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Common driver platform for 12V and 48V systems simplifies EV starter generator design

Dec 29, 2022      View: 260

Belt-driven starter generators are integral to hybrid electric vehicle (HEV) and electric vehicle (EV) systems because they help reduce carbon emissions from internal combustion engines. Starter-generator systems play multiple roles in electric vehicle architecture. They are responsible for starting the engine, providing an electrical boost to the engine, and generating charging voltage during deceleration or coasting, thereby reducing wear and tear on the mechanical braking system while improving overall system efficiency.

Regardless of their architecture or location, starter generator systems have proven to be integral to vehicle electrification. Starter generators can be used in multiple locations within a vehicle. Figure 1 shows the locations of the major starter generator systems. The P0 and P1 positions are typically less than 20 kW. The P0 systems are becoming very common because they are the easiest to implement, require less redesign, and are cost-effective. The P1 position offers similar advantages while eliminating belt losses, improving performance, and reducing wear.

Starter Generator Circuit Implementation

The starter-generator system consists of several electrical and mechanical components. The inverter provides the electrical boost and DC-DC converter that converts the mechanical energy in the rotor into electrical energy in an energy harvesting mode. The system is also responsible for the crank position in the idle stop system and the high starting torque required for cold starts. On the mechanical side, the starter generator consists of a stator connected to a three-phase inverter and a rotor that generates a magnetic field through slip rings and brushes that allow DC to flow through the rotor windings. Newer designs using permanent magnet motors eliminate the need for excitation coils. Still, this approach presents other safety challenges because the magnetization cannot be turned off in the event of a fault.

Universal Drive, Current Sensor, and Motor Position Angle Sensor Solutions for 12V and 48V Systems

Belt-driven starter generator (BSG) systems for 12V and 48V power rails. 12V BSG systems do not offer the same power benefits as 48V starter generators. Typically, 12V systems are limited to < 10 kW, while 48V systems can produce up to 25 kW or more. As the power increases, so do the requirements for the gate driver and the current sensor. For the P0/P1 position, using a common architecture for both 12V and 48V cells is advantageous, requiring minimal additional components or redesign. Using a common architecture reduces design time and cost of materials (BOM) and provides a single platform for bolt-on BSG systems in 12V and 48V systems.

The AMT49502 half-bridge gate driver can operate over a voltage range of 5.5 V to 80 V, making it a versatile platform for BSG applications operating on either the 12V or the 48V supply rail. The device's charge pump regulator provides a gate drive for two N-channel MOSFETs. Figure 3 shows a functional block diagram of the half-bridge design. Only a single power supply is required, and all internal logic is created by the on-chip logic power regulator powered by the charge pump regulator. The regulator is responsible for providing a regulated 11 V voltage to the floating bootstrap capacitor, ensuring that the high-side MOSFET has a gate voltage of 11 V and a battery voltage of 5.5 V. The charge pump regulator also provides the internal logic, thereby reducing the total power consumption of the chip. Minimizing power consumption is the key to operating at 48 V without buck regulation.

NEVSEMI also offers a wide range of current sensing options, all with similar analog interfaces that can be fed back to the microprocessor for redundant full-field-orientation control (FOC). The AMT49502 has an integrated high-performance current-sense amplifier for low-power systems that measures current through a low-side current shunt. As power increases, Hall-effect-based current sensors offer much lower power consumption and smaller size than the necessary shunt resistors. Their current isolation also means they can be placed on the high side, low side, or in-phase, providing flexibility at the system level for control and short-circuit detection. The ACS71240 provides an accurate, efficient, and small solution for typical currents in the rotor coil. The most common solutions for much higher currents in motor phases are the ACS70310/1 or ACS37612/10 coreless solutions within the C-core. All of these solutions provide redundancy and built-in diagnostics. Both the ACS71240 and ACS37610 provide built-in overcurrent detection, and the ACS37610 provides overheat detection. Each MOSFET in the AMT49502 can be independently controlled using the logic inputs and auxiliary ENABLE input, providing separate paths to disable the bridge or activate sleep mode. The serial peripheral interface (SPI) port can then read diagnostic information and set functional parameters.

NEVSEMI offers a complete portfolio of magnetic angle sensors for various motor position sensing applications to support starter generator designs further. The high-resolution A1333 and AAS33001 angle sensors provide rotating motor position information and can be part of a sinusoidally commutated motor control scheme. This motor control solution brings high efficiency and improved torque performance to starting generators. In addition, NEVSEMI can support traditional block commutation motor control methods with a complete portfolio of magnetic Hall sensors.

Overall, the BSG designed with the AMT49502, current sensor IC and motor position sensor can be used in both 12V and 48V systems and easily tuned for power.

