How to build a functionally safe small 48V, 30kW MHEV motor drive system

[Guide]Global initiatives to reduce greenhouse gas (GHG) emissions have promoted the development of automobiles, requiring automakers to increase the electrification level of new car powertrains. Light hybrid electric vehicles (MHEV) use a 48V motor drive system to help reduce GHG emissions from internal combustion engines (ICE), which has become an attractive option to achieve compliance because the implementation cost of such vehicles is much lower than that of full-scale vehicles. Hybrid electric vehicles. This white paper describes how to use DRV3255-Q1 48V BLDC motor driver to achieve automotive safety integrity level D (ASIL D) functional safety in MHEV, while providing up to 30kW motor power and high integration to help reduce layout space.

1 Introduction

Global initiatives to reduce greenhouse gas (GHG) emissions have promoted the development of automobiles, requiring automakers to increase the electrification of new car powertrains. Light hybrid electric vehicles (MHEV) use a 48V motor drive system to help reduce GHG emissions from internal combustion engines (ICE), which has become an attractive option to achieve compliance because the implementation cost of such vehicles is much lower than that of full-scale vehicles. Hybrid electric vehicles. This white paper describes how to use DRV3255-Q1 48V BLDC motor driver to achieve automotive safety integrity level D (ASIL D) functional safety in MHEV, while providing up to 30kW motor power and high integration to help reduce layout space.

2 The use of MHEV and 48V motor drive system

In countries/regions such as the United States, Japan, China, and the European Union, a number of global initiatives aimed at reducing greenhouse gas (GHG) emissions have promoted the development of automobiles.For example, the National Highway Traffic Safety Administration under the U.S. Department of Transportation issued its final environmental impact statement in March 2020[1], Which stipulates vehicle fuel economy indicators from 2021 to 2026. According to the estimates in Article 2.2.2.3 of the statement, by 2026, the average fuel economy index of the combined passenger car and truck company will reach 40.4mpg. The EU also pledged to reduce GHG emissions by 40% by 2030 (compared to 1990) in accordance with the Paris Agreement (COP21)[2].

There are several ways for automakers to achieve the goal of reducing GHG emissions. One way is to make a light hybrid electric vehicle (MHEV) that uses a 48V motor drive system. Since the internal combustion engine (ICE) emits GHG during the combustion process, the ICE in the MHEV will be turned off when the vehicle is coasting, which helps to reduce the GHG emissions of the ICE. In this case, the 48V motor drive system will charge the 48V battery to power the vehicle. MHEV is an attractive option for automakers to achieve GHG emission reduction targets, because the cost of implementation of this type of vehicle is much lower than that of a full hybrid electric vehicle, and it has design flexibility.

The 48V motor drive system in MHEV can be fixed to the transmission system in different positions according to the design goals. Figure 2-1 shows the connection points on the transmission system.

How to build a functionally safe small 48V, 30kW MHEV motor drive system

Figure 2-1. Motor drive system connection point on the transmission system (48V)

When the 48V motor drive system is located at P0 or P1, it can be used as a starter generator because it is connected to the ICE and has the functions of a starter and a generator. When located at P2, P3 or P4, the 48V motor drive system is used as a motor generator.

3 Design Challenges of 48V Motor Drive System

There are many factors that affect the successful design of a 48V motor drive system: high-power motor drive, safety and small size. High-power motor drives are very important to help achieve GHG emission reduction. Since the 48V motor generates electricity during the coasting and braking of the vehicle, and when used as a starter generator, it will also provide power assistance when the engine is started. Therefore, ensuring functional safety is of the utmost importance. In addition, in the limited space of the engine compartment, the 48V motor system is placed close to the ICE, and it is necessary to achieve a small size.

For automotive powertrain applications, a typical 48V motor drive system requires 10kW to 30kW of electrical power. 48V and 12V dual power supply system can support this level of high-power motor drive. There are many different power architectures for high-power 48V motor drivers.

Figure 3-1 shows the most common architecture of a 48V motor driver. Connect a 48V battery to the motor, and then use a DC-to-DC step-down converter to convert the 48V down to 12V and supply it to the motor driver, power management integrated circuit and microcontroller. The 12V battery and the 12V power generated by the DC-to-DC step-down converter are connected through a second machine to ensure that the 12V power required for motor control can be provided. The voltage of the 48V power supply should follow the standards specified by the International Organization for Standardization (ISO) 21780 (as shown in Figure 3-2).

How to build a functionally safe small 48V, 30kW MHEV motor drive system

Figure 3-1. Common power architecture of high-power 48V motor driver

How to build a functionally safe small 48V, 30kW MHEV motor drive system

Figure 3-2. The 48V voltage level specified by ISO 21780

4 Precautions for high-power motor drive

As shown in Figure 3-1, a 48V high-power motor driver can drive an external metal oxide semiconductor field effect transistor (MOSFET) to rotate the motor. In order to support power from 10kW to 30kW, these external MOSFETs need to support currents from 200A to 600A or more. Minimizing the RDS(on) of the MOSFET can help reduce heat dissipation and conduction loss; in some cases, it is advisable to connect multiple MOSFETs in parallel in each channel because it helps to dissipate heat from each MOSFET . Therefore, the total gate charge of the MOSFET is very large, between 300nC and 700nC. In extreme cases such as power up to 30kW, the total gate charge of the MOSFET may be as high as 1,000nC.

