BLDC Motor Control in Automotive Environment Rainer Boehringer Abstract Automotive environments that approach the operational limits of semiconductor devices are a challenge for system designers. Under-the-hood applications require a wide supply voltage range and have high maximum junction temperatures. Designers must integrate increasing functionality within their electronic units, hence the ICs need to provide higher device integration levels. The environment is also subject to stronger EMC radiation levels. Actuators close to the turbocharger are typical examples for such hightemperature applications. These actuators serve to adjust the flaps of exhaust gas recirculation systems, the so-called waste gate. Further examples are coolant or oil pumps operating at 125°C and more. Remove Belt-Driven Actuators With limited engineering resources and more stringent CO2 emission requirements, designers need to consider all the power appliances within a car. It is no longer sufficient to optimize just the engine. Continuously operating loads waste a lot of energy. Loads driven by the engine's belt are using power, even if not needed. It would be preferable to operate the water pump or the cooling fan, for example, according to 21 actual requirements. Driving uphill the fan and water pump must dissipate plenty of heat. When driving downhill the motor is running in fuel cut-off and minimal heat is generated. Unlike belt-driven devices, you can control electronic actuators according to real demand while considering all relevant parameters. Unlike DC motors, BLDC (brushless DC) motors allow precise control over a wide dynamic range of revolution speed. BLDC motors help to efficiently and flexibly control loads according to the power actually needed. This is why electronically-commutated actuators should be your first choice for automotive applications such as power steering, HVAC (heating, ventilation and air conditioning) fans, power windows, and all kind of pumps. Automotive Requirements High Integration Level A typical BLDC motor control application comprises various functions. There is the microcontroller (MCU), high-current external MOSFETs, a pre-driver to switch those external MOSFETs, the power supply plus a voltage regulator for the digital supply of the ECU (engine control unit), and a communication interface to the car (see figure 1). © 2013 / www.atmel.com Kommunikation Communication Schnittstelle Interface Mikrocontroller Microcontroller Versorgung Supply Integrierter Integrated Charge Pump Gatetreiber Gate Driver PMWSteuerung Control PWM via mittels Power PowerStage Stage Controller Controller Input Logic Input Logic Emergency Shut Down E Diagnose Diagnosis LINUART LIN UART LIN Transceiver Transceiver LIN System System Management Management 5V 5V Regulator Regulator WD WD Timer Timer B6 Bridge obere Externe Mosfets 3 High-side 3 obere Gate Drivers Treiber Gate BLDC Motor 33 Low-side untere Gate Gate Drivers Treiber untere Externe Mosfets Position Detection Figure 1. BLDC System Architecture IC manufacturers integrate as many functions as possible to ease your design effort. A higher integration level also saves space. If the chip has an integrated LIN (local interconnect network) physical layer function, it does not need a discrete LIN transceiver. If you reduce the size of the electronic park brake control, you might have room to add ESP (Electronic Stability Control) functions on your ECU. A watchdog timer is mandatory in automotive safety applications. For failsafe reasons, it needs to be on a different die than the MCU. Since the watchdog timer consists of digital logic and a counter, Atmel® integrated this function onto the MOSFET gate-driver chip to save cost and space. is still running. The inductance of the alternator windings creates high-energy pulses with voltage peaks up to 120V. This voltage is limited by a central load-dump protection unit. The protected load dump output voltage depends on the individual OEM requirements (typical example 36V). Low operating voltages also challenge electric motor controller systems. The most critical low-voltage condition occurs during car start. Activating the ignition key or starting the engine after the start/stop function can drop the battery voltage as low as 4.5V. This is called crank pulse (figure 2). The ECU must function properly during this crank pulse. You can achieve this with electrolytic capacitors that you size according to the lowest voltage and longest time expected for the crank pulse. Automotive Supply Voltage Range A wide supply voltage range is a key criterion for applications within an automotive