Designed for Harsh Environments

Starting a generator system generates high voltages on the inverter bridge. In generator mode, the purpose of the inverter is to convert the three-phase current into DC voltage and current that can be applied as a charge to a 12V or 48V battery system. Ultimately, the voltage generated by the motor is based on the speed. For the inverter bridge, it is important to withstand the voltage transients that exist during high-speed rotation and the transition from drive and generator modes. The gate driver must be robust enough to handle the high current and voltage transients present in the system. By designing the gate driver to withstand these transients, developers save valuable design time and minimize the additional cost of adding a high-voltage clamp to protect the system. When the high-side MOSFET is turned off in generator mode

Relative to the phase node, the AMT49502 gate driver can withstand -8 V on the low-side gate and -18 V on the high-side driver, as shown in Figure 4. The robust transient performance and intelligent control algorithms ensure that even high-power systems will not damage the inverter. The EV components must be robust enough to handle negative voltage transients and pass the manufacturer's EMF requirements. Start-up generator inverters need to switch quickly to maintain efficiency while minimizing emissions. They must also limit the intensity of electromagnetic radiation to meet stringent OEM requirements.

To meet the tradeoff between high efficiency and low EMF, the AMT49502 driver uses a segmented programmable current gate drive topology that allows control of all MOSFETs on and off in the system. MOSFET off-to-on and on-to-off transitions are controlled, as shown in Figure 5. All parameters are programmed through the SPI port.

When the gate driver is commanded to turn on, the current I 1 provides a duration t 1 on the high-side or low-side gate terminal. These parameters should normally be set to charge the MOSFET input capacitor quickly to the start of the Miller zone since the drain-source voltage does not change during this time. After that, the source current on GH or GL is set to the value of I 2 and remains at that value while the MOSFET transitions through the Miller region and reaches full conduction.

The on-off transition of the MOSFET is controlled, as shown in Figure 5. When the gate driver is commanded to turn off, the current I 1 is absorbed by the high-side or low-side gate terminal for a duration of t1. These parameters should normally be set to quickly discharge the MOSFET input capacitance to the beginning of the Miller zone since the drain-source voltage does not change during this time. After that, the current absorbed by the high-side or low-side gate terminal is set to a value of I 2 and remains there as the MOSFET transitions through the Miller zone and reaches a fully off state.

Fully controlling MOSFET switching increases efficiency and reduces EMI. Reducing dead time and the time required for the MOSFET to reach its Vt improves inverter performance by minimizing high-side and low-side MOSFET switching times and improves sinusoidal current fidelity. Programmable current control of the MOSFET voltage swing in the Miller region limits radiation while maintaining efficient switching times.

The ASIL D-certified AMT49100 three-phase gate driver is available for pure 48V systems. The AMT49100 provides additional diagnostics and the ability to verify each diagnostic using the built-in test circuitry. This additional diagnostic and verification capability for single-driver designs provides functional safety by notifying the engine control unit (ECU) of various faults.

Some 48V designs may benefit from ultra-small gate drivers. For example, the 10 to 100 V A89500 half-bridge gate driver in a 3 mm × 3 mm DFN package is small enough to reduce overall printed circuit board (PCB) space. The device can be used for excitation coil drivers and inverters with proper safety analysis. The driver is powered directly from the 8 to 13 V gate supply, and the field effect transistor (FET) bridge is connected directly to the 48 V battery. For more information, see Block Diagram 6.


Designed for Safety

A starter generator failure can cause the Li-ion battery pack to overcharge, which can be dangerous if the battery is shorted. For this reason, the starter generator circuit must comply with ISO 26262 standards, which typically require a "B" rating. For example, a failure of the inverter bridge while the generator is still spinning at high speed can lead to an overcharge situation. In a five-phase system, one solution is to effectively remove the magnetic field from the rotor by disabling the excitation coil drive. In this implementation, design is critical to developing a fail-safe system. If the gate driver in this system is designed specifically for safety, the requirements can be more easily achieved. For example, the AMT49502 was designed according to an ISO 26262-certified development process, and the device is certified as ASIL B compliant.

Each half-bridge driver has an advanced set of diagnostics that includes nearly two dozen diagnostic functions, including load-surge detection, MOSFET short-circuits protection, gate drive under-voltage, bridge power supply overvoltage, temperature warnings, and other conditions. IC diagnostics provide the system controller with the information necessary to monitor operation and make decisions about the actions taken by the system to ensure fail-safe operation. Figure 7 shows the diagnostic features supported by the AMT49502 gate driver.

Similarly, NEVSEMI offers Hall-effect current sensing and motor positioning solutions designed with safety in mind. In the current sensor portfolio, the ACS71240, ACS70310/1, and ACS37612/10 are QMs with available safety-related documentation and are used in applications with system-level ratings up to ASIL D. In the angle sensor portfolio, the A1333 and AAS33001 are available as ASIL B, or D rated safety components independent of single-chip and dual-chip products.

Conclusion

BSG systems are becoming more common in HEV motor control designs because they are easy to implement, have a similar footprint to existing alternator systems, and do not require major modifications to the powertrain (in positions P2 - P4). As starter alternator systems continue to evolve, further integration may impact the role of the BSG over time. Looking forward, 48V systems may dominate the P3 - P4 positions.

As the electrical revolution transforms the automotive industry, electrification will continue to gain market share, and 12V solutions will give way to higher battery voltages. A common platform for 12V and 48V systems will simplify and streamline the transition to 48V solutions. Starter-generator systems will also benefit from industry-leading safety diagnostics, redundancy provided by independent bridge control, current sensing, and motor position sensors with robust transient performance.

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