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

1.3.3 RCD and Diode Clamp

Using a resistor and capacitor are commonly used for the clamp for flybacks, the design philosophy for how to pick resistor and capacitor values is covered. The resistor and capacitor is generally a value that is found through testing, but starting values can be obtained. To figure out the resistor and capacitor needed for the RCD clamp, first pick how much the node is allowed to overshoot. Use Equation 14 to find the voltage of the clamp.

Equation 14. V c l a m p = K c l a m p × N p s × ( V o u t + V D i o d e )

Kclamp is recommended to be 1.5, because this value allows for only around 50% overshoot. Knowing the parasitic inductance of the transformer and how much the snubber voltage is allowed to change over the switching cycle, allows the user to determine the starting values for the resistor and capacitor using Equation 15 and Equation 16.

Equation 15. R c l a m p = V c l a m p 2 1 2 × L l e a k a g e × I P r i P e a k 2 × V c l a m p V c l a m p - N p s × ( V o u t + V D i o d e ) × f o s c
Equation 16. C c l a m p = V c l a m p Δ V c l a m p × V c l a m p × R c l a m p × f o s c

A starting value of 10% is generally used for ΔVclamp. Testing determined that a resistor value of 1 kΩ and a capacitor value 15 nF was required.