Digitally controlled power supply - a perspective on slope compensation
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Digitally controlled power supply - a perspective on slope compensation

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Digitally controlled power supply - a perspective on slope compensation Presentation Transcript

  • 1. Digitally controlled power supply Perspective on Slope Compensation www.controltrix.comcopyright 2011 controltrix corp www. controltrix.com
  • 2. Slope compensation basics Switched mode power converter operating under: • Peak current mode control • Fixed PWM frequency • Continuous conduction mode • Duty > 50% Results in Sub-harmonic oscillations and instability !!!!copyright 2011 controltrix corp www. controltrix.com
  • 3. Slope compensation basics. In depth analysis : http://www.ti.com/lit/an/slua101/slua101.pdfcopyright 2011 controltrix corp www. controltrix.com
  • 4. Sub - harmonic oscillations Expected duty: 83.3%, ripple: 1A, valley current =4A, duty max = 95%copyright 2011 controltrix corp www. controltrix.com
  • 5. Std Solution Use slope compensationcopyright 2011 controltrix corp www. controltrix.com
  • 6. Slope choices Merits De-Merits m = m2 / 2 • Behaves like average • Convergence issues at current mode control higher duty ratios (~ 0.9) m = 2 * m2 • Guaranteed • Voltage mode control convergence equivalent • Poor dynamic line regulation m = m2 • Fastest single cycle • Not true current mode convergence control. • Poor dynamic line regulationcopyright 2011 controltrix corp www. controltrix.com
  • 7. Digital Power adopting analog legacy • Analog circuit with switch or extra DSP PWM • Fixed slope value (m) • Wide range input and output fixed • slope m is suboptimal • Finding m1, m2 hard with inductance changes with loadcopyright 2011 controltrix corp www. controltrix.com
  • 8. Other Methods • Find intersection point of slope m1 and m in Software • Defeats peak current mode control • Sub harmonic oscillations still present • Needs noise free ADC measurement of valley current (hard)copyright 2011 controltrix corp www. controltrix.com
  • 9. Proposed method Features • Only Software / firmware no extra hardware • Few extra mips (<1us extra execution per cycle @40 mips / 40MHz proc) • Wide input / output voltage acceptable • Maintains nearly stable effective dutycopyright 2011 controltrix corp www. controltrix.com
  • 10. Proposed method Features. • Supports all duty ratios • Reduces instability (current ripple deviation) by factor of 10X or more • Higher duties more improvement factor • True peak current mode control dynamics • Patent pendingcopyright 2011 controltrix corp www. controltrix.com
  • 11. Tradeoffs • May need extra PWM channel. (Depends on DSC peripheral availability) • Bounded overshoot above current limit. • Periodic sub harmonics still present (smaller amplitude)copyright 2011 controltrix corp www. controltrix.com
  • 12. Test conditions • 4A valley expected at steady state. • 5A peak (current reference). • 83.3% duty. • 95% max duty. • 1 unit period. • Figure of merit : minimum valley current.copyright 2011 controltrix corp www. controltrix.com
  • 13. Simulation Results Baseline : valley min 0.5 A (ideal 4A) Proposed : valley min 3.8 A (ideal 4A)copyright 2011 controltrix corp www. controltrix.com
  • 14. Thank You consulting@controltrix.comcopyright 2011 controltrix corp www. controltrix.com