White Paper. Digital Optocouplers Deliver Low Power Consumption and High Isolation for Automotive Applications. He Junhua. Abstract.

Transcription

1 Digital Optocouplers Deliver Low Power Consumption and High Isolation for Automotive Applications White Paper He Junhua Abstract Avago s new AEC Q100 grade optocouplers, the ACPL-K4xT/K7xT series has significantly lower LED driving current compared to conventional optocouplers, thus lowering system power consumption. Additionally, the unique stretched surface-mount package reduces the optocoupler s footprint and volume while improving the insulation clearance and creepage, thus increasing the insolation voltage rating. The ACPL-K4xT//K7xT family of digital optocouplers can be widely applied in applications such as IPM drivers, CAN bus interfaces and discrete digital-signal isolation in HEV, PHEV and EV systems. Introduction As automotive systems employ ever-increasing amounts of electronic and electrical components, the systems become more susceptible to transients and short circuits, additional protection must be included to prevent critical systems from damage. This becomes especially critical when an electric motor provides the drive for a hybrid or all-electric vehicle. In such cases, high voltages of up to hundreds of volts can be introduced into the powertrain system. Additionally, beside high-voltage power conversion, data communications subsystems such as CAN (Control Area Network) networks must be protected so the high-voltages do not feed through to other subsystems such as the ECU (Engine Control Unit), BMS (Battery Management System), Climate Control System, etc. Some sample interface circuit diagrams illustrating how the optocouplers are employed in the applications are included.

2 High Reliability LEDs have benefitted from over 35 years of improvement and refined manufacturing processes. Avago leverages that experience to deliver highly-reliable LED optocouplers that serve many markets --industrial, military and space applications to name a few. Key to the reliability is the LED fabrication technology developed by Avago that achieves automotive AEC Q100 grade performance. Avago optocouplers are stress tested at 150 C and a LED drive current of 20 ma. The parameter of current transfer ratio (CTR) is proportional to LED light output. A CTR vs Time chart, such as shown in Figure 1 [1], indirectly measures LED degradation over lifetime. As shown in the graph, there is almost no degradation after 5000 hours of stress testing. Projections based on this measurement extrapolate to a lifetime of more than 100 years of operation in automotive applications. Table 1. Low LED Driving Current 1.4 Part Number Single Channel Dual Channel Low Power Consumption Propagation Delay Time Max. Typical LED On Current ACPL-K43T ACPL-K44T 1.0 μs 10 ma 5 μs 1.5 ma 10 μs 0.8 ma ACPL-K71T ACPL-K74T 35 ns 10 ma ACPL-K72T ACPL-K75T 100 ns 4 ma Figure 1. High performance of LED after 5000 hours of 150 C stress Besides the extended lifetime, Avago digital optocouplers have a significantly lower LED drive current vs competitive optocouplers (see Table 1). For example, the 1 Mbps (non-return-to-zero-coding) capable ACPL-K43T/44T series optocouplers [2] are available in three typical drive current ratings: 0.8 ma, 1.5 ma and 10 ma. Additionally, Avago s highspeed CMOS optocoupler family, the ACPL-K72T/75T, requires just a 4 ma drive level. Such relatively low current levels benefit system design since no additional current buffers are needed to drive the LEDs directly from a microcontroller s output pin. Let s look at a real power consumption example using the ACPL-K43T. The system conditions to determine the power consumption are as follows: input LED On current 1.5 ma at 50% duty cycle (), output pull-up resistor 10 k, and an output power-supply voltage of : P = P I + P O = I F x V F x + (I CCL x + I CCH x (1 )) x V CC = 1.5 ma x 1.4V x 50% + (15 μa x 50% μa x 50%) x 5 V = 1.1 mw Note: P i = Input Power, P o = Output Power, I F = LED forward current, V F = LED forward voltage, I CCL = Output low current current, I CCH = Output high current, V CC = Output supply voltage. Additional power consumed through pull-up resistor: P RL = (5 V) 2 / 10 k x 50% = 1.25 mw Normalized Current Transfer Ratio Hot Temperature Operating Life Stress (khours) Ave Ave+3σ Ave-3σ 2

