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

2.2 PSR Design Challenges

As shown in Figure 2-4, Compared to SSR control, the PSR control method is much simpler which eliminates the optocoupler, TL431, and related resistors and capacitors circuit.

GUID-20230807-SS0I-7XM1-MWVK-CTLXXSTDJPKF-low.svgFigure 2-4 Simplified PSR Circuit Schematic

As shown in Figure 2-5, it saves BoM cost and PCB size, and the low counts of components increase mean time between failures (MTBF). In addition, for applications with high surge or isolation voltage requirements, reducing the number of components crossing the isolation barrier reduces the number of areas that could potentially break down, and also reduces the isolation voltage requirements of the components, which also has lowers total cost.

GUID-20230807-SS0I-FJWC-JN9D-2PFLCHDWDFFC-low.svgFigure 2-5 PSR Control Example Circuit Based on UCC28704

However, SSR control also has some drawbacks. Most PSR controllers use knee point sampling for feedback, which is a method that samples the voltage at the time when the inductor current is at its lowest value. So, the reflected output voltage sampling on the auxiliary winding occurs only once in each PWM cycle and in between switching cycles there is no monitoring of the voltage value. Therefore, transient response is slower than that of SSR control, where the output voltage is constantly monitored.

In addition, PSR has poorer cross regulation performance than that of SSR in multiple outputs application scenario, especially the loads attached to each winding vary significantly. Because for PSR controller’s crossing regulation, the most heavily loaded output is typically chosen to implement the feedback and determines the control loop’s response. The other outputs may be poorly regulated if lightly loaded.

In short, conventional Flyback designs, either SSR or PSR could bring some design challenges for micro inverter application. Moreover, for other issues in common, Flyback controller normally requires an extra auxiliary winding for controller powering and the switching frequency is usually less than 150KHz. So, the big transformer size could be challenging for micro inverter requiring small size. Fortunately, TI has an alternative design that can solve these challenges will be discussed in the following sections.