Digital Control ConceptsDigital Control ConceptsFor Power Supply EngineersFor Power Supply EngineersRobert V. WhiteStaff EngineerWorldwide Technology Group

Presentation OverviewPresentation OverviewPresentation 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 PWMAnalog PWMAnalog PWM

Analog PWMAnalog PWMAnalog PWM

VoltageScaling

ErrorDetermination

Control LoopCharacteristics

SwitchTimingControl

Reference

Classical Digital PWMClassical Digital PWMClassical Digital PWM

Classical Digital PWMClassical Digital PWMClassical Digital PWM

Resolution+ Accuracy +

Speed =$$$

Typical DSP/Processor=> Lots Of MIPs

=> $$$

Counter Approach+ High Precision

= Very Fast Clock= $$$

Re-Thinking Digital Control For PowerReRe--Thinking Digital Control For PowerThinking Digital Control For Power

Step 1: Digitize Only The ErrorStep 1: Digitize Only The ErrorStep 1: Digitize Only The Error

Step 1: Digitize Only The ErrorStep 1: Digitize Only The ErrorStep 1: Digitize Only The Error

How Many Bits Are Needed?

3? 5? 7?

Thinking About Error BinsThinking About Error BinsThinking About Error Bins

Non-Linear Error BinsNonNon--Linear Error BinsLinear 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 CostDigitization?

ImprovedNonlinearControl?

Step 2: Simplify Calculation EngineStep 2: Simplify Calculation EngineStep 2: Simplify Calculation Engine

Lookup Table,Fixed Configuration Filters,

State Machines

$10DSP

Step 3: Low Cost Digital PWMStep 3: Low Cost Digital PWMStep 3: Low Cost Digital PWM

Example:Combination Counter

And Delay Line

The Point?Re-Thinking Digital

Control ForPower ConversionIs Lowering Cost

And Simplifying Design

Avoiding Limit CyclingAvoiding Limit CyclingAvoiding Limit Cycling

ZERO ERROR BIN

+1 ERROR BIN

–1 ERROR BIN

+2 ERROR BIN

–2 ERROR BIN

+3 ERROR BIN

–3 ERROR BIN

OUTPUT VOLTAGED/A ERROR BINS

COARSE RESOLUTION DPWMPOSSIBLE OUTPUT VOLTAGES

Avoiding Limit CyclingAvoiding Limit CyclingAvoiding Limit Cycling

VOUTNOMINAL

DUTYCYCLE

N

N+1

PWM

Avoiding Limit CyclingAvoiding Limit CyclingAvoiding Limit Cycling

ZERO ERROR BIN

+1 ERROR BIN

–1 ERROR BIN

+2 ERROR BIN

–2 ERROR BIN

+3 ERROR BIN

–3 ERROR BIN

OUTPUT VOLTAGED/A ERROR BINS

FINE RESOLUTION DPWMPOSSIBLE OUTPUT VOLTAGES

Avoiding Limit CyclingAvoiding Limit CyclingAvoiding Limit Cycling

ZERO ERROR BIN

+1 ERROR BIN

–1 ERROR BIN

+2 ERROR BIN

–2 ERROR BIN

+3 ERROR BIN

–3 ERROR BIN

OUTPUT VOLTAGED/A ERROR BINS

FINE RESOLUTION DPWMPOSSIBLE 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 OutputSampling the OutputSampling the Output

Once Per Cycle Can GiveGood Information On Average Value

Oversampling Is NotNecessarily Helpful

ComputationTime!

Everyone Is Terrified Of…Everyone Is Terrified Of…Everyone Is Terrified Of…

Z Transforms Programming

[ ] [ ] nc c

nV z v n z

∞−

=−∞

= ∑ main(){printf(“hello,world\n”);}

REALITY!Converter Designers Will Work

Through Computer Based GUI Interfaces

Winning Application #1:Efficiency OptimizationWinning Application #1:Winning Application #1:Efficiency OptimizationEfficiency Optimization

Digital Control Offers Opportunities To Optimize Operation To Minimize LossesExample: 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 LoopsWinning Application #2:Winning Application #2:Self Tuning Control LoopsSelf Tuning Control Loops

A Converter With A Digital Control LoopCan Act As Its Own Network AnalyzerBased On Measurement Of The System, A Device Can Optimize Its Loop ResponsePOLs– 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 ManagementWinning Application #3:Winning Application #3:Digital Power ManagementDigital 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 WillSimplify IC Design And Lower Cost

Improved Fault Management– Digital Control Enables Graceful Transition

Between Normal And Abnormal Operating Modes

References – For More InformationReferences References –– For More InformationFor 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