3 Stretched Insulation Voltage CREEPAGE (A) 8.0 mm THICKNESS THROUGH INSULATION (C) 0.08 mm CC CLEARANCE (B) 8.0 mm Figure 2. Stretched SO-8 package Electrical isolation for safety is required by worldwide regulatory organizations when high voltages are present in systems. Optocouplers are key isolation components that transmit control signals yet provide high isolation between lowvoltage circuits and high-voltage devices. Common semiconductor component electrical safety standards applicable to optocouplers include IEC , DIN/EN and UL Appropriate optocouplers can be selected based on the equipment safety requirements. Some of the key parameters of the equipment safety ratings that are required for the selection of optocouplers are working voltage, installation class and insulation level. A reinforced insulation level is usually required for electrical equipment powered from AC line voltage by safety standards for industrial, home, office and IT equipment. In addition to insulation voltage rating, basic insulation parameters including external clearance, creepage and distance-throughinsulation (DTI, also referred-to as internal clearance) and comparative tracking index (CTI) are specified by some equipment safety standards. Table 2 shows Avago digital optocouplers SO-5 and Stretched SO-8 package insulation parameters, while Figure 2 illustrates the physical aspects of the parameters for the Stretched SO-8 package. Table 2. Optocoupler Insulation Parameters Package Style Example P/N External Creepage (mm) External Clearance (mm) Internal Clearance (mm) UL 1577, Input-Output Momentary Withstand Voltage, VISO / 1 minute SSO-8 ACPL-K43T SO-5 ACPL-M43T IEC , DIN/EN , Max. Working Insulation Voltage, VIORM (VPEAK) The SSO-8 package increases external creepage and clearance to more than 8 mm, and has a maximum working insulation voltage of up to 1140 VPEAK. For higher battery voltage and inverter bus voltage operations, SSO-8 package provides more insulation space. As an example, equipment safety standard IEC [3], Safety of Insulation Technology Equipment Including Electrical Business Equipment, provides requirement regarding creepage and clearance. Referring to table 3, working voltages of 200 and 400 V require different clearance and creepage under conditions of pollution degree 2, material group IIIa (for optocoupler ACPL-K43T) and installation category II. Thus a coupler such as the ACPL- M43T would be a good fit for 200 V battery systems, and the ACPL-K44T for 400 V battery systems. Table 3. IEC on Insulation Clearance and Creepage Pollution degree 2, material group IIIa and installation category II Working Voltage, V Basic Reinforced Basic Reinforced Clearance, mm Creepage, mm

4 In a traditional gasoline engine car, the low-voltage (12-V) battery uses the chassis for its ground, which means that the current path actually passes through the body of the car. In an electric vehicle, usually there are two voltage systems: high voltage and low voltage. The high-voltage battery and its connected devices in an EV is a floating system, which is completely electrically isolated from the chassis and the low-voltage system. In fact, some devices, such as the motor inverter and battery charger, will not function if they detect a current leakage over a preset threshold level to the chassis. A communication bus (i.e. CAN bus) transmits a signal from controllers powered from the low-voltage system to the various high-voltage devices. The main interface points for each block are shown in Figure 3, and at each point optocouplers are used to provide isolation boundaries to ensure no leakage current reaches the chassis. Three main optocoupler applications in an electric vehicle include the IPM Driven Inverter, the CAN Bus Isolation, and the SPI Isolation. Engine ECU Auxiliary Battery Communication Bus Engine Transmission Motor Generator Inverter Oil Pump HVAC - Converter BMS HV Battery HV Bus Charger Figure 3. Electrical system in an EV/HEV IPM Driven Inverter Indicate optocoupler isolation places. The inverter in an electric vehicle transforms the battery voltage so the batteries can drive an AC induction motor or a brushless motor. IGBTs (insulated-gate bipolar transistors) are the common switching power devices, and the IPM (smart, intelligent integrated power module) [4] hybrid module provides both the power to the IGBTs and the gate-driver circuits in a single compact package. Most IPMs, like Fuji Electric s Econo 7MBP150TEA060, also incorporate functions such as over-temperature detection, over-current sensing and under-voltage protection to feed back fault alarms to the microcontroller. A typical inverter system is shown in VDD FLT H_U ACPL-K49T 330p VccU_1 2.5k IPM HV Battery LV Battery Vreg _U MCU _U FLT L 310 ACPL-K43T ACPL-K43T k Vcc_1 1.9k 2.5k U V W M VccU_1 VccV_1 VccW_1 Vcc_1 330p ACPL-K49T Electrical isolation boundary Figure 4. Optocouplers provide isolation between the inverter s MCU and IPM (U-phase details shown, connections to V and W phases between the IPM and the MCU are not shown). 4

