TIDUF57 November   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations [Required Topic]
    3. 2.3 Highlighted Products
      1. 2.3.1 LMG3624
  9. 3System Design Theory
    1. 3.1 Quasi-Resonant Operation
    2. 3.2 Transformer Design
    3. 3.3 GaN FET Switching Device
    4. 3.4 Current Sense Emulation Resistor
  10. 4Hardware, Testing Requirements, and Test Results
    1. 4.1 Required Hardware
      1. 4.1.1 Hardware
      2. 4.1.2 Testing Equipment
    2. 4.2 Test Setup
    3. 4.3 Test Results
      1. 4.3.1 Efficiency Results
      2. 4.3.2 Thermal Results
      3. 4.3.3 Switching Waveforms
      4. 4.3.4 Switching Transients
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
    2. 5.2 Tools and Software
    3. 5.3 Documentation Support [Required Topic]
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

3.3 GaN FET Switching Device

As the quasi-resonant converter is designed to minimize the transformer size with high switching frequencies, the switching device must be able to support high-frequency operation without suffering major additional power loss penalties.

There are two types of losses that must be considered when making the selection for the switching device: frequency-dependent switching losses, and current-dependent conduction losses.

In the quasi-resonant flyback application, the main component of the switching loss can be attributed to the turn-on stored energy loss, as described by using Equation 12:

Equation 12. P L O S S , T U R N - O N - E N E R G Y = = 1 2 C D × V V A L L E Y 2 × f S W

Here, the CD term is highly-dependent on the output capacitance of the switching device. The LMG3624 integrated GaN FET is chosen to optimize for this switching loss, considering that the effective output capacitance, COSS, is drastically lower than a counterpart Silicon FET with a similar on-resistance.

GUID-20221210-SS0I-LNWW-ZPMV-HLVZ4GCRFLGB-low.svg Figure 3-3 LMG3624 COSS vs VDS Characteristic

Figure 3-3 shows the COSS vs VDS characteristic for the LMG3624 device.

When switching from 400 V to 0 V, the effective energy-related output capacitance , CO,ERof the device is only 29 pF. Using Equation 12 and assuming fSW= 150 kHz, this value of CO,ER yields a turn-on stored energy loss of 348 mW, or roughly 0.53% of the system efficiency. In practice, the design is switching lower than 400 V, due to the valley-switching operation, yielding higher power-loss savings.

The conduction loss component can be calculated with Equation 13:

Equation 13. P C O N D U C T I O N = I R M S 2 × R D S ( o n )

where

  • IRMS is the RMS current through the GaN device (1.2 A)
  • RDS(on) is the on-resistance (170 mΩ)

In this 65-W application, the worst-case conduction loss at the 90 VAC input comes out to be 261 mW, or 0.40% of the system efficiency

When the main components of the switching loss and conduction loss are considered, the GaN device contributes less than 0.9% of the entire system losses, enabling a simple thermal design and a high system efficiency.