Digital Control Concepts Digital Control Concepts For Power Supply Engineers For Power Supply Engineers Robert V. White Staff Engineer Worldwide Technology Group
Presentation Overview Presentation Overview Presentation Overview � Control Techniques – Traditional Analog – Traditional Digital – Modern Digital � Examples Of Digital Advantages – Dead Time Optimization – Automatic Compensation Tuning Digital Control ≠ Digital Power Management
Analog PWM Analog PWM Analog PWM
Analog PWM Analog PWM Analog PWM Control Loop Characteristics Voltage Scaling Switch Timing Control Reference Error Determination
Classical Digital PWM Classical Digital PWM Classical Digital PWM
Classical Digital PWM Classical Digital PWM Classical Digital PWM Typical DSP/Processor => Lots Of MIPs => $$$ Resolution+ Counter Approach Accuracy + + High Precision Speed = = Very Fast Clock $$$ = $$$
Re- -Thinking Digital Control For Power Thinking Digital Control For Power Re Re-Thinking Digital Control For Power
Step 1: Digitize Only The Error Step 1: Digitize Only The Error Step 1: Digitize Only The Error
Step 1: Digitize Only The Error Step 1: Digitize Only The Error Step 1: Digitize Only The Error How Many Bits Are Needed? 3? 5? 7?
Thinking About Error Bins Thinking About Error Bins Thinking About Error Bins
Non- -Linear Error Bins Linear Error Bins Non Non-Linear Error Bins +10 × DELTA Opportunities For: +3 ERROR BIN Lower Cost Digitization? +5 × DELTA Improved +2 ERROR BIN Nonlinear Control? +2.0 × DELTA +1 ERROR BIN +0.5 × DELTA VOUT NOMINAL ZERO ERROR BIN –1 ERROR BIN –2.0 × DELTA
Step 2: Simplify Calculation Engine Step 2: Simplify Calculation Engine Step 2: Simplify Calculation Engine Lookup Table, Fixed Configuration Filters, State Machines $10 DSP
Step 3: Low Cost Digital PWM Step 3: Low Cost Digital PWM Step 3: Low Cost Digital PWM Example: Combination Counter And Delay Line
The Point? Re-Thinking Digital Control For Power Conversion Is Lowering Cost And Simplifying Design
Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling COARSE RESOLUTION DPWM OUTPUT VOLTAGE POSSIBLE OUTPUT VOLTAGES D/A ERROR BINS +3 ERROR BIN +2 ERROR BIN +1 ERROR BIN ZERO ERROR BIN –1 ERROR BIN –2 ERROR BIN –3 ERROR BIN
Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling N+1 DUTY CYCLE N PWM VOUT NOMINAL
Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling FINE RESOLUTION DPWM OUTPUT VOLTAGE POSSIBLE OUTPUT VOLTAGES D/A ERROR BINS +3 ERROR BIN +2 ERROR BIN +1 ERROR BIN ZERO ERROR BIN –1 ERROR BIN –2 ERROR BIN –3 ERROR BIN
Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling FINE RESOLUTION DPWM OUTPUT VOLTAGE POSSIBLE OUTPUT VOLTAGES D/A ERROR BINS +3 ERROR BIN Rule: Minimum Δ VOUT < Error Bin Size +2 ERROR BIN +1 ERROR BIN For A Buck Regulator: ZERO ERROR BIN Maximum Setpoint Error = ±0.1% => –1 ERROR BIN Duty Cycle Resolution < 0.2% => –2 ERROR BIN 9 Bits Time Resolution Per Cycle –3 ERROR BIN
Sampling the Output Sampling the Output Sampling the Output Once Per Cycle Can Give Good Information On Average Value Computation Time! Oversampling Is Not Necessarily Helpful
Everyone Is Terrified Of… Everyone Is Terrified Of… Everyone Is Terrified Of… Z Transforms Programming ∞ = ∑ [ ] [ ] main(){ − n V z v n z c c printf(“hello,world\n”); =−∞ n } REALITY! Converter Designers Will Work Through Computer Based GUI Interfaces
Winning Application #1: Winning Application #1: Winning Application #1: Efficiency Optimization Efficiency Optimization Efficiency Optimization � Digital Control Offers Opportunities To Optimize Operation To Minimize Losses � Example: Buck Converter Dead Time (CoPEC) – Start With Excessive Deadtime – Slowly Minimize While Watching Duty Cycle – Minimum Duty Cycle => Minimum Losses � Example: On Bus Voltage – Monitor Load Of POLs Powering A Board – Light Load? Lower Bus Voltage – Heavy Load? Increase Bus Voltage
Winning Application #2: Winning Application #2: Winning Application #2: Self Tuning Control Loops Self Tuning Control Loops Self Tuning Control Loops � A Converter With A Digital Control Loop Can Act As Its Own Network Analyzer � Based On Measurement Of The System, A Device Can Optimize Its Loop Response � POLs – Compensate For Actual Capacitive Loading – Adjust As Components Age � Front End Power Supplies – Compensation For Initial Conditions – Compensate For Configuration Changes Over Time
Winning Application #3: Winning Application #3: Winning Application #3: Digital Power Management Digital Power Management Digital Power Management � Digital Control ≠ Digital Power Management – But They Marry Very Well! – Passing Digital Values From User’s GUI Directly To A Digital Control Loop Will Simplify IC Design And Lower Cost � Improved Fault Management – Digital Control Enables Graceful Transition Between Normal And Abnormal Operating Modes
References – – For More Information For More Information References References – For More Information � Colorado Power Electronics Center (CoPEC) – In My Opinion, Leading The Way In Digital Control And Silicon Integration Research Useful To Industry – Much Of This Presentation Based On Their Work – http://ece-www.colorado.edu/~pwrelect/ � Seth Sanders/UC Berkeley – Limit Cycling Paper Is Classic – http://www-power.eecs.berkeley.edu/sanders.html
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