5 Figure 4. In this system diagram, the isolation circuitry sits between a MCU and IPM. Six ACPL-K43T digital optocouplers isolate the IPM s six gate driver inputs (three upper and three lower IGBTs for the three power phases). Optocouplers are shown only for phase U to reduce the figure complexity; additional optocouplers are required for phase V and phase W. Four ACPL-K49T [5] optocouplers isolate the fault feedback signals developed by the IPM (three for each upper phases and one for the lower phases). Power supply distributions are recommended as shown on the right portion of Figure 4. The 5-V supply for the optocouplers is directly derived from 12-V battery via a voltage regulator; The Vcc_1 power is generated by an isolated / converter that supplies all the low channel circuits; three additional isolated / converters output 15 V VccU/VccV/ VccW which are floating to each other, and power the upper channel circuit individually. CAN Bus Isolation The CAN bus network within a car connects the host control ECU and local subsystems such as the climate controller, battery management system (BMS), traction Inverter, oil/hydraulic pump and other subsystems. Optocouplers as shown in Figure 5 provide a galvanic isolation boundary between the local device with possible high voltage, and low voltage CAN bus in the systems. This ensures stable operation, eliminates electrical noise and prevents interference between subsystems. V DD TXD ACPL-K72T x2 V IO TXD V CC Vreg LV BAT INH RXD MCU c/w CAN I/O RXD V BAT STB NXP TJA1041 CANH I/O EN SPLIT CAN BUS I/O CANL ERR ACPL-K49T x3 Electrical isolation boundary Figure 5. Optocouplers provide isolation between local device MCU and the CAN bus transceiver One example of the CAN transceiver subsystems shows how the interface connections are configured to an NXP TJA1041 CAN controller (Figure 5). Two ACPL-K72T optocouplers transmit and receive data between MCU and TJA1041. With a maximum propagation delay time of 100 ns across a -40 C to 125 C operating temperature range, the ACPL-K72T has a wide performance range that encompasses all 3 ranks of high speed CAN data rate of 125 kbps, 250 kbps and 500 kbps in vehicle applications.[6] Three low speed ACPL-K49T optocouplers are communicating command, status or feedback signal for transceiver s pin ofstb, EN anderr. From LV battery, a voltage regulator derives 5 V power to transceiver and an isolated - converter supplies 5 V to MCU. 5

6 SPI Isolation Serial Peripheral Interface bus (SPI) is usually applied to between processor and its peripheral chips, such as battery cell voltage monitoring in a vehicle. Vehicle s high voltage battery is stacked of multi cells, which quantity is up to more than a hundred cells. To balance battery cell voltage during charging or monitor cell energy during battery output power to drive the motor, each cell s voltage is measured individually, after the analog voltage acquired, A within the measurement module convert it into digital data and communicate to BMS MCU as master device via SPI bus. LV Battery HV Battery Cells Battery Cell Voltage Measurement control A c/w SPI CS SCK SDI ACPL-K49T CS CLK MOSI V CC BMS MCU SDO MISO ACPL-K72T x3 Electrical isolation boundary Figure 6. Optocouplers isolate the SPI bus signals A typical 4-wire SPI interface consists of clock, data in/out and chip selection channels as shown in Figure 6. Three high speed ACPL-K72T optocouplers interface data in/out signal and clock frequency up to 2 MHz, one low speed ACPL-K49T transmit chip selection status from master/mcu to slave device/a. One isolated - converter supply 5 V power to slave side circuits. The isolation boundary provides insulation for low voltage circuits to high voltage battery and prevents noise interference from battery voltage fluctuation. Summary Avago s broad portfolio of digital optocouplers meets the automotive isolation/insulation application requirements from IPM driver to data communication. Additionally, the company s innovative new optocouplers feature low power consumption and high insulation values to provide enhanced high-voltage safety insulation and data integrity to automotive electronic systems, which ensure safety and reliability to automotive drivers and passengers with no compromise. 6

7 References [1] High Reliability LED Technology in R2Coupler Optocouplers for High Temperature Applications in HEVs and EVs, Roy Tan, White Paper AV EN, Avago Technologies. April 21, [2] ACPL-K43T, ACPL-K44T Automotive Wide Operating Temperature 1MBd Digital Optocoupler in a Stretched 8-Pin Surface Mount Plastic Package, Data sheet AV EN, Avago Technologies. September, [3] Avago Regulatory Guide to Isolation Circuits, White Paper AV EN, Avago Technologies. February 26, [4] Optocouplers Isolated Circuit for Intelligent Power Modules (IPM) and Gate Drivers, Application Note 1 AV EN, Avago Technologies. September 2, [5] ACPL-K49T Wide Operating Temperature Automotive R2CouplerTM 20 kbd Digital Optocoupler Configurable as Low Power, Low Leakage Phototransistor, Data sheet AV EN, Avago Technologies. September, [6] J2284/1/2/3 High Speed CAN (HSC) for Vehicle Applications at 125 kbps/250 kbps/500 kbps, SAE International For product information and a complete list of distributors, please go to our web site: Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies in the United States and other countries. Data subject to change. Copyright Avago Technologies. All rights reserved. AV EN - November 17, 2011