a much wider range of operation.

LCC Resonant Operating Frequency Range Figure 3-5 LCC Resonant Operating Frequency Range

By supporting a wider range, the LCC approach is much more suitable to battery charging and LED lighting applications where the output voltage can vary. The downside of the LCC approach is that it is less efficient as the input RMS currents are higher than the LLC at the same power level.

Table 3-2 shows a quick comparison of the primary current and switching frequency.

Table 3-2 LLC vs LCC Primary Current and Switching Frequency Comparison
VOUT IOUT LLCIPRI (ARMS) LCCIPRI (ARMS) LLCfSW (kHz) LCCfSW (kHz)
200 1 1.73 2.69 131 122
100 1 0.775 1.65 196 130

Looking at the switching frequencies, the LCC barely moves when VOUT changes from 100 to 200V. Whereas the LLC sees almost a 33% reduction in switching frequency. The real impact if the LCC topology is the higher input currents.

So the LLC topology is more desirable from an overall efficiency perspective but the potential to enter burst mode when the battery voltage is low was a difficult design hurdle. Until now.