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

3.3 Rectifying Diode

The rectifying diode must be rated to handle the reverse bias voltage during the on-time as well as the secondary load current.

The reverse voltage across the diode during the off-time is:

Equation 33. V D = V o u t 2 + N 2 N 1 × V i n - V o u t 1 = 3.3   V + 1 1 × 36   V - 5   V = 34.3   V

The forward voltage drop across the diode when it is conducting is also important to consider as it limits the maximum secondary output voltage. Neglecting conduction losses associated with the DCR of the inductor, in order to achieve 3.3 V at the secondary output, the forward voltage drop across the diode when it is fully conducting should be:

Equation 34. V f = N 2 N 1 × V o u t 1 - V o u t 2 = 1 1 × 5   V - 3.3   V = 1.7   V

The MURA160 ultra-fast rectifying diode is chosen because of its 1-V forward voltage drop at 500 mA and low junction capacitance. Schottky diodes are commonly used as the secondary rectifier diode due to their low forward voltage drop and fast reverse recovery time. Because this application calls for a higher forward voltage drop to achieve the desired secondary output voltage, the ultra-fast rectifier diode is preferred.

Another easily overlooked yet important parameter is the diode junction capacitance. This junction capacitance forms a resonant LC tank with the leakage inductance from the secondary winding of the coupled inductor which causes high frequency ringing to occur on the switch node when the high-side MOSFET changes state from OFF to ON. This is another reason why an ultra-fast rectifier diode is chosen instead of the typical Schottky diode for this application. The smaller junction capacitance of the ultra-fast diode results in smaller peak overshoot and higher resonant frequency making it easier to filter. This ringing can be mitigated with a simple RC snubber in parallel with the diode. See Section 3.7 for more information.