TIDT328 april   2023

 

  1.   1
  2.   Description
  3.   Features
  4.   Applications
  5. 1Test Prerequisites
    1. 1.1 Voltage and Current Requirements
    2. 1.2 Considerations
    3. 1.3 Dimensions
  6. 2Testing and Results
    1. 2.1 Efficiency Graphs
    2. 2.2 Load Regulation
    3. 2.3 Thermal Images
      1. 2.3.1 Unmodulated 60 VDC - Output
      2. 2.3.2 60 VAC - Output Modulated With 120-Hz Sinus (0 V – 5 V)
      3. 2.3.3 Conclusion
    4. 2.4 Bode Plot at Maximum Duty Cycle
  7. 3Waveforms
    1. 3.1 Switching
      1. 3.1.1 Transistor Q1 Operating in Deep DCM
        1. 3.1.1.1 Drain to Source
        2. 3.1.1.2 Gate to Source
      2. 3.1.2 Diode D3 (Referenced to VOUT)
    2. 3.2 Output Voltage Ripple
    3. 3.3 Input Voltage Ripple
    4. 3.4 Start-Up Sequence
    5. 3.5 Shutdown Sequence
  8.   A Modulating the Output Voltage
    1.     A.1 Revision B
      1.      A.1.1 Bode Plot
      2.      A.1.2 Simulation
      3.      A.1.3 Measured Waveforms
        1.       A.1.3.1 Sinus 40 Hz
        2.       A.1.3.2 Sinus 100 Hz
        3.       A.1.3.3 Sawtooth 1
        4.       A.1.3.4 Sawtooth 2
        5.       A.1.3.5 Pure Triangle
        6.       A.1.3.6 Conclusion
    2.     A.2 Revision C
      1.      A.2.1 Bode Plot
      2.      A.2.2 Measured Waveforms
        1.       A.2.2.1 Sinus 120 Hz
        2.       A.2.2.2 Sawtooth 1
        3.       A.2.2.3 Sawtooth 2
        4.       A.2.2.4 Pure Triangle
      3.      A.2.3 Analysis Capacitor 1 µF, 100 V, X7R, 1206
        1.       A.2.3.1 DC-Bias
        2.       A.2.3.2 Resistance (ESR)
        3.       A.2.3.3 Reactance
        4.       A.2.3.4 Impedance

Description

This automotive single-ended primary inductor converter (SEPIC) using the LM5156-Q1 or LM51561-Q1 devices is designed to support a modulated output voltage, in this design a sinusoidal waveform 60 VPP at up to 120 Hz. Descriptions of two of the dynamic limitations with this reference design follow:

  1. A SEPIC operating in continuous-conduction mode (CCM) is dynamically limited by the right half plane zero (RHPZ)
  2. By using a nonsynchronous rectification at low output voltages, the discharge current at the output capacitor by resistive load gets small, the output voltage cannot follow the desired modulating waveform anymore

The magnetizing inductance is minimized to operate the power stage in discontinuous mode (DCM), moving the RHPZ at the lowest input voltage and maximum load (worst case) beyond 300 kHz. However, the dynamics are still limited by the gain bandwidth of the error amplifier and the Nyquist criteria. A loop bandwidth of 10 kHz is sufficient for only 100 Hz at the output. Furthermore, DCM removes Qrr ringing at the semiconductors and the dual inductor gets fairly small. The disadvantage is the high ripple current resulting in magnetizing losses, AC losses at the winding, and a large output ripple voltage. In this situation this disadvantage is acceptable (for such a low maximum output power of 18 W).
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