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

1.2 Typical Power Tree and Design Requirements

Figure 1-4 shows a typical power tree of micro inverter.

GUID-20230807-SS0I-WSHJ-6LRS-NRKN0LM0HKS0-low.svgFigure 1-4 Typical Power Tree of Micro Inverters Auxiliary Power Supply

Outputs of all PV panels are connected together through the Oring diodes as the input source of the auxiliary power supply. Based on the voltage characteristics of the PV panel, the input voltage range is normally 16 to 60 volts.

Because the auxiliary power supply needs to provide multiple isolated voltages for the inverter gate drivers, the Flyback topology is commonly used, not only because it can easily realize multiple outputs, but also its counts of external components that can achieve competitive bill-of-materials (BOM) cost. For the single-phase inverter gate driver, the conventional flyback converter normally has one primary winding and three secondary windings, of which two windings for the upper and lower gate drivers, and one for the primary circuit use that includes MPPT dc-dc stage's non-isolated driver, relay and other system circuits.

Due to Flyback converter’s cross regulation issue, other system circuits are not powered by the Flyback converter. Additional dc/dc converter or LDOs either on the primary side or secondary side are used to provide the required tightly regulated bias supply voltages to those circuits. Usually, a primary buck is used to produce 5 V for the buffer, amps, Comps and hall circuits etc. A secondary buck converter is used to produce 3.3 V and 1.2 V for the DSP. An LDO is used as well to provide 3.3 V output without switching noise for the wireless communication module (such as Sub-1G).

Table 1-1 lists a 7-watts design requirements example of the auxiliary power supply.

Table 1-1 Design Requirements Example of the Auxiliary Power Supply
Specifications Requirements
Input Voltage Range 16 V approximately 60 V
Output 1 12 V/5 W (non-isolated)
Output 2 12 V/1 W (isolated)
Output 3 12 V/1 W (isolated)
Thermal Allows max 30°C up above Ta (Ta max = 80°C).
Transformer Core Prefer EP10 or smaller, accepts max EP13.
Frequency Target around 300 KHz (250 approximately 400KHz).
Efficiency > 85%
Cross Regulation < 10%
Reliability Solar products need high reliability and long-life time

The following section includes the conventional Flyback design based on the above design requirements.