SLVAF95 april   2023 TPS7H5001-SP

 

  1.   Abstract
  2.   Trademarks
  3.   Description
  4.   Features
  5.   Applications
  6. 1System Overview
    1. 1.1 Block Diagram
    2. 1.2 Design Considerations
    3. 1.3 System Design Theory
      1. 1.3.1 Switching Frequency
      2. 1.3.2 Transformer
      3. 1.3.3 RCD and Diode Clamp
      4. 1.3.4 Output Diode and MOSFET
      5. 1.3.5 Output Filter and Capacitance
      6. 1.3.6 Compensation
      7. 1.3.7 Controller Passives
  7. 2Test Results
    1. 2.1 Testing and Results
      1. 2.1.1 Test Setup
      2. 2.1.2 Test Results
        1. 2.1.2.1 Efficiency
        2. 2.1.2.2 Frequency Response
        3. 2.1.2.3 Thermal Characteristics
        4. 2.1.2.4 Output Voltage Ripple
        5. 2.1.2.5 Load Step
        6. 2.1.2.6 Start-Up
        7. 2.1.2.7 Shutdown
        8. 2.1.2.8 Component Stresses
  8. 3Design Files
    1. 3.1 Schematics
    2. 3.2 Bill of Materials
    3. 3.3 Assembly Drawings
  9. 4Related Documentation

1.3.5 Output Filter and Capacitance

For most designs, a ripple voltage is picked and the output capacitance is figured out from that value. The TPS7H5001-SP design started similar to that using Equation 19 through Equation 22.

Equation 19. C o u t > I o u t × D V R i p p l e × f o s c
Equation 20. C o u t > 10   A × 0 . 33 50   m V × 500   k H z = 133   μ F
Equation 21. C o u t > Δ I s t e p 2 π × Δ V o u t × f c o
Equation 22. C o u t > 10   A 2 π × 0 .15   V × 10 . 0   k H z = 1   m F

A value of around 1145 µF was chosen to keep output voltage ripple low. Note that the output voltage ripple in the design was further decreased by adding an output filter and by adding an inductor after a small portion of the output capacitance. Six ceramic capacitors were picked to be placed before the output filter and then the large tantalum capacitors with some small ceramics were added to be part of the output filter. The initial ceramics help with the initial current ripple, but have a very large output voltage ripple. This voltage ripple is attenuated by the inductor and capacitor combination placed between the ceramic capacitors and the output. Equation 23 through Equation 28 allow for finding the amount of attenuation that comes from a specific output filter inductance. An inductance of 500 nH was chosen to attenuate the output voltage ripple.

Equation 23. F r e s o n a n t = 1 2 π × L F i l t e r × C o B u l k
Equation 24. F r e s o n a n t = 1 2 π × 0 . 5   n H × 1127   μ F = 6 . 7   k H z
Equation 25. F Z e r o = 1 2 π × C o B u l k × E S R o B u l k
Equation 26. F Z e r o = 1 2 π × 1127   μ F × 0 . 009   Ω = 15 . 69   k H z
Equation 27. A t t e n u a t i o n f s w = 40 × log 10 f o s c f r e s o n a n t - 20 × log 10 f o s c f z e r o
Equation 28. A t t e n u a t i o n f s w = 40 × log 10 200   k H z 6 . 7   k H z - 20 × log 10 200   k H z 15 . 69   k H z   = 36 . 88   d B

Sometimes the output filter can cause peaking at high frequencies, this can be damped by adding a resistor in parallel with the inductor. For the UC1843A-SP design, 0.5 Ω was used as a very conservative value. Calculate the resistance needed to damp the peaking using Equation 29 through Equation 32.

Equation 29. ω o = 2 ( C o C e r m + C o B u l k ) L F i l t e r × C o C e r m × C o B u l k
Equation 30. ω o = 2 ( 19   μ F + 1127   μ F ) 500   n H × 19   μ F × 1127   μ F = 463   k H z
Equation 31. R F i l t e r = R o × L F i l t e r × ( C o C e r m + C o B u l k ) - L F i l t e r ω o R o × ( C o C e r m + C o B u l k ) ω o - L F i l t e r × C o C e r m
Equation 32. R F i l t e r = 0 . 5 × 500   n H × ( 19   μ F + 1127   μ F ) - 500   n H 463   k H z 0 . 5 × ( 19   μ F + 1127   μ F ) 463   k H z - 500   n H × 19   μ F = 0 . 232   Ω