It is important to optimize the heat dissipation caused by switching losses and ensure that the entire solution complies with electromagnetic compatibility (EMC) specifications. The rise and fall times of MOSFETVDS determine the switching losses. Shorter rise and fall times can reduce switching losses, but will affect EMC performance. Figure 4-1 shows the relationship between the gate charge of the MOSFET and the fall time during MOSFET switching.

How to build a functionally safe small 48V, 30kW MHEV motor drive system

Figure 4-1. Relationship between VDS fall time and gate charge

As shown in Figure 3-2, the 48V battery may exceed the nominal voltage, and its transient overshoot may be higher than the 60V limit. On the contrary, since the reverse recovery time of the MOSFET parasitic diode will cause a slow response, the phase connection pins of the motor driver must be able to withstand negative transient voltages. It is difficult to choose a motor driver that can maintain normal operation at a voltage higher than 48V and still withstand negative voltage.

The integrated DRV3255-Q1 48V BLDC motor driver is designed to drive high gate charge MOSFETs: the peak source current output by the gate driver is 3.5A, and the peak sink current output by the gate driver is 4.5A. With such a high current drive capability, even when the gate charge is 1,000nC, the rise and fall times of the MOSFET VDS can be shortened. In addition, DRV3255-Q1 can select the gate driver output current level. This device can help system designers to fine-tune the rise and fall times with adjustable current levels to optimize the switching loss (which affects heat dissipation) and EMC performance.

The maximum operating voltage of the DRV3255-Q1 high-side MOSFET gate driver bootstrap pin is 105V. At the same time, combined with the continuous working maximum motor power pin voltage of 90V, DRV3255-Q1 can achieve true 90V operation when rotating a 48V motor. The negative transient voltage rating of the bootstrap pin, high-side MOSFET source sense pin, and low-side MOSFET source sense pin is –15V.

5 Safety and dimensional precautions of 48V motor drive system

When using a safe high-power motor driver, a protection mechanism is required, because the rated current flowing through the motor may exceed 200A. One of the key issues with 48V motor drive systems is that the motor may generate unnecessary power, which can lead to overvoltage conditions that can damage the system. The system should have a functional MOSFET control mechanism (to ensure correct turn-on or turn-off) in order to protect the system from further damage caused by overvoltage conditions. This type of protection usually requires external safety logic and comparators.

DRV3255-Q1 integrates active short-circuit logic, allowing system designers to determine the response to MOSFET short-circuits. The device can be configured to enable all high-side MOSFETs or all low-side MOSFETs or dynamically enable all low-side or high-side MOSFETs instead of disabling all MOSFETs in response to fault conditions. The response delay time of the active short-circuit input device can be programmed and configured through the serial peripheral interface (SPI) register. In addition, DRV3255-Q1 provides complete diagnostic coverage and is designed in accordance with the ISO 26262 standard, helping to achieve functional safety motor drive systems up to automotive safety integrity level D (ASIL D).

Figure 5-1 shows a block diagram of a typical motor driver designed for a 48V high-power motor driver. To realize a safe and reliable motor drive system, clamp diodes, external drive circuits, sink resistors and diodes, comparators, and external safety logic are required. DRV3255-Q1 integrates external logic and comparators, supports voltages up to 105V on the bootstrap pin, can handle negative transient voltages as low as -15V, and provides optional high-current gate driver current output. As shown in Figure 5-2, using DRV3255-Q1 to design a 48V high-power motor drive system that can remove the dashed box components. This method can simplify the design and reduce the number of components on the circuit board, thereby realizing a compact design suitable for the limited space of the engine compartment.

How to build a functionally safe small 48V, 30kW MHEV motor drive system

Figure 5-1. Block diagram of the motor driver

How to build a functionally safe small 48V, 30kW MHEV motor drive system

Figure 5-2. Simplified DRV3255-Q1 motor driver block diagram

6 Conclusion

The 48V motor drive system is designed to reduce the GHG emissions of MHEV. As system designers, they need to design a 48V motor drive system with small size and functional safety with high power (10kW to 30kW). DRV3255-Q1 has optional high gate drive current, high voltage transient support, active short-circuit logic and functional safety compliance, which helps to design high-power, functionally safe small 48V MHEV motor drive systems.

7 References

1. “Final Environmental Impact Statement – ​​The Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule for Model Years 2021-2026 Passenger Cars and Light Trucks,” National Highway Traffic Safety Administration, US Department of Transportation, March 2020 (docket no. NHTSA -2017-0069).

2. “Worldwide Emission Standards and Related Regulations – Passenger Cars/Light and Medium duty Vehicles”, CPT Group GmbH, May 2019.

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