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

3.2 Transformer Design

The benefit of GaN is the capability of higher switching frequency with lower losses. To improve the overall efficiency for high-density designs, the transformer design plays a key role.

The worst-case system efficiency occurs at the lowest input voltage condition of 90 VAC. For a flyback topology, the main switch turns on to store energy in the magnetized inductance (LMAG), and transfers the energy to the output when the device turns off. During the demagnetizing period, the secondary winding is clamped to the output voltage, VOUT, and the reflected voltage VRF from secondary to primary is calculated using Equation 5:

Equation 5. V R F = V O U T × N P S

where

  • NPS is the turn ratio between primary and secondary

Equation 6 calculates the maximum duty cycle:

Equation 6. D M A X = V R F V D C _ M I N + V R F

where

  • VDC_MIN is the minimum DC voltage at lowest AC input voltage condition

For the operation of QR flyback, the main device turns on after the magnetizing energy is discharged and the switch node voltage resonates to the lowest point. This means that the switching FET turns on with Zero Voltage Switch (ZVS) if VRF > VDC and the conduction loss dominates the overall efficiency.

Suppose the running frequency, fRUN_MIN is the minimum frequency at VDC_MIN, and the primary inductance of the transformer, LP is calculated with Equation 7:

Equation 7. L P = V D C _ M I N × D M A X 2 × η 2 × f R U N _ M I N × P O U T

where

  • POUT is the output power and η is the efficiency of the system

At this condition, the peak current, IPK_MAX is found using Equation 8:

Equation 8. I P K _ M A X = 2 × P O U T V D C _ M I N × D M A X × η

The primary RMS current, IRMS is determined with Equation 9:

Equation 9. I R M S = D M A X 3 × I P K _ M A X

These calculations show that, for a fixed output power, the conduction loss of the device depends on VDC_MIN and DMAX, which is only related to the turns ratio, NPS. In summary, for a QR flyback design, the worst case is at the lowest AC input voltage. Under this condition, the switching device turns on with ZVS, which means conduction loss dominates the system losses, but these losses are related to the turns ratio only. In other words, a higher turns ratio leads to lower IRMS. The loss on the switching device is fixed.

To facilitate the discussion on selection of switching frequency, Equation 7 can be re-written as:

Equation 10. L P × f R U N _ M I N = V D C _ M I N × D M A X 2 × η 2 × P O U T

The above shows higher frequency leads to lower a inductance value. In the transformer design, the flux density must be minimized to prevent saturation of the ferrite core. The maximum flux density, BMAX is found using Equation 11:

Equation 11. B M A X = L MAG × I P K _ M A X A E × N P

where

  • AE is the effective area of the ferrite core which depends on the core shape
  • NP is the number of turns on the primary winding

From Equation 11, if Bmax and AE are held constant, the higher running frequency leads to a lower LMAG value with lower turns of winding. In this case, Litz wire can be used with more strands to reduce the copper loss to achieve better efficiency and thermal result.

In this reference design, the turns ratio is chosen to be 32:5 with a split primary winding method to minimize the leakage inductance. The primary winding is 0.1 mm × 15P Litz wire and the secondary winding is 0.05 mm × 320P triple isolation Litz wire.