SNVAA87 august   2023 LMR38020

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Micro Inverter System
    2. 1.2 Typical Power Tree and Design Requirements
  5. 2Conventional Flyback Design Challenges
    1. 2.1 SSR Design Challenges
    2. 2.2 PSR Design Challenges
  6. 3New Fly-Buck Design
    1. 3.1 LMR38020 Overview
    2. 3.2 Comparison with Conventional Flyback
    3. 3.3 Design Considerations
    4. 3.4 LMR38020 Fly-Buck Design Example
  7. 4Bench Test and Result
    1. 4.1 Start Up
    2. 4.2 Typical Switching Waveforms Under Steady State
    3. 4.3 Efficiency
    4. 4.4 Load Regulation
    5. 4.5 Short Circuit
    6. 4.6 Thermal Performance
  8. 5Summary
  9. 6References

3.2 Comparison with Conventional Flyback

The Fly-Buck™ is also known as the isolated buck converter, where the isolated output is generated by adding a coupled winding to the filter inductor of a buck converter. The circuit on the secondary side looks similar to a Flyback, but the primary side is a synchronous buck converter. This makes Fly-Buck™ naturally primary side regulation, and achieve isolated output regulation effortlessly via winding coupling. Comparing with the conventional PSR Flyback, Fly-Buck™ has the following key advantages and detailed differences comparison between Fly-Buck™ and Flyback are listed in Table 3-1.

  • Fly-Buck™ eliminates the auxiliary sensing winding and provide an accurate primary output. This makes Fly-Buck™ have fewer components count, smaller design size and lower cost. Fly-Buck™ usually has high efficiency comparing with PSR Flyback.
  • Fly-Buck™ has continuous current flow in the primary side due to the synchronous buck configuration, and the maximum voltage across MOSFET drain to source equals input voltage. Therefore, there is no need of snubber circuit. On the contrary, the primary side current in Flyback is discontinuous and the winding generates excessive voltage stress on the MOSFET and snubber is a must-have circuit.
Table 3-1 Comparison Between Fly-Buck and Flyback
Specifications Fly-Buck Flyback
Input The minimum input voltage has to be larger than the primary output voltage, while the secondary output voltage can be higher or lower than input voltage (isolated buck topology). The minimum input voltage can be smaller or larger than the output voltage (isolated buck-boost topology).
Output Primary side output is a non-isolated output and can be only positive.
Secondary side output is isolated, and can be positive or negative.		 Primary side output is an non-isolated output for controller powering and also sensing in PSR.
Secondary side dual outputs are isolated, and can be positive or negative.
Switch Stress MOSFETs are rated at Vinmax. Need consider flyback voltage on the primary low-side MOSFET, rated at Vinmax + (Vout / N) (where N = Ns / Np), which means higher switch stress.
Transformer 3-winding transformer, smaller, lower leakage. 4-winding transformer, bigger, larger leakage.
Size Smaller design size, smaller transformer size (typically). Larger design size, larger transformer size (typically).
Cost Fewer components count, lower cost. More components count, higher cost.
Performance Good regulation achievable, +/-5% on both primary and secondary outputs.
Relatively better power transfer efficiency.		 PSR has poorer cross regulation performance. High output accuracy can be achieved if using SSR with optocoupler, but large deviation under light load condition.
Less efficient as only utilizing off-time transferring power.

About the input specification of Fly-Buck™, it needs to be kept in mind that the minimum input voltage has to be higher than the targeted primary output voltage. Very often this results in a high duty cycle when the input voltage is low. That needs attention because high duty cycle results in very high peak current during power transferring to the secondary side.

For example, in micro inverter application, the output voltage of the PV panels (as input voltage of the auxiliary power) could be very low in dawn and dusk as the sun light is dim, or at some time the PV panels are under heavy shade. So, it needs more considerations on the switch MOSFET selection.