digitally controlled power supply - a perspective on slope compensation
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copyright 2011 controltrix corp www. controltrix.com
Digitally controlled power supply
Perspective on Slope Compensation
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copyright 2011 controltrix corp www. controltrix.com
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 !!!!
Slope compensation basics
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In depth analysis : http://www.ti.com/lit/an/slua101/slua101.pdf
Slope compensation basics.
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Sub - harmonic oscillations
Expected duty: 83.3%, ripple: 1A, valley current =4A, duty max = 95%
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Use slope compensation
Std Solution
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Slope choicesMerits De-Merits
m = m2 / 2 • Behaves like average current mode control
• Convergence issues at higher duty ratios (~ 0.9)
m = 2 * m2 • Guaranteed convergence
• Voltage mode control equivalent• Poor dynamic line regulation
m = m2 • Fastest single cycle convergence
• Not true current mode control. • Poor dynamic line regulation
copyright 2011 controltrix corp www. controltrix.com
• 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
Digital Power adopting analog legacy
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• 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)
Other Methods
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• 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
Proposed method Features
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• 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
Proposed method Features.
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• May need extra PWM channel. (Depends on DSC peripheral availability)• Bounded overshoot above current limit.• Periodic sub harmonics still present (smaller amplitude)
Tradeoffs
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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
Simulation ResultsBaseline : valley min 0.5 A (ideal 4A)
Proposed : valley min 3.8 A (ideal 4A)
