Digital Control Concepts Digital Control Concepts For Power Supply - - PowerPoint PPT Presentation

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

Voltage Scaling Error Determination Control Loop Characteristics Switch Timing Control Reference

Classical Digital PWM Classical Digital PWM Classical Digital PWM

Classical Digital PWM Classical Digital PWM Classical Digital PWM

Resolution+ Accuracy + Speed = $$$ Typical DSP/Processor => Lots Of MIPs => $$$ Counter Approach + High Precision = Very Fast Clock = $$$

Re-Thinking Digital Control For Power Re Re-

• Thinking Digital Control For Power

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 Non Non-

• Linear Error Bins

Linear Error Bins

VOUT NOMINAL ZERO ERROR BIN +0.5 × DELTA +1 ERROR BIN +2 ERROR BIN +3 ERROR BIN +2.0 × DELTA +5 × DELTA +10 × DELTA –1 ERROR BIN –2.0 × DELTA

Opportunities For: Lower Cost Digitization? Improved Nonlinear Control?

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

ZERO ERROR BIN +1 ERROR BIN –1 ERROR BIN +2 ERROR BIN –2 ERROR BIN +3 ERROR BIN –3 ERROR BIN OUTPUT VOLTAGE D/A ERROR BINS COARSE RESOLUTION DPWM POSSIBLE OUTPUT VOLTAGES

Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling

VOUT NOMINAL DUTY CYCLE N N+1 PWM

Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling

ZERO ERROR BIN +1 ERROR BIN –1 ERROR BIN +2 ERROR BIN –2 ERROR BIN +3 ERROR BIN –3 ERROR BIN OUTPUT VOLTAGE D/A ERROR BINS FINE RESOLUTION DPWM POSSIBLE OUTPUT VOLTAGES

Avoiding Limit Cycling Avoiding Limit Cycling Avoiding Limit Cycling

ZERO ERROR BIN +1 ERROR BIN –1 ERROR BIN +2 ERROR BIN –2 ERROR BIN +3 ERROR BIN –3 ERROR BIN OUTPUT VOLTAGE D/A ERROR BINS FINE RESOLUTION DPWM POSSIBLE OUTPUT VOLTAGES

Rule: Minimum ΔVOUT < Error Bin Size For A Buck Regulator: Maximum Setpoint Error = ±0.1% => Duty Cycle Resolution < 0.2% => 9 Bits Time Resolution Per Cycle

Sampling the Output Sampling the Output Sampling the Output

Once Per Cycle Can Give Good Information On Average Value Oversampling Is Not Necessarily Helpful Computation Time!

Everyone Is Terrified Of… Everyone Is Terrified Of… Everyone Is Terrified Of… Z Transforms Programming

[ ] [ ]

n c c n

V z v n z

∞ − =−∞

= ∑

main(){ printf(“hello,world\n”); }

REALITY! Converter Designers Will Work Through Computer Based GUI Interfaces

Winning Application #1: Efficiency Optimization Winning Application #1: Winning Application #1: 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: Self Tuning Control Loops Winning Application #2: Winning Application #2: 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: Digital Power Management Winning Application #3: Winning Application #3: 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 References References – – For More Information 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