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

4.4 The Prototype Hardware

Figure 4-5 3D View of Complete Charger Design

Test results from 200W charger with PFC, LLC and GaN on both stages:

Efficiency Versus VACFigure 4-6 Efficiency Versus VAC
Efficiency Versus VOUTFigure 4-7 Efficiency Versus VOUT

Since in this design LF burst mode is impacting the peak-peak charging current ripple, we measured at what conditions LF burst mode stops and the converter runs continuously or with HF bursts. Below are some graphs showing the thresholds (when the curve collapses, then at this point LF bursts are off).

Figure 4-8 Entry Level and Frequency Versus VOUT and IOUT

The waveforms below describing the main characteristics of switch node and resonant current, in continuous mode and burst mode. In all possible working modes and points, versus VIN, VOUT and IOUT.

The switching frequency is shifting from 170 kHz up to 380 kHz in normal operation, therefore it is a critical factor to select a controller and switching elements that can operate in this frequencies and deliver high efficiency and low loss. During burst mode, as well as soft start, the controller can even reach 1 MHz frequency, shown in Figure 4-9. With TI new controller UCC256611 and GaN extremely high-speed capability this is not a problem.

Figure 4-9 Steady State Operation vs Startup

A more detailed description of the behavior during burst start is better shown and explained in the ZCS Prevention During Startup figure in the UCC25661x Family 750kHz Wide VIN/VOUT Range LLC Controller Optimized for Light-Load Efficiency Data Sheet, where TON extensions are used to wait until the current in resonant inductor has the right polarity.

ZCS Prevention During Startup Figure 4-10 ZCS Prevention During Startup