Designing High-Performance And Power Efficient 3-Phase .
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsBy John T. Lee, Carlos Ribeiro, and Miguel Mendoza; Micrel, Inc.SynopsisToday motor control systems are used by engineers use for both digital and analog technologiesto conquer past challenges including motor speed control, rotation direction, drift and motorfatigue. The application of MCUs has enabled this generation of engineers the opportunity todynamically control motor actions so that they respond to environmental stresses and conditions.This will help to provide a longer operational lifetime and reduce maintenance which meanslower cost. Currently, motor manufactures are gravitating towards 3-Phase BLDC motorsbecause they provide more torque for less power and have a longer operating time due to nodirect contact from the commutator and electrical terminals such as is found in the brushedmotors. Regrettably, the use of 3-Phase motor control adds additional complexity compared tobrushed DC or AC motors and the relationship between digital and analog components becomesvery important.This whitepaper discusses a summary of the concerns that should be factored in when usinganalog components and microcontroller in a 3-Phase BLDC motor application. It also covers thesuitable power management devices and power level shifters that enable the microcontrollers todrive motors form power sources from 12V all the way up to 300V DC voltages.Who Needs BLDC Motors Anyway?Recently, designers have been favoring the use of a higher efficiency BLDC motor. This trendholds true for numerous markets and a diverse array of applications. Presently, applications canuse or are already using BLDC motors as a replacement for dated AC motors or mechanicalpump technology. Important benefits of using BLDC motors include: Higher efficiency (75% vs. 40% of an AC motor) Less heat generated Higher reliability (no brushes to wear out) Safer to operate in a dangerous environment (no brush dust generated as is found withbrushed motors).April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsThe use of BLDC motors in key sub-systems also reduces the overall system weight. As theBLDC motor is commutated entirely electronically, it is much simpler to control the torque andRPM of the motor and at much higher speeds. Governments the world over are dealing with veryreal power deficits that are caused by insufficient electrical power grids. Moreover, there aremany regions of the world that must routinely deal with blackouts during peak demand seasons.The result is that those countries are now either giving subsidies or preparing to hand outsubsidies for the more efficient use of BLDC motors.Key ParametersSize and WeightEfficiencySpeed ControlAccuracy and SpeedTorque ControlAC Motors100%40% - 50%Difficult3% - 5%PoorBLDC Motors55%70% -75%Easy & Linear0.5%ExcellentTable 1. Advantages of Brushless DC MotorsStrategic Market Segments and ApplicationsAutomotive. There are many instances in automotive markets where mechanical and hydraulicpumps/movement controls are being replaced. Applications for this trend cover everything fromfuel pumps, power steering, seat control, automotive HVAC (Heating Venting and AirConditioning), moon roof movement, windshield wiper motors, and more. It has been calculatedthat each one of these functions can save up to one mile per gallon by switching to BLDCmotors. The reasoning behind this movement is obviously fuel savings and power efficiency.April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsFigure 1. Window Lift Functional Block DiagramHome Appliances. There are quite a few appliances in the home appliance market that canbenefit from the use of high efficiency BLDC motors. These include pumps, fans, airconditioners, blenders, hand power tools, and other kitchen appliances.Figure 2. Blender Motor Control Functional Block DiagramApril 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsIndustrial Systems. Most pumps, fans, air conditioners, mixers, and HVAC units require amotor drive. The EU has issued an edict that requires all new industrial appliances to use the 3Phase “Inverter Drive” of the BLDC motors.Figure 3. Air Conditioning Functional Block DiagramApril 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsWhite Goods. The electricity use in many clothes washers and dryers can be reduced with theadoption of higher efficiency BLDC motors.Figure 4. Washer Motor Functional DiagramSegmentsAutomotiveDriving ForceFuel SavingsHigher Efficiency and ReliabilityWhite GoodsEU-Drive Towards Clean Energyand Power EfficiencyIndustrialEU-“Inverter Drive” Power EfficiencyHome ApplianceClean Energy and Power EfficiencyGovernment SubsidyApplicationsElectrical PumpsPower SteeringWipersWashersDryersRefrigeratorsAir ACTable 2. Key Areas for Brushless DC Motor DrivesApril 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsWhat Drives BLDC Motors?There are several methods for driving a BLDC motor. Some fundamental system requirementsare listed below:a. Power Transistors. These are usually MOSFETs or IGBTs that can tolerate highvoltages (matched to the requirements of the motor). Most home appliances use motorsthat produce half to three quarter of a horsepower (1hp 734W). Hence, typical currentcapabilities of up to 10A are utilized. For a high voltage system ( 350V typically),IGBTS can be used.b. MOSFET/IGBT Drivers. Generally, a set of MOSFET or IGBT drivers are utilized.One could choose either three “Half Bridge” drivers or 3-Phase drivers. These solutionsmust be able to handle two times the motor voltage to manage the back-electromotiveforce (EMF) generated from the motors. Moreover, these drivers should provideprotection of the