Technical Article: Optimizing 48V Mild Hybrid Electric Vehicle Motor Drive Design

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Technical Article: Optimizing 48V Mild Hybrid Electric Vehicle Motor Drive Design [Copy link]

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The ultimate goal of manufacturers in creating mild hybrid electric vehicles (MHEVs) is to reduce greenhouse gas (GHG) emissions. Mild hybrid electric vehicles consist of a 48V motor drive system connected to the vehicle's transmission system. To reduce GHG emissions, the internal combustion engine (ICE) in a mild hybrid electric vehicle is turned off when the vehicle is coasting, and the 48V motor system charges the 48V battery to power the vehicle. In this article, I will discuss a design approach for a 48V motor drive that provides high-power motor drive, achieves functional safety, and is smaller in size.

Precautions for high-power motor drive

For automotive powertrain applications, a typical 48V motor drive system requires 10kW to 30kW of electrical power. Traditional 12V battery systems cannot meet this power level, so a 48V architecture must be used to support high-power motor drives.

Read the white paper, “How to Build a Small, Functionally Safe 48V, 30kW Mild Hybrid Electric Vehicle Motor Drive System”, to learn more about how to solve the key design challenges in the drive circuit of the motor drive system.

As shown in Figure 1, the 48V motor driver controls external metal-oxide semiconductor field-effect transistors (MOSFETs) to rotate the motor. These external MOSFETs must support currents of more than 600A to achieve the 30kW power target. Effectively reducing the RDS(on) of the MOSFET reduces heat dissipation and conduction losses, and in some cases, paralleling multiple MOSFETs in each channel will help spread the heat, as described in the application note "Driving Parallel MOSFETs with the DRV3255-Q1". The total gate charge of the MOSFET can be as high as 1,000nC.

Designers also need to optimize the power dissipation caused by switching losses to make the entire solution meet automotive electromagnetic compatibility (EMC) regulations. High gate current gate drivers such as the DRV3255-Q1 can drive high gate charge MOSFETs with peak source currents up to 3.5A and peak sink currents up to 4.5A. Such high output currents enable short rise and fall times even at gate charges of 1,000nC. Selectable gate driver output current levels allow you to fine-tune rise and fall times to optimize between switching losses and electromagnetic compatibility (EMC).

Figure 1: The most common power architecture for high-power 48V motor drives

Even though the battery’s nominal voltage is 48V, the supply voltage can vary significantly due to transient conditions during operation; see the voltage levels specified by the International Organization for Standardization (ISO) 21780 in Figure 2. In addition, the motor driver pins need to be able to withstand negative transient voltages given the reverse recovery time of the MOSFET parasitic body diode.

Figure 2: Voltage levels for 48V systems as specified in ISO 21780

With a 105-V tolerant high-side bootstrap pin, the DRV3255-Q1 supports true continuous operation at 90 V and transients up to 95 V. The bootstrapped high-side MOSFET source and low-side MOSFET source are rated for –15-V transients, providing the robust protection required for high-power motor drive systems.

Functional Safety Considerations for 48V Motor Drives

48V motor drive systems run the risk of generating unnecessary power dissipation, which can cause an overvoltage condition that can damage the system. The normal system response is to turn on all high-side or low-side MOSFETs to recirculate the motor current and avoid drawing more current. If a fault occurs, the system must have a mechanism to switch the functional MOSFETs appropriately to avoid further damage. Implementing this type of protection typically requires external logic and comparators.

With the active short-circuit logic integrated in the DRV3255-Q1, you can decide how to respond when a fault condition is detected. Instead of responding to a fault condition by disabling all MOSFETs, the logic can be configured to enable all high-side MOSFETs, enable all low-side MOSFETs, or dynamically switch between low-side and high-side MOSFETs, depending on the fault condition. In addition, the DRV3255-Q1 meets the functional safety standards specified by ISO 26262 and includes diagnostic and protection features to support ASIL D-level functional safety motor drive systems.

Sizing Considerations for 48V Motor Drivers

The limited space in the engine compartment requires a small board size for the 48V motor drive system. Figure 3 shows a typical motor driver block diagram of a traditional 48V high-power motor drive design. To implement a safe motor drive system with robust protection features, clamping diodes, external drive circuits, sink resistors and diodes, comparators, and external safety logic are required. These external components increase board space and system cost.

Figure 3: Typical 48V high-power motor driver block diagram

Using the DRV3255-Q1, significant advantages can be provided to effectively reduce the overall board size by integrating external logic and comparators, adjustable high current gate drivers, and support for large voltage transients without the need for additional external components, as shown in Figure 4.

Figure 4: Simplified DRV3255-Q1 motor driver block diagram

As 48V mild hybrid electric vehicles become more common, are you considering a mild hybrid electric vehicle for your next car?

Keywords: 48V motor driver 48V starter generator