Control System Design for Power Converters - microchip.com

1 1 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 1 Control System Design for Power Converters2 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 2 Learning objectivesOImportance of Control systemOdesign of Control systemsOTricks and tips to improve designToday we will be talking about basics of Control System Design for Power learning objectives are as follows. Importance of Control System Design , Design of Control systems and tricks and tips to improve the design3 2008 microchip Technology Incorporated.

2 All Rights Reserved. Control System Design for Power Converters Slide 3 Introduction to Control SystemsOState variable : physical quantity that needs to be controlled speed of carOManipulated input : A variable whose quantity can be set which controls the state variable. Pressure on gas pedal OLoad or disturbance : external influence that effects state variable gradient of the roadOReference : Desired Value of state variable * notationOFeedback : Actual Value of state variable readingIntroduction to Control System . Imagine you are driving a car and want to Control the speed of the car. The speed of the car is the state variableThe only thing that can be controlled is the pressure on the gas pedal.

3 The pressure on the gas pedal is the manipulated input The System acts in response to the manipulated input The desired speed is the reference for the systemExternal factors like road gradient and wind drag are disturbancesThe feedback is the measured speed using speedometer4 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 4 Open loopOThe input command is blindly set hoping the System to be at desired value OEasy to implementOExternal variables like load/drift from ideal conditions like payload spoil the partyOLeads to over designed passive componentsOAdd bulk and costSYSO(s)I(s)

4 One method to Control the System is open loop implementationThe manipulated input is blindly set hoping the state variable will be at the desired valueIt is easy to implement as no measurement devices of the actual state of the System is requiredExternal factors like wind and gradient and variables like payload spoil the partyTo prevent effects of external influences, leads to over designed passive System which adds bulk and 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 5 Closed loopOFeedback is taken using sensorsOError between desired value and feedback is used to Control the command OAllows System to respond to external variables (isolate state from external influences)OReduce passive component size/costODesign needs careful considerationFeedback is taken from using sensors which measure the desired output feedback is used to generate error between the reference.

5 This error is then used to set the drive or the manipulated input. The block that takes in the error as input and generates the drive or the manipulated input is known as the controller. Closed loop systems can respond to changes in external reduce bulk and cost of passive componentsClosed loop Control System Design needs some mathematical 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 6 Closed loop Control systemPlantGp(s)V(s)D(s)Feedback /sensor H(s)V*-ControllerGc(s)e(s)Physical systemControl system7 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 7 Closed loop Control System Design objectivesOGiven : a System with input commands and output OControl a physical quantity (state) to a varying desired value within a specified time OMinimize effect of external influences OMitigate effect of measurement errors/noiseOEstimation as sensor replacement8 2008 microchip Technology Incorporated.

6 All Rights Reserved. Control System Design for Power Converters Slide 8 Metrics for evaluation OCommand tracking : Ability of output to respond to varying input rejection : Ability to isolate output from variation in loadOLine Regulation : Ability to isolate output from variation in input ONoise rejection : Ability to reject measurement noise/errorsControl System efficacy can be evaluated on 4 metricsCommand tracking : Ability of output to respond to varying input rejection : Ability to isolate output from variation in loadLine Regulation : Ability to isolate output from variation in input Noise rejection : Ability to reject measurement noise/errors9 2008 microchip Technology Incorporated.

7 All Rights Reserved. Control System Design for Power Converters Slide 9 Closed loop Control systemFrequency response of a typical Control systemBandwidth or eigen valueClosed loop Control System response is frequency domain is shown here. Typically it looks like a low pass cutoff frequency or bandwidth is the frequency over which the controller is not able to track changes in reference 10 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 10 How to Control a systemOA polynomial of s is selected for controllerOTypically + Kp + Ki/s is used for controllerODifferentiation leads to noise amplification Caution OEstimators may be used for derivative termsTOMORROWTODAYYESTERDAYHow Do you Control the System and Why PID?

8 ?WHAT DO I HAVE IN HAND . Feed Back . I take the feed back either with amplification or attenuation and do a simple compare ( P) I LOOK AT THE Trend of my feedback with Past set of feedbacks and then Control ( Integrate) I look at the rate of change of my feed back and Control ( Differentiate ) This Control Module is acting upon the System Transfer Function . Which means Adding Gains to the System Moving or adding Poles / Zeros for better response and stability 11 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 11 Buck topologyBuck converterOVo = Ic/sCOIL= VL/sLODVin Vo = VLOIC= IL IoZVT , multiphase buckand synchronoussingle phase buck stages can be modeled as buck drivesBuck converterA buck converter is a Power converter that converts higher voltage to a lower voltage using switches and inductor and capacitor.

9 The basic algebraic equations are shown in the slide. The capacitor acts like an integrator for the current that flows into it. The output of the integrator is the capacitor voltage. The inductor acts like an integrator for the voltage that gets applied across it. The output of the integrator is the current. The voltage at the point where inductor and switches are connected is goal is to Control the output voltage to desired possibly changing values under varying conditions of line and load12 2008 microchip Technology Incorporated. All Rights Reserved. Control System Design for Power Converters Slide 12 Tricks and tips for insightORepresent physical elements as integrationsONatural characteristics of elements To Control Vo, Ic needs to be manipulated by controllerOController should emulate a current To Control IL, VL needs to be manipulated by controllerOController should emulate voltage source O information quantities should be interpreted as having physical units OMay lead to more than standard PID and more MATHOB etter performanceDesigning Control System gets simplified if characteristics of elements and respect for physical quantities13 2008 microchip Technology Incorporated.

10 All Rights Reserved. Control System Design for Power Converters Slide 13 Buck LC block diagram1/sL1/sC--IoVxVoILObserving the following rules should make the System Design lot easierRepresent physical elements as integrationsNatural characteristics of elements To Control Vo, Ic needs to be manipulated by controllerController should emulate a current To Control IL, VL needs to be manipulated by controller Controller should emulate voltage source information quantities should be interpreted as having physical units May lead to more than standard PID and more MATHB etter performance14 2008 microchip Technology Incorporated. All Rights Reserved.