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

3.4 Current Sense Emulation Resistor

The current-sense resistor RCS is determined after initially calculating a value for a traditional current-sense resistor, RCS(trad). An initial value for RCS(trad) can be selected with considerations for the feedback voltage and other gain factors in the feedback loop as explained by Equation 14:

Equation 14. V F B = K F B × I P K × R C S ( T R A D )

where

  • VFB is the feedback voltage input to the controller
  • KFB is a generic variables to account for additional gains in the feedback loop
  • IPK is the peak current through the FET

RCS is then determined by solving for the traditional current-sense design resistance, RCS(trad), and multiplying by the inverse of GCSE, the current sense gain, equal to 1,036 mA/A. The traditional current-sense design creates the current-sense voltage, VCS(trad), by passing the GaN power FET drain current, ID, through RCS(trad). The LMG3624 creates the current-sense voltage, VCS, by passing the CS pin output current, ICS, through RCS. The current-sense voltage must be the same for both designs.

Equation 15. VCS = ICS × RCS = VCS(trad) = ID × RCS(trad)
Equation 16. RCS = ID / ICS × RCS(trad) = 1 / GCSE × RCS(trad)
Equation 17. RCS = 1,036 × RCS(trad)