power transistors via timing and switching controls that ensure the toptransistors are turned off before the bottom transistors turn on.c. Feedback Elements/Control. Designers should have some sort of feedback element inall servo control systems. Examples include optical sensors, hall effect sensors,tachometers, and sensorless EMF sense—the least costly of all. Various feedbackmethods are useful, depending upon the precision required, RPM, and torque needed.Many consumer applications generally seek to use the sensorless technique of back-EMFsense.d. Analog to Digital Converter. In many situations, an A/D device is needed in order toconvert the analog signals to digital code, which are then sent to the system MCU.e. MCU. All closed-loop control systems (BLDC motors are nearly always in this group)require an MCU, which handles the servo loop control, calculations, corrections, PIDcontrols, and sensor management. These digital controllers are usually 16-bit, but lesssophisticated applications can use 8-bit controllers.f. Analog Power/Regulators/References. In addition to the above-mentioned components,many systems contain axillary power, regulation, voltage conversion, and other analogdevices such as supervisors, LDOs, DC-to-DC converters, and OP amps.April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsMIC6270EMF FeedbackFeedback Loop toDetect Position,Speed, andDirectionMIC4604Half Bridge DriverPhase AProvide Protection andAppropriate Voltage andCurrent to Drive theMOSFETON or OFFMOSFETHigh VoltageSwitch(ON / OFF)ZNEO Core MCUGain CalculationsPosition and Velocity CalculationsPID Control SystemStabilty CompensationPhase DetechBack EMF CalculationsSensor ManagementADC I/0s3V DCMIC5235LDOProvides ConstantVoltage Despite ofInput VoltageVariationsMIC4604Half Bridge DriverPhase BMOSFETMIC4604Half Bridge DriverPhase CMOSFET12V DCMIC4682Buck RegulatorProvides RegulatedOutput Voltage thatis Lower Than theInput Voltage withHigh Efficiency24V DCMIC38HC44FlybackProvides RegulatedOutput DC VoltageIsolated SupplyBridgeRectifier & PFCRectifies AC to DCand Maintains theCurrent in Phase withthe Voltage110V to220V ACACFigure 5. Typical Functional Block Diagram of a 24V Brushless DC Motor ControlMicrel Advantages in Motor Drivea. Power Drivers. Micrel maintains a broad line of MOSFET/IGBT drivers. Keyparameters include fast pulse delay, high peak current for gate charge/control, and up to85V operation. Some examples include theMIC4604 family that can withstand up to 85Vof back-EMF motor voltage.b. Voltage Reference and Supervisors. Micrel offers an extensive line of these deviceswhich are critical to the operations of MCU. Examples include: the MIC811, MIC2775,and the MIC1232 voltage supervisors.c. OP Amps/Comparators. Micrel has a line of low power op amps and comparators.These devices are critical to the precision feedback control of servo systems. Examplesinclude the MIC6270, MIC841N, and the MIC833.d. LDO. Micrel offers the broadest line of LDOs in the industry including the fastesttransient LDO, lowest input LDO, lowest dropout LDO, and highest current LDO.Examples include the MIC49150, MIC29150, MIC5235, and the MIC5283.e. DC-to-DC Switching Regulators. Micrel also offers an extensive line of DC-to-DCconvertors with the highest efficiencies. These are used in auxiliary power applications.Examples include the MIC2605 Boost and MIC4682 Buck (Step-Down) switchingregulators.April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsBasic Theory of Operation for 3-Phase Brushless DC MotorsBrushless DC (BLDC) motors are synchronous motors thathave permanent magnets residing on the rotor and coilwindings. These produce electrical magnets on the stator of themotor (see Figure 6). Electrical terminals are directly connectedto the stator windings; hence there are no brushes or mechanicalcontacts to the rotor such as in brushed motors. BLDC motorsuse DC power and a switching circuit to generate bi-directionalFigure 6. Cross-section of BLDCmotorcurrents on the stator windings. The switching circuit entails theuse of both a high-side switch and low-side switch for each winding, which totals six switchesfor one BLDC motor.300VSUPPLYQ6Q5300VSUPPLYQ2Q4Q1Q3Figure 7. Switching Circuit Using IGBTsModern motor designs use solid state switches such as MOSFETs or IGBTs (see Figure 7)depending upon the speed and voltage requirements of the motor when compared to relays. Cost,reliability, and size must all be factored in. The switching currents produce the appropriatemagnetic field polarity that attracts opposite polarities and repels equal polarities. This, in turn,produces a magnetic force that rotates the rotor. Using permanent magnets on the rotor providesthe designer with a mechanical advantage: the reduction of both size and weight. BLDC motorsfeature improved thermal characteristics compared to brush motors and induction motors,making them ideal choice for the next generation of power savings for mechanical systems.BLDC generally use three phases (windings) with each phase having a conducting interval of120 degrees (see Figure 8).April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsABACBCAC ABCBCABABCACFigureSix StepCommutationFigure3. 