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 The Relationship Between Output Voltage (VOUT) and Switching Frequency (fS) in an LLC Resonant Controller

  • To maintain resonance, the LLC topology has a 50% duty cycle, which leaves frequency modulation (FM) as the only tool to regulate output voltage.
  • The voltage gain of the Resonant Tank depends on the switching frequency.
  • The controller adjusts the switching frequency (fS) to maintain required output voltage despite load or input variations.

Nominal Output Voltage (VO) →The resonant tank operates at its resonant frequency (fR)

Equation 3. f R =   1 2 π L R   ×   C R

At resonance (fS = fR) → voltage gain is close to 1, ZVS is maintained, highest efficiency point

Higher Output Voltage (VOUT) → Decrease Switching Frequency (fS)

To boost the output voltage, the controller decreases switching frequency (operate closer to or just below resonant frequency (fR)) to increase voltage gain of the resonant tank. More power is transferred to the output, raising the voltage.

Below resonance (fS < fR) → voltage gain > 1, useful for boost mode.

Battery Charging CC-CV Profile for Traditional LLC Figure 3-2 Battery Charging CC-CV Profile for Traditional LLC
LLC Resonant Operating Frequency Range Figure 3-3 LLC Resonant Operating Frequency Range

Lower Output Voltage (VOUT) → Increase Switching Frequency(fS)

To reduce output voltage, the controller increases switching frequency to lower voltage gain of the resonant tank. Less power is transferred, lowering the output voltage.

Above resonance (fS > fR) → voltage gain < 1, converter regulates lower output voltages. but ZVS may be lost (can happen if the magnetizing current is not sufficient)

Table 3-1 Switching Frequency vs Output Voltage
Output Voltage Change Switching Frequency Change Reason
Output ↓ Frequency ↑ Increase gain
Output ↑ Frequency ↓ Decrease gain