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

5 QA, QB, QC, QD FET Selection

The FETs to drive the HBridge (QA..QD) need to selected based on maximum drain to source voltage (VdsQA_max) and peak drain to source current (IdsQA_max).

Equation 30. VdsQA_max≥VINMAX=410 V 
Equation 31. IdsQA_max≥IPP=3.3 A

The FETs then need to be selected based on efficiency goals and FET power dissipation (PQA) and is a trial an error process. Equations 32 through 38 are used to estimate PQA based on FET data sheet parameters. To meet our efficiency goals, we selected a 20 A, 650 V, CoolMOS FETs from Infineon that had an estimated PQA of 2.1 W and would enable us to hit our efficiency goals.

In this design, to meet efficiency and voltage requirements 20 A, 650 V, CoolMOS FETs from Infineon were chosen for QA..QD.

FET drain to source on resistance:

Equation 32. R d s ( o n ) Q A = 0.220   Ω

FET Specified COSS:

Equation 33. C O S S _ ⁡ Q A _ ⁡ S P E C = 780   p F

Voltage across drain-to-source (VdsQA) where COSS was measured, data sheet parameter:

Equation 34. V d s Q A = 25 V

Calculate average Coss [2]:

Equation 35. C O S S _ ⁡ Q A _ ⁡ A V G = C O S S _ ⁡ Q A _ ⁡ S P E C V d s Q A V I N M A X ≈ 193   p F

QA FET gate charge:

Equation 36. Q A g = 15   n C

Voltage applied to FET gate to activate FET:

Equation 37. V g = 12   V

Calculate QA losses (PQA) based on Rds(on)QA and gate charge (QAg):

Equation 38. P Q A = I P R M S 2 × R d s ( o n ) Q A + 2 × Q A g × V g × f s 2 ≈ 2.1   W

Recalculate power budget:

Equation 39. P B U D G E T = P B U D G E T - 4 × P Q A ≈ 29.7   W