SNVA790A October   2020  – July 2022 LMR36520

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2 Fly-Buck Converter Device Operation
    1. 2.1 Output Current Equations and Considerations
  5. 3LMR36520 Fly-Buck Converter Design
    1. 3.1 Coupled Inductor
    2. 3.2 Primary Output Capacitor
    3. 3.3 Rectifying Diode
    4. 3.4 Secondary Output Capacitor
    5. 3.5 Preload Resistor
    6. 3.6 Zener Diode
    7. 3.7 Snubber Circuit
  6. 4Experimental Results
    1. 4.1 Steady State
    2. 4.2 Secondary Output Voltage
    3. 4.3 Load Transient
    4. 4.4 Start-up
    5. 4.5 Output Current
  7. 5Conclusion
  8. 6References
  9. 7Revision History

3.4 Secondary Output Capacitor

The secondary output capacitor must be able to supply the secondary load current during the on-time of the converter. To calculate the minimum capacitance required, use Equation 35:

Equation 35. C o u t 2 = I o u t 2 × D m a x f s w × ∆ V o u t 2 = 0.5   A × 0.5 400000   H z × 0.033   V = 17.8   u F

A 22-μF capacitor rated at 16 V is selected to account for de-rating. An additional 220-nF, high-frequency capacitor rated at 10 V is also selected to reduce high frequency noise on the secondary output.

Once again, the piecewise linear method is used to approximate the rms current through the secondary output capacitor bank in order to determine the rms current ratings of these components.

Equation 36. i c o u t 2 _ r m s = a 2 + b 2 + a × b 3 × t o f f T + a 2 + a 2 + a × a 3 × t o n T
Equation 37. a = - I o u t 2 = - 0.5   A
Equation 38. b = I s e c ⁡ _ p k - I o u t 2 = 1.5   A - 0.5   A = 1   A
Equation 39. i c o u t 2 _ r m s = 0.645   A r m s