environment. Both high as well as low battery voltages are a challenge for the ECU. It needs to withstand a high operating voltage during operation conditions such as jump-starting and load dump. Starting an engine with an external starter battery is called jump start. The worst-case jump start is off a 24V truck battery with 12 instead of 6 lead acid cells. This creates a maximum voltage requirement of 28V. Load dump occurs when a mechanic disconnects the battery while the engine Automotive Compilation Vol. 10 U Crank Pulse Cycle 6.5V 4.5V t Figure 2. Typical Crank Pulse Waveform 22 External MOSFETs There are both N-channel and P-channel high-current MOSFET switches. For the same die size, an N-channel MOSFET will have half the on-resistance (RDSon) compared to a P-channel device. Since die size is the fundamental factor of the part’s cost, N-channel MOSFETs are the preferred solution in most cases. The control voltage that begins to turn on a MOSFET is called the gate threshold VGth. This voltage drops at high temperatures. In a hot engine compartment, logic-level MOSFETs may not switch off completely, whereas non-logiclevel MOSFETs guarantee safe and proper switch-off. To create this gate drive voltage, chip designers use an integrated charge pump (figure 4). In addition, the charge pump helps to stabilize the drive of the external low-side MOSFETs. Non-logic-level MOSFETs require a gate voltage of 8V. If you derive the low-side gate drive directly from the battery you cannot maintain 8V during a crank pulse event. A 2-stage charge pump solves this issue. The charge pump output voltage is transferred by the VG regulator to the lowside gate circuitry (see figure 3). Integrated Charge Pump vs. Bootstrap To turn on a high-side MOSFET, you need to raise the gate voltage above the supply voltage the MOSFET is switching (see figure 3). The ATA6843/44 charge pump is similar to a Dickson charge pump with its 2-stage architecture (see figure 4). You can generate the output voltage of a 2-stage charge pump to a maximum value two times higher than the input supply voltage. The 2-stage configuration enables a reliable gate supply voltage range for the external MOSFETs. The MOSFET gates are protected from load dump and the gate drive voltage is maintained during a crank pulse event. Closing the high-side switch increases voltage on motor phase A to the level of the battery supply voltage VSupply. This means the voltage on the source pin of the MOSFET is at VSupply. The gate threshold voltage, VGth, is always relative to the FET source pin. Hence, the gate voltage VGSH needs to reach a level of at least VSupply plus VGth. Competing products often use bootstrap gate drive techniques. Bootstrap circuits will double the power supply voltage. But bootstrapping will not maintain gate drive during a low-voltage crank pulse condition. Bootstrap circuits need an oscillating motor drive output to work. If the motor output is fully on or fully off the bootstrap circuit cannot keep its Gate Drive Charge Pump U VSupply VGSH VGSL H1 S1 High-side Switch VGSH VG Regulator L1 VSupply Motor Phase A Low-side Switch VPHASE VGSH VGSL t Figure 3. Gate Drive 23 © 2013 / www.atmel.com TDS 3034B 7 May 2013 15:33:09 Charge Pump Voltage Supply Voltage GND level VSupply CPOUT CCP1 T1 CCP2 CCPOUT T2 GND Figure 4. ATA6844 Charge Pump Wafeforms storage capacitor charged. Only a free-running charge pump is able to provide a stable output voltage above battery supply no matter what the motor duty cycle is. Engineers often believe that a charge pump is a complicated device and difficult to design into their application. Atmel developed the ATA6843/44's integrated charge pump to drive six N-channel MOSFETs. The chip only requires three external ceramic capacitors. The on-chip charge pump guarantees to easily create a reliable BLDC gate drive system. There is no additional effort for comparators, chopping, or switching. You don’t have to agonize over complex design issues. The Atmel engineers considered EMC (electromagnetic compatibly) radiation when they developed the ATA6843/44's internal push/pull stages. They included sufficient cross-conduction times to keep emissions low so you can meet strict automotive regulations. Automotive Compilation Vol. 10 Reverse-Voltage Protection The integrated charge pump allows you to implement a reverse-voltage protection circuit (figure 5). This requires a single external N-channel MOSFET wired in the reverse direction. At power-on the N-channel MOSFET conducts via its intrinsic body diode. This starts the integrated charge pump. Since the motor is not operating, the supply current is low. The intrinsic body diode can power the chip without overheating. As soon as the charge pump voltage exceeds the protection MOSFETs' gate threshold, the MOSFET is driven into active mode and conducts through its low on resistance. The charge pump can now also provide the gate drive voltage to the motor MOSFETs. An NPN transistor plus a diode in series protects against fast negative voltages. When the battery input goes negative relative to chassis common, it turns on the NPN transistor. The transistor then clamps the MOSFET gate and source together. 