This document discusses slope compensation in digitally controlled power supplies. Slope compensation is needed to prevent sub-harmonic oscillations when operating in peak current mode control with duty cycles over 50%. The document proposes a new software-based method for dynamically adjusting slope compensation that maintains stability across all duty ratios without requiring extra hardware. Simulation results show the proposed method reduces current ripple deviation by over 10x compared to standard fixed-slope approaches.

1. Digitally controlled power supply
Perspective on Slope Compensation
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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 !!!!
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3. Slope compensation basics.
In depth analysis :
http://www.ti.com/lit/an/slua101/slua101.pdf
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4. Sub - harmonic oscillations
Expected duty:
83.3%, ripple: 1A, valley current =4A, duty max = 95%
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5. Std Solution
Use slope compensation
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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
regulation
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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 load
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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)
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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 duty
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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 pending
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11. Tradeoffs
• May need extra PWM channel.
(Depends on DSC peripheral availability)
• Bounded overshoot above current limit.
• Periodic sub harmonics still present (smaller amplitude)
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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.
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13. Simulation Results
Baseline : valley min 0.5 A (ideal 4A)
Proposed : valley min 3.8 A (ideal 4A)
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14. Thank You
consulting@controltrix.com
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