68.StepCommutationBecause the current is bi-directional, each phase has two steps per conducting interval. This iscalled six-step commutation. For example, the commutation phase sequence can be AB-AC-BCBA-CA-CB. Each conducting stage is one step and only two windings conduct current at anytime, leaving the third winding floating. The un-energized winding can be used as a feedbackcontrol that forms the basis for the characteristics needed for sensorless control algorithms.To keep the magnetic field in the stator moving ahead of the rotor and to create optimal torque,the transition from one sector to another must happen at exact rotor positions. Maximum torqueis attained via the switching circuit commutating every 60 degrees. All the switching controlalgorithms are implanted in the MCU. The microcontroller can control the switching circuitthrough the MOSFET drivers, which contain the appropriate response times, such as prop delays,rise and fall times, and drive capability including the gate drive voltage and current sync requiredto turn the MOSFET/IGBT to the “ON” or “OFF” state.The rotor position is crucial in determining the correct moment to commutate the motor winding.In applications where precision is required, hall sensors or a tachometer are used to calculate theposition, speed, and torque of the rotor. In applications where cost is the most importantconsideration, the sensorless technique—which is the calculation of the back electromotive force(EMF)—can be used to calculate position, speed, and torque.April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsBack-EMF is defined as thevoltage that is created in the statorwinding by the permanent magnet.This occurs when the rotor of themotor is turning. There are threevital back-EMF characteristics thatcan be utilized for control andfeedback signals. The first is thatthe magnitude of the back-EMF beproportional to the speed of theFigure 9. Crossing Eventmotor. For this reason, designersuse MOSFET drivers that can operate on at least two times the voltage supply. The second factoris that as speed increases, the slope of the back-EMF signal becomes greater. The third is that theback-EMF signal is symmetrical around the Crossing Event, as illustrated in Figure 9. Detectingthe Crossing Event precisely is the key to implementing the back-EMF algorithm. The backEMF analog signal can be translated to the MCU per a mixed signal circuit using high voltage opamps and analog-to-digital converters which are widely used in most modern microcontrollers.There needs to be at least one ADC per winding, for a total of three ADCs.When using sensorless control, the start sequence is critical because the MCU doesn’t innatelyknow the rotor’s initial rotor position. The first step initiates the motor energizing two windingsat a time while taking several measurements from the back-EMF feedback loops and it does thisuntil a precise position can be determined.Figure 10. Closed Control LoopApril 17, 2014Revision 2.1
Designing High-Performance and Power Efficient3-Phase Brushless DC Motor Control SystemsBLDC motors generally operate using a closed loop control system that requires an MCU. TheMCU performs the servo loop control, calculations, corrections, PID controls, and sensormanagement using back-EMF, a hall sensor, or tachometer (see Figure 10). These digitalcontrollers are generally 8-bits or higher and require EEPROM to store the firmware that createsthe algorithms necessary to set the desired motor speeds, direction, and to maintain motorstability. The MCU often offers ADCs that allow for sensorless motor control architecture. Thisarchitecture saves on valuable cost and board space. The MCU features construability and theflexibility necessary to optimize the algorithms for the application. The analog ICs can give theMCU efficient power supply, voltage regulation, voltage references, the ability to driveMOSFETs or IGBTs, and fault protection. Both technologies can operate 3-Phase BLDC motorsefficiently and at a comparable price point versus induction motors and brushed motors.ConclusionThe need to migrate towards higher efficiency BLDC motors in many markets and applicationsis becoming increasingly commonplace. This is due to certain key benefits: Higher efficiency (75% vs. 40% of an AC motor) Lower heat generation Higher reliability (no electrical contacts) Safer to operate in a dangerous environment (no brush dust generated as in brushedmotors).By using the BLDC motor in key sub-systems, the overall weight can also be reduced. Thismeans that the application can offer better fuel economy in vehicles. As the BLDC motor isentirely commutated electronically, it is much easier to control the torque and RPM of the motorand at much higher speeds. Around the world, many countries are facing insufficient power dueto electrical power grid deficiencies. To be certain, a small number of countries are now eithergiving subsidies or getting ready to provide subsidies for the more efficient use of BLDC motors.The BLDC deployment is but one of the many trends addressing the green initiative to save theworld’s precious resources without adversely impacting our way of life.April 17, 2014Revision 2.1
Designing High-Performance and Power Efficient 3-Phase Brushless DC Motor Control Systems April 17, 2014 Revision 2.1 The use of BLDC motors in key sub-systems also reduces the overall system weight. As the BLDC motor is commutated entirely electronically, it is much simpler to control the torque and RPM of the motor and at much higher speeds.
Designing High-Performance, Low-EMI Automotive Power Supplies 2.3.1 Dropout Performance The dropout level is defined as the difference between the output voltage and the minimum input voltage maintaining regulation. The dropout performance is dependent on the RDS_ON of the high-side FET, maximum duty cycle, and inductor DCR.
Designing your Tesla Coil September 2003 Rev 2 www.spacecatlighting.com Designing your Tesla Coil Introduction When I was in the process of designing my first tesla coil in May of 2002, I was hardpressed to find one source that comprehensibly described the process of designing a tesla coil from scratch.
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