24 + CCPOUT CCP2 PBAT CPHI2 CPHI1 CPLO1 CCP1 CPOUT CVINT VG VINT VBAT VMODE CVINT CPLO2 Battery CVCC VCC VCC Regulator VG Regulator Charge Pump /RESET WD VINT Regulator /IH1-3 DG1 DG2 WD Timer WDEN GND SCREF EN LIN LINGND LIN RWD ADC High-side Driver 2 H2 High-side Driver 1 H1 S1 S3 CC Timer RCC Low-side Driver 1 L1 Low-side Driver 2 L2 Low-side Driver 3 L3 PGND RX DAC Driver Control Atmel ATA6843/44 DG3 H3 S2 Supervisor: Short Circuit Overtemperature Undervoltage /COAST TX Oscillator VBG CC SLEEP Control Logic RWD Microcontroller IL1-3 High-side Driver 3 CCC VCC U V W LIN KL 15 Figure 5. ATA6843/44 Application Schematic High-Temperature Operation The AEC-Q100 standard defines ambient temperature ranges for automotive applications. Grade 1 covers ambient temperatures of -40°C to 125°C, grade 0 is suitable for under-the-hood applications up to 150°C ambient temperature. Atmel manufactures motor driver ICs on its own BCD-on-SOI (bipolar/CMOS/DMOS on silicon-on-insulator) technology. This enables the IC to operate at junction temperatures up to 200°C and ambient temperatures up to 150°C. SOI technology offers specific design advantages. There is significant cross-talk reduction between power and digital circuits on the same die, as well as easy integration of highquality power devices, and immunity to radiation. Compared to devices manufactured with existing bulk technology, devices manufactured with SOI wafers achieve a higher level of integration and processing speed at reduced power consumption. The isolation of devices through an oxide layer 25 eliminates problems of parasitic capacitance and latch up, thus minimizing internal coupling. This helps to simplify the design and reduce risks. Switching the MOSFETs MOS gate threshold voltages typically decrease at higher junction temperatures. This disturbs the IC-internal timing and may prevent the MOS switches from turning off. Atmel adjusts the gate threshold voltage of the high-temperature BCD-on-SOI process to meet these high-temperature gate threshold requirements. Likewise, the decreasing gate threshold voltage affects the external MOSFETs. The higher the temperature the lower the MOSFET gate threshold voltage. One solution is to apply non-logic level MOSFETs with higher gate threshold voltages instead of P-channel MOSFETs. At the same die size, N-channel MOSFETs have only half the RDSon resistance. The reduction of thermal dissipation is an important benefit, in particular for applications in hot environments. © 2013 / www.atmel.com Support Tools: The Development Kit Diagnostics and Monitoring In automotive applications monitoring and diagnostic functions are mandatory. Voltage failures, thermal overload, or overcurrent are events that require immediate action, for example, instant stop or motor driver unlock, independent of any microcontroller operation. In addition, ECUs must provide feedback on malfunctions to the host controller, and record them in central failure protocols to enable appropriate countermeasures. One emergency shut-down function is the coast feature. The ATA6844's COAST pin enables the motor to rotate in coasting mode by activating a single input pin. In case of emergency, all six output gate drivers immediately switch to off mode, the external MOSFETs are deactivated, and the motor will coast to a stop. The ATA6844-DK development kit enables designers to take first steps in high-temperature BLDC motor control. It consists of two connected boards plus a standard BLDC motor. The + CCPOUT PBAT CPHI1 VG CPOUT CCP2 CPLO1 CCP1 CPHI2 CVG VINT VBAT CVINT CPLO2 Battery Figure 6. Development Kit ATA6844-DK CVCC VMODE VCC SCREF DG1 DG2 3.3/5V VCC Regulator 13V Regulator Supervisor: Short Circuit Overtemperature Undervoltage Charge Pump VINT 5V Regulator VBG Oscillator DG3 High-side Driver 3 H3 High-side Driver 2 H2 High-side Driver 1 H1 S3 Logic Control WD Atmel ATA6843/44 /IH1-3 IL1-3 EN1 WD Timer LIN CC Timer RWD L1 Low-side Driver 2 L2 Low-side Driver 3 L3 CC WDEN RWD GND LINGND EN2 LIN LIN TX Low-side Driver 1 RCC Back-EMF Conditioning