SLUA560D June   2011  – March 2022 UCC28950 , UCC28950-Q1 , UCC28951 , UCC28951-Q1

 

  1.   Trademarks
  2. 1 Design Specifications
  3. 2 Functional Schematic
  4. 3 Power Budget
  5. 4 Transformer Calculations (T1)
  6. 5 QA, QB, QC, QD FET Selection
  7. 6 Selecting LS
  8. 7 Output Inductor Selection (LOUT)
  9. 8 Output Capacitance (COUT)
  10. 9 Select FETs QE and QF
  11. 10Input Capacitance (CIN)
  12. 11Setting Up the Current Sense (CS) Network (CT, RS, RRE, DA)
  13. 12Voltage Loop and Slope Compensation
  14. 13Setting Turn-on Delays to Achieve Zero Voltage Switching (ZVS)
  15. 14Turning SR FETs-off Under Light Load Conditions
  16. 15600 W FSFB Detailed Schematic and Test Data
  17. 16References
  18. 17Revision History

8 Output Capacitance (COUT)

The output capacitor is being selected based on holdup time (tHU) and load transient requirements.

tHU is the time it takes LOUT to change 90% of its full load current:

Equation 50. tHU=LOUT×POUT×0.9VOUTVOUT=7.5 μs

During load transients most of the current will immediately go through the capacitors equivalent series resistance (ESRCOUT). The following equations are used to select ESRCOUT and COUT based on a 90% load step in current. The ESR is selected for 90% of the allowable transient voltage (VTRAN), while the output capacitance (COUT) is selected for 10% of VTRAN.

Equation 51. ESRCOUT≤VTRAN×0.9POUT×0.9VOUT=12 mΩ
Equation 52. COUT≥POUT×0.9×tHUVOUTVTRAN×0.1≈5.6 mF

Before selecting the output capacitance it is also required to calculate the output capacitor RMS current (ICOUT_RMS).

Equation 53. ICOUT_⁡RMS=ΔILOUT3≈5.8 A

To meet our design requirements five 1500-µF, aluminum electrolytic capacitors were chosen for the design from United Chemi-Con, part number EKY-160ELL152MJ30S. These capacitors had an ESR of 31 mΩ. The five capacitors when used in series will have an equivalent ESR of 6.2 mOhm which meets are design requirements.

Number of output capacitors:

Equation 54. n = 5

Total output capacitance:

Equation 55. COUT=1500uF×n≈7500 uF

Effective output capacitance ESR:

Equation 56. ESRCOUT=31mΩn=6.2 mΩ

Calculate output capacitor loss (PCOUT):

Equation 57. PCOUT=ICOUT_⁡RMS2×ESRCOUT≈0.21 W

Recalculate remaining Power Budget:

Equation 58. PBUDGET=PBUDGET-PCOUT≈25.2 W