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
copyright 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 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)
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.
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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
copyright 2011 controltrix corp www. controltrix.com