PGND /RESET RX S1 S2 COAST CCC Power Board Controlller Board ATmega32M1 UART ATmega32U2 Figure 7. Evaluation Kit Block Diagram Automotive Compilation Vol. 10 26 power board handles all BLDC functions except the MCU microcontroller unit. Six discrete N-channel FETs are arranged in a BLDC bridge architecture. An Atmel SBC ATA6844 handles the basic electronic control unit functions, a lowdropout regulator, LIN transceiver, and window watchdog. The controller board features the Atmel ATmega32M1 8-bit AVR® MCU dedicated to BLDC motor control. An ATmega32U2 microcontroller is on the board for debugging. Power Board The power board comprises all the BLDC motor control functions: • Six N-channel MOSFETs arranged as a B6 bridge supply the motor current. The output terminals U, V, and W attach to the motor connector to operate the included BLDC motor. • For emergency purposes, you can adjust the shortcircuit shutdown current with potentiometer SCREF. • For EMC purposes, you can modify the serial resistors to achieve gate voltage shaping to adjust the slew rates of the discrete MOSFETs. Controller Board While an actual automotive application will have the BLDC microcontroller placed close to the gate driver chip, this kit has the MCU on a separate board to increase flexibility. All MCU signals required to drive the power board are available on the interface connector. This approach enables the customer to use any motor control MCU by simply connecting the relevant control signals to the interface connector. All Atmel MCU evaluation boards, e.g. STK®600, can be used. The controller board provides three debugging methods. The standard debug interface is a UART interface. The Tx and Rx connections are accessible via jumper connectors. Since the ATA6844 has a LIN transceiver, diagnostics can also be done via the LIN interface. Thirdly, the on-board ATmega32U2 enables RS232 interfacing. The MCU's output USB interface can be directly connected to a PC and controlled by a hyper terminal application. For further information please refer to the ATA6844-DK application note at http://www.atmel.com/tools/ATA6844-DK.aspx. • 82mOhm shunt for motor current measurement can be adjusted for various motor current loads • Charge pump for external gate voltage supply • 3 capacitors for complete charge pump function • Test pin CPOUT allows access to the charge pump output voltage • The charge pump output voltage is also used to implement reverse battery protection. Typical supply voltage is 12V. A seventh MOSFET the same size of the B6 bridge MOSFETs is controlled by the charge pump output. The reverse voltage protection control circuit ensures fast switch off during any negative supply voltage spikes. Motor position feedback is a key feature of BLDC applications. The ATA6844-DK offers both Hall sensor feedback and B-EMF (back-electromotive force) feedback. The option can be set via jumpers. For Hall sensor feedback the jumpers directly connect the motor Hall sensor output signals to the microcontroller. The microcontroller uses these digital Hall signal outputs for commutation. A resistor and capacitor network provide for B-EMF feedback position detection. For this mode the jumpers connect 3 motor control signals and their dedicated neutral point signals to the microcontroller interface. 12V Supply, GND Reverse Voltage Protection N-Channel MOSFET B6 Bridge N-Channel MOSFETs B asis Board Motor Connector Reverse Voltage Protection Control Shunt resistor Gate Control Charge Pump capacitors Short Circuit Threshold adjustment Controller Board Position Feedback: Back-EMF Signal Conditioning Controller Board Interface Connector Motor controller ATmega32M1 Control / Diagnosis Interface ATmega 32U2 Operating Status Jumpers to select MCU communication channel - LIN - UART - USB Figure 8. Application Board Top View, Functional Blocks 27 © 2013 / www.atmel.com Summary The Atmel ATA6843/44 integrated gate driver enables to overcome the challenges of present day automotive BLDC designs. It lets you design such applications utilizing fewer external components. These motor driver devices feature a high maximum junction temperature to meet the strict automotive grade 0 requirements for under-the-hoodapplications. The IC has an integrated 2-stage charge pump ensuring that designers can easily create a reliable BLDC gate drive system without any additional design effort. The development kit allows engineers to quickly get familiar with high-temperature BLDC motor control. For further information on AVR motor control designs, see http://www.atmel.com/products/AVR/mc/?family_id=607. Automotive Compilation Vol. 10 28