SNVA559C September   2012  – February 2019 LM2574 , LM2575-N , LM2575HV , LM2576 , LM2576HV , LM2577

 

  1.   Switching regulator fundamentals
    1.     Trademarks
    2. 1 Switching Fundamentals
      1. 1.1 The Law of Inductance
      2. 1.2 Transformer Operation
      3. 1.3 Pulse Width Modulation (PWM)
    3. 2 Switching Converter Topologies
      1. 2.1  Buck Regulator
      2. 2.2  Continuous vs Discontinuous Operation
      3. 2.3  Boost Regulator
      4. 2.4  Output Current and Load Power
      5. 2.5  Buck-Boost (Inverting) Regulator
      6. 2.6  Flyback Regulator
      7. 2.7  Generating Multiple Outputs
      8. 2.8  Push-Pull Converter
      9. 2.9  Half-Bridge Converter
      10. 2.10 Full-Bridge Converter
    4. 3 Application Hints for Switching Regulators
      1. 3.1 Capacitor Parasitics Affecting Switching Regulator Performance
        1. 3.1.1 Input Capacitors
        2. 3.1.2 Output Capacitor ESR Effects
        3. 3.1.3 Bypass Capacitors
      2. 3.2 Proper Grounding
      3. 3.3 Transformer/Inductor Cores and Radiated Noise
      4. 3.4 Measuring Output Ripple Voltage
      5. 3.5 Measuring Regulator Efficiency of DC/DC Converters
      6. 3.6 Measuring Regulator Efficiency of Offline Converters
    5. 4 Application Circuits
      1. 4.1 LM2577: A Complete Flyback/Boost Regulator IC
        1. 4.1.1 Increasing Available Load Power in an LM2577 Boost Regulator
      2. 4.2 LM2577 Negative Buck Regulator
      3. 4.3 LM2577 Three-Output, Isolated Flyback Regulator
      4. 4.4 LM2575 and LM2576 Buck Regulators
      5. 4.5 Low Dropout, High Efficiency 5-V/3-A Buck Regulator
    6. 5 References and Related Products
  2.   Revision History

3.1 Capacitor Parasitics Affecting Switching Regulator Performance

All capacitors contain parasitic elements which make their performance less than ideal (see Figure 16).

capacitor_parasitics_snva559.gifFigure 16. Capacitor Parasitics

Summary of Effects of Parasitics:

  • ESR: The equivalent series resistance (ESR) causes internal heating due to power dissipation as the ripple current flows into and out of the capacitor. The capacitor can fail if ripple current exceeds maximum ratings.

    • Excessive output voltage ripple will result from high ESR, and regulator loop instability is also possible. ESR is highly dependent on temperature, increasing very quickly at temperatures below about 10°C.

  • ESL: The effective series inductance (ESL) limits the high frequency effectiveness of the capacitor. High ESL is the reason electrolytic capacitors need to be bypassed by film or ceramic capacitors to provide good high-frequency performance.

    • The ESR, ESL, and C within the capacitor form a resonant circuit, whose frequency of resonance should be as high as possible. Switching regulators generate ripple voltages on their outputs with very high frequency (>10 MHz) components, which can cause ringing on the output voltage if the capacitor resonant frequency is low enough to be near these frequencies.