Precautions for high-power motor drives
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, therefore a 48V architecture must be adopted to support high-power motor drives.
Read the white paper "How to Build a Functionally Safe Small 48V, 30kW Mild Hybrid Electric Vehicle Motor Drive System" to learn in detail how to solve major design challenges in the drive circuit of the motor drive system.
As shown in Figure 1, a 48V motor driver controls external metal-oxide-semiconductor field-effect transistors (MOSFETs) to rotate the motor. These external MOSFETs must support currents of over 600A to achieve a power target of 30kW. Effectively reducing the RDS(on) of the MOSFETs reduces heat dissipation and conduction losses. In some cases, paralleling multiple MOSFETs in each channel helps to distribute heat, as described in the application manual "Driving Parallel MOSFETs Using the DRV3255-Q1". The total gate charge of the MOSFETs can be as high as 1,000nC.
Designers also need to optimize power dissipation caused by switching losses to ensure the entire solution complies with automotive electromagnetic compatibility (EMC) specifications. 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. Even with a gate charge of 1,000nC, such high output currents allow for very short rise and fall times. Selectable gate driver output current levels allow for fine-tuning of rise and fall times, thus optimizing between switching losses and EMC.
Figure 1: The most common power architecture for high-power 48V motor drivers
Even if the battery's nominal voltage is 48V, the supply voltage can vary significantly due to transient conditions during operation; please refer to the voltage levels specified in International Organization for Standardization (ISO) 21780 in Figure 2. Furthermore, considering the reverse recovery time of the MOSFET parasitic diode, the motor driver pins need to be able to withstand negative transient voltages.
Figure 2: Voltage level of a 48V system as specified in ISO 21780
With its high-side bootstrap pin capable of withstanding 105V, the DRV3255-Q1 can support true continuous operation at 90V and transient voltages up to 95V. The bootstrap high-side MOSFET source and low-side MOSFET source are rated for -15V transient voltages, providing the robust protection required for high-power motor driver systems.
Functional safety precautions for 48V motor drivers
48V motor drive systems pose a risk of generating unnecessary power consumption, which can lead to overvoltage conditions and damage the system. A normal system response is to turn on all high-side or low-side MOSFETs to recirculate motor current and prevent further current generation. In the event of a fault, the system must have a mechanism to appropriately switch functional MOSFETs to prevent further damage. Implementing such protection typically requires external logic and comparators.
The active short-circuit logic integrated into the DRV3255-Q1 allows you to determine how to respond when a fault condition is detected. This 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), rather than responding by disabling all MOSFETs. Furthermore, the DRV3255-Q1 complies with the functional safety standard specified in ISO 26262 and includes diagnostic and protection functions to support ASIL D-level functional safety motor drive systems.
Size Considerations for 48V Motor Drivers
Limited space in the engine compartment necessitates a small circuit board size for the 48V motor drive system. Figure 3 shows a typical block diagram of a conventional 48V high-power motor drive design. Implementing a safety motor drive system with robust protection requires clamping diodes, external drive circuitry, bus resistors and diodes, comparators, and external safety logic. These external components increase board space and system cost.
Figure 3: Block diagram of a typical 48V high-power motor driver
By adopting the DRV3255-Q1, significant advantages can be provided to effectively reduce the overall board size through the integration of 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 block diagram of the DRV3255-Q1 motor driver
With 48V mild hybrid electric vehicles becoming increasingly common, have you considered adopting a mild hybrid electric vehicle for your next car?
