Integrated Power Factor Correction (PFC) and …

1 2008 Microchip Technology 1AN1208 INTRODUCTION In recent years, the motor control industry has beenfocusing on designing Power efficient motor controldrives for a wide variety of applications. The consumerdemand for improved Power quality standards is drivingthis trend. The Power quality can be enhanced byimplementing Power Factor Correction (PFC), andefficient control of a motor can be realized usingSensorless Field Oriented Control (FOC) appliance industry often requires low-costimplementation of these algorithms. This can beachieved by integrating PFC and Sensorless FOCalgorithms on a single Digital Signal Controller (DSC). This application note describes the process ofintegrating two complex applications: PFC and Sensor-less FOC.

2 These applications are implemented on aPermanent Magnet Synchronous Motor (PMSM). Inaddition, this application note also describes the inte-gration of the algorithms, lists the necessary hardwarerequirements, and provides the guidelines to optimizethe development Integrated solution is based on these applicationnotes: AN1106, Power Factor Correction in Power Conversion Applications Using the dsPIC DSC AN1078, Sensorless Field Oriented Control of PMSM Motors Using dsPIC30F or dsPIC33F Digital Signal ControllersThe application note AN1106, describes the PowerFactor Correction (PFC) method. The application noteAN1078, describes the Sensorless Field OrientedControl (FOC) method. The detailed digital design andimplementation techniques are provided in theseapplication notes.

3 This application note is anaddendum to the above application notes. The Integrated application is implemented on thefollowing families of dsPIC DSC devices: dsPIC30F dsPIC33 FThe low cost and high performance capabilities of theDSC, combined with a wide variety of Power electronicperipherals such as the Analog-to-Digital Converter(ADC) and the Pulse Width Modulator (PWM), enablethe digital design and the implementation of such acomplex application to be simpler and PFC and Motor ControlThe majority of motor control systems often use PFCas the first stage of the system. Without an input PFCstage, the current drawn will have significant harmoniccontent due to the presence of switching elements ofthe inverter.

4 In addition, since motor loads are highlyinductive, the input currents will induce significantreactive Power into the input system, thereby reducingoverall efficiency of the system. A PFC stage which isa front-end converter of a motor control application,provides better output voltage regulation and reducesharmonic content of the input current boost converter topology with average currentmode control is the preferred method for implementingdigital PFC in these applications. The dual shunt Sensorless FOC method is a speedcontrol technique that drives the PMSM motor. TheSensorless FOC technique overcomes restrictionsplaced on some applications that cannot deployposition or speed sensors.

5 The speed and position ofthe PMSM motor are estimated by measuring phasecurrents. With a constant rotor magnetic field producedby a permanent magnet on the rotor, the PMSM is veryefficient when used in appliances. When comparedwith induction motors, PMSM motors are morepowerful for the same given size. They are also lessnoisy than DC motors, since brushes are not , the PMSM motor is chosen for thisapplication. Why Use a Digital Signal Controller?The dsPIC DSC devices are ideal for a variety of com-plex applications running multiple algorithms at differ-ent frequencies and using multiple peripherals to drivethe various circuits. These applications ( , washingmachines, refrigerators, and air conditioners) use vari-ous motor control peripherals to precisely control thespeed of the motor at various operating loads.

6 Theintegrated PFC and Sensorless FOC system uses thefollowing peripherals: Pulse Width Modulator (PWM) Analog-to-Digital Converter (ADC) Quadrature Encoder Interface (QEI)Author:Vinaya SkandaMicrochip Technology Power Factor Correction (PFC) and Sensorless Field Oriented Control (FOC) SystemAN1208DS01208A-page 2 2008 Microchip Technology peripherals offer the following major features: Multiple sources to trigger the ADC Input Conversion Capability up to 1 Msps rate Methods to simultaneous sample multiple analog channels Fault detection and handling capability Comprehensive single-cycle DSP instructions ( , MAC)SYSTEM OVERVIEWF igure 1 shows a block diagram of the Integrated PFCand Sensorless FOC first stage is a rectifier stage that converts the inputline voltage into a rectified AC voltage.

7 The rectified ACvoltage is the input to the second stage, which is theboost converter stage. During the second stage, the boost converter booststhe input voltage and shapes the inductor currentsimilar to that of the rectified AC voltage. This isachieved by implementing digital Power Factor correc-tion. The Average Current Mode Control method isused to implement PFC on a dsPIC DSC device. In thiscontrol method, the output DC voltage is controlled byvarying the average value of the current amplitude sig-nal. The current amplitude signal is calculated digitally. The third and the final stage of the Integrated system isa three-phase inverter stage that converts the DCvoltage into a three-phase voltage.

8 The convertedthree-phase voltage is the input to the PMSM stage is controlled by implementing the Sensor-less FOC strategy on the dsPIC DSC device. TheSensorless FOC controls the stator currents flowinginto the PMSM to meet the desired speed and torquerequirements of the system. The position and speedinformation is estimated by executing mathematicaloperations on the dsPIC DSC. The Integrated system uses five compensators toimplement PFC and Sensorless FOC technique. ThePFC technique uses two compensators to control thevoltage and current control loops, and the SensorlessFOC technique uses three compensators to control thespeed control loop, torque control loop, and flux controlloop.

9 All of the compensators are realized byimplementing Proportional-Integral (PI) 1: Integrated PFC AND SENSORLESS FOC SYSTEM BLOCK DIAGRAMK2K1K3K4K5 Analog-to-Digital ConverterPower Factor CorrectionSensorless Field Oriented ControlPWM GeneratorPFC PWM Duty CycleInverter PWM Duty CycleIACVACVDCIaIbAmplifier GainsLN152634AA12345 6 LDCPMSMPWM Generator 2008 Microchip Technology 3AN1208A NOVEL APPROACH FOR DIGITAL IMPLEMENTATION OF PFC AND SENSORLESS FOC ALGORITHMSF igure 2 shows a block diagram of the PFC and Sensorless FOC control loops implemented digitally using the dsPIC DSC 2:DIGITAL PFC AND SENSORLESS FOC BLOCK DIAGRAMS peed ControlId ControlBridge RectifierBoost ConverterThree-Phase InverterVoltage ControlPWM Current Control++Iq Control+++d -qtoSVMPWM / Estimator tod -qa, b, cto PWM VACVACVDCPWMIAC1 VAVGVAVG+++++V V I I IqIdIaIb02 Stator System3 Stator System2 Rotor System Ref VDCREF1 AC abc+ Sensorless Field Oriented Control (FOC) SystemPower Factor Correction (PFC) AN1208DS01208A-page 4 2008 Microchip Technology Power Factor CorrectionThe inductor current (IAC), input rectified AC voltage(VAC), and DC Output Voltage (VDC) are used asfeedback signals to implement the digital PFC.

10 Thesesignals are scaled by hardware gains and are input tothe analog channels of the ADC module. The PFC algorithm uses three control loops: thevoltage control loop, current control loop, and thevoltage feed forward control loop. The voltage compensator uses the reference voltageand actual output voltage as inputs to compute theerror and compensate for the variations in outputvoltage. The output voltage is controlled by varying theaverage value of the current amplitude current amplitude signal is calculated digitally bycomputing the product of the rectified input voltage, thevoltage error compensator output, and the voltage feed-forward compensator output. The rectified input voltage is multiplied to enable thecurrent signal to have the same shape as the inputvoltage waveshape.