TIDUEJ8C January   2019  – May 2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 MSPM0G1506
      2. 2.3.2 LMG2100R044
      3. 2.3.3 INA241
      4. 2.3.4 TPSM365
      5. 2.3.5 TMP303
    4. 2.4 System Design Theory
      1. 2.4.1 MPPT Operation
      2. 2.4.2 Buck Converter
        1. 2.4.2.1 Output Inductance
        2. 2.4.2.2 Input Capacitance
      3. 2.4.3 Current Sense Amplifier
        1. 2.4.3.1 Shunt Resistor Selection
        2. 2.4.3.2 Current Measurement Resolution
        3. 2.4.3.3 Shunt Resistor Power Dissipation
      4. 2.4.4 Switching Regulator
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Required Hardware and Software
      1. 3.1.1 Hardware
        1. 3.1.1.1 TIDA-010042
        2. 3.1.1.2 ITECH-IT6724H
        3. 3.1.1.3 Chroma, 63107A
      2. 3.1.2 Software Flow
    2. 3.2 Testing and Results
      1. 3.2.1 Test Setup
      2. 3.2.2 Test Results
  10. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Loop Inductances
      2. 4.3.2 Current Sense Amplifiers
      3. 4.3.3 Trace Widths
      4. 4.3.4 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
    7. 4.7 Software Files
  11. 5Related Documentation
    1. 5.1 Trademarks
    2. 5.2 Support Resources
  12. 6About the Author
  13. 7Revision History

2.4.2.2 Input Capacitance

Select input capacitors carefully to both reduce the size and satisfy the big ripple current capability (for more information, see the How to select input capacitors for a buck converter analog applications journal).

To get a satisfied MPPT effect, such as 99.5% of maximum power tracking, make sure the input ripple voltage is small. For many panels, when the Vpanel is within 97.5% – 102.5% of the Vmpp, the output power of the panel is above 99.5% of maximum power. In a 12V battery system, make sure the Vmpp of the panel is higher than 12V, so the input ripple voltage can be within 0.3V. In a 24V battery system, make sure the Vmpp of the panel is higher than 24V, so the input ripple voltage can be within 0.6V. So, 0.3V is taken as the maximum input ripple voltage (ΔVin).

TIDA-010042 Input Current WaveformFigure 2-12 Input Current Waveform

The AC current flowing through the input capacitors results in the input voltage ripple. Even the majority of the ripple current goes through MLCC, thanks to the low equivalent series resistance (ESR), ripple voltage results from this can be ignored. The remaining ripple current flows through the electrolytic capacitor, if there is any in the system. Although electrolytic capacitor has much bigger ESR, the AC current is relatively small. As a result, the overall effect on the input voltage ripple is negligible.

Use Equation 3 to estimate the required effective capacitance that meet the ripple voltage requirement. At 50% duty cycle, the CIN is biggest.

Equation 3. CIN≥D×(1-D)×IO∆Vin×fsw

Where Io is 16A and fsw is 250kHz, CIN needs to be bigger than 53μF. Considering DC bias effect of the MLCC as the voltage increases, the actual value taken needs to be larger depending on the practical situation.

Besides, the input capacitors also need to meet the thermal stress caused by the ripple current, the bigger footprint, the lower temperature rise. Use Equation 4 to calculate the root mean square (RMS) current of the input ripple current.

Equation 4. Iin_rms=IO×D×1-D+112×(VOL×fsw×IO)2×(1-D)2×D

Duty cycle has a significant impact on the input RMS ripple current. Figure 2-13 is a plot of , from which the largest ripple current RMS can be observed. The largest ripple current occurs when the duty cycle is 0.5. The maximum value of Iin_rms is 8.2A. To reduce the temperature rise of the MLCC, the 1210 footprint is chosen. Meanwhile, it is better to parallel multiple capacitors with small capacity than just use one with bigger capacity.

TIDA-010042 Input RMS, Load Current Ratio vs Duty CycleFigure 2-13 Input RMS, Load Current Ratio vs Duty Cycle

Place additional small MLCCs with low equivalent series inductance (ESL) and low ESR as close as possible to the input side of the FETs, especially using GaN devices with high di/dt and dv/dt slope. These MLCCs can greatly alleviate overshoot of the switching node waveform without sacrificing efficiency.

Bulk capacitors like aluminum electrolytic capacitors can also be added to satisfy the transient response if the response speed of the system is important. Because of the high ESR of electrolytic capacitors, the ripple current can be approximated by dividing the input ripple voltage by the ESR. Also, the waveform is triangular, so the RMS value can be estimated with Equation 5.

Equation 5. Ibulk_rms=123×∆VinESR

Care is needed when selecting a bulk capacitor due to its low tolerance for RMS current.