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

3.5 Preload Resistor

A preload resistor can be placed in parallel with the secondary output capacitor bank in order to minimize overshoot above the target output voltage, especially at higher input voltages. A good starting point for this resistor value is to choose a resistor that will discharge around 5 mA of current when at the target output voltage. The exact value of this resistor will need to be experimentally determined based on the given application. The power consumed by this preload resistor can be calculated using:

Equation 40. P p r e l o a d = I 2 R p r e l o a d

The preload resistor should be properly sized in order to properly handle the expected power dissipation.