Digitally controlled power supply - a perspective on slope compensation

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Digitally controlled power supply - a perspective on slope compensation

  1. 1. Digitally controlled power supply Perspective on Slope Compensation www.controltrix.comcopyright 2011 controltrix corp www. controltrix.com
  2. 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. 3. Slope compensation basics. In depth analysis : http://www.ti.com/lit/an/slua101/slua101.pdfcopyright 2011 controltrix corp www. controltrix.com
  4. 4. Sub - harmonic oscillations Expected duty: 83.3%, ripple: 1A, valley current =4A, duty max = 95%copyright 2011 controltrix corp www. controltrix.com
  5. 5. Std Solution Use slope compensationcopyright 2011 controltrix corp www. controltrix.com
  6. 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. 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. 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. 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. 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. 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. 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. 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. 14. Thank You consulting@controltrix.comcopyright 2011 controltrix corp www. controltrix.com

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