TIDUF59 March   2024

 

  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
      1. 2.2.1 PFC Inductance Design
      2. 2.2.2 Configuration of CS pin in LMG3622
      3. 2.2.3 AHB Topology and the VCC Design
      4. 2.2.4 LMG2610 for AHB Topology
    3. 2.3 Highlighted Products
      1. 2.3.1 UCC28056
      2. 2.3.2 LMG3622
      3. 2.3.3 LMG2610
  9. 3Hardware, Test Requirements, and Test Results
    1. 3.1 Hardware
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Switching Waveform
        1. 3.3.1.1 Switching Waveform on the PFC Stage
        2. 3.3.1.2 Switching Waveform on the AHB Stage
      2. 3.3.2 Efficiency Test Result
      3. 3.3.3 Thermal Test Result
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
      3. 4.1.3 Layout Prints {Optional Section}
    2. 4.2 Tools
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

2.2.1 PFC Inductance Design

Based on the TM control method, the system works at ZVS when the input DC voltage is less than half of the output voltage. In PFC stage, the lowest efficiency was determined to occur at the lowest line voltage. The system does not have much switching loss, and the conduction loss dominates the system.

For TM PFC, the peak current was found at the phase angle equal to 90°, Equation 1 determines the value.

Equation 7. I P E A K = 2 × 2 × P I N V A C

where

  • PIN is the input power which is the output power divides by the overall efficiency
  • VAC is the input RMS voltage

The output voltage was set as VOUT, the duty cycle at this point is found with Equation 2.

Equation 2. D u t y = V O U T - 2 × V A C V O U T

The target frequency is set at the lowest input AC voltage as FREQ_MIN. Use Equation 3 to calculate the inductance value.

Equation 3. L P F C = 2 × V A C I P E A K × D u t y F R E Q _ M I N

The flux density of the core material BMAX is found using Equation 4.

Equation 4. B M A X = L P F C × I P E A K A e × N

where

  • Ae is the effective area of the core material
  • N is the number of turns of the winding

Based on Equation 4, the peak current is fixed, and the Ae depends on the core shape. To keep the same flux density with the same core size, the turns N is proportional to the LPFC value. From a system point of view, make the system run at higher frequency by implementing the GaN HEMT. Reducing the turns with thicker wire to minimize the copper loss minimizes the LPFC value.

For this 140W design, the target efficiency is 93%, the input power is 150.54W. At 90V input voltage, assume the output voltage is 390V and the minimum frequency is 100kHz, IPEAK is 4.731A. The duty cycle is 67.4%, and the inductance is 181μH.

In this design, the RM10 core size with 3C95 material from Ferroxcube is selected. Set the inductance value as 185μH with 30 turns by 0.1mm × 40P Litz wire to reduce the copper loss.

As the LPFC value is fixed, the turn on time, Ton, can be calculated, at any angle θ as done in Equation 5.

Equation 5. T o n θ = L P F C × I P E A K × sin θ 2 × V A C × sin θ

Equation 5 shows the Ton time is constant at any phase angle.

The RMS current of the switching device IRMS can be calculated as shown in Equation 6.

Equation 6. I R M S = 1 6 - 4 2 9 π × V A C V O U T × I P E A K

According to this formula, the RMS current is a constant which relates to the VAC, VOUT, and PIN only. From the device point of view, the conduction loss can only be reduced by lowering the RDS(on).