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

3.6 Measuring Regulator Efficiency of Offline Converters

Offline converters are powered directly from the AC line, by using a bridge rectifier and capacitive filter to generate an unregulated DC voltage for conversion (see Figure 13 and Figure 14).

Measuring the total power drawn from the AC source is fairly difficult because of the power factor. If both the voltage and current are sinusoidal, power factor is defined as the cosine of the phase angle between the voltage and current waveforms.

The capacitive-input filter in an offline converter causes the input current to be very non-sinusoidal. The current flows in narrow, high-amplitude pulses (called Haversine pulses) which requires that the power factor be redefined in such cases.

For capacitive-input filter converters, power factor is defined as:

Equation 12. P.F. = PREAL / PAPPARENT

The real power drawn from the source (PREAL) is the power (in Watts) which equals the sum of the load power and regulator internal losses.

The apparent power (PAPPARENT) is equal to the RMS input current times the RMS input voltage. Rewritten, the importance of power factor is shown

Equation 13. IIN (RMS) = PREAL / (VIN (RMS) × PF)

The RMS input current that the AC line must supply (for a given real power in Watts) increases directly as the power factor reduces from unity. Power factor for single-phase AC-powered converters is typically about 0.6. If three-phase power is used, the power factor is about 0.9.

If the efficiency of an offline converter is to be measured, power analyzers are available which will measure and display input voltage, input power, and power factor. These are fairly expensive, so they may not be available to the designer.

Another method which will give good results is to measure the power after the rectifier bridge and input capacitor (where the voltage and current are DC). This method is shown in Figure 24.

measuring_input_power_in_offline_converter_snva559.gifFigure 24. Measuring Input Power in Offline Converter

The current flowing from CIN to the converter should be very nearly DC, and the average value can be readily measured or approximated (see previous section).

The total power drawn from the AC source is the sum of the power supplied by CIN (which is VIN × IDC) and the power dissipated in the input bridge rectifier. The power in the bridge rectifier is easily estimated, and is actually negligible in most offline designs.