【Introduction】As the industry replaces mechanical relays with solid-state switches in high-current motor control applications, smart gate drivers in the FLEXMOS™ portfolio can be paired with external MOSFETs. Onsemi’s pre-driver solutions enable scalable design flexibility to meet different load requirements, extending motor drive applications from one motor to as many as seven, making these products ideal for a wide range of automotive body electronics applications ideal choice.
The main functions of the pre-driver IC are as follows:
1. Interface between microcontroller (uC) and external