SLVUBB4B November   2017  – February 2023

 

  1.   Abstract
  2.   Trademarks
  3. 1 Topologies Window
  4. 2 FET Losses Calculator
  5. 3 Load Step Calculator
  6. 4 Capacitor Current Sharing Calculator
  7. 5 AC/DC Bulk Capacitor Calculator
  8. 6 RCD-Snubber Calculator for Flyback Converters
  9. 7 RC-Snubber Calculator
  10. 8 Output Voltage Resistor Divider
  11. 9 Dynamic Analog Output Voltage Scaling
  12. 10Dynamic Digital Output Voltage Scaling
  13. 11Unit Converter
  14. 12Loop Calculator
    1. 12.1 Inputs
    2. 12.2 Transfer Functions
      1. 12.2.1  Output Impedance Transfer Function
      2. 12.2.2  Transfer Function VMC Buck Power Stage
      3. 12.2.3  Transfer Function CMC Buck Power Stage
      4. 12.2.4  Transfer Function CMC Boost Power Stage
      5. 12.2.5  Transfer Function CMC Inverting Buck-Boost Power Stage
      6. 12.2.6  Transfer Function CMC Forward Power Stage
      7. 12.2.7  Transfer Function CMC Flyback Power Stage
      8. 12.2.8  Transfer Function Closed Loop
        1. 12.2.8.1 Transfer Function Type II Compensation Network
        2. 12.2.8.2 Transfer Function Type II Transconductance Compensation Network
        3. 12.2.8.3 Transfer Function Type III Compensation Network
      9. 12.2.9  Transfer Function Isolated Type II Compensation Network With a Zener Clamp
      10. 12.2.10 Transfer Function Isolated Type II Compensation Network Without a Zener Clamp
  15. 13Filter Designer
    1. 13.1 Impedances
    2. 13.2 Transfer Functions
    3. 13.3 Filter Output Impedance
    4. 13.4 Damping Factor
  16. 14Additional Information
  17. 15Revision History

6 RCD-Snubber Calculator for Flyback Converters

In Flyback converters the output voltage is reflected from the secondary to the primary side. Additionally, parasitics caused by the layout and the Flyback transformer leakage inductance can cause a voltage spike followed by ringing when the MOSFET is turning off. The voltage spike and the ringing can be limited by implementing an RCD-snubber circuit in parallel to the primary winding. The energy of the high-frequency ringing is dissipated in the RCD-network. The RCD-Snubber Calculator for Flyback converters in Power Stage Designer helps the designer choose the starting values for snubber resistor and capacitor based on the user’s inputs, which follow:

  • Sum of output voltage and rectifier voltage
  • Flyback transformer turns ratio
  • Leakage inductance
  • Maximum primary current
  • Switching frequency
  • Permitted voltage overshoot as a factor
  • Snubber capacitor voltage ripple in percent

#SLVUBB42043 shows the RCD-Snubber Calculator for Flyback Converters window.

GUID-7D1307DC-90A3-4A4C-B145-22D35F762140-low.png Figure 6-1 RCD-Snubber Calculator for Flyback Converters Window

Vsnub is the reflected output voltage plus the permitted overshoot caused by transformer leakage inductance and switching node parasitics. Thus Ksnub has a value greater than 1. TI recommends a value of 1.5 for most applications, permitting 50% overshoot (see [1]). See #SLVUBB45069.

Equation 14. V s n u b = K s n u b × N p N s × V o u t + V f

Starting Snubber resistance:

Equation 15. R s n u b = V s n u b 2 1 2 × L l e a k × I m a x , p r i 2 × V s n u b V s n u b - N p N s × V o u t + V f × f s w i t c h

Starting Snubber capacitance:

Equation 16. C s n u b = V s n u b ∆ V s n u b × V s n u b × R s n u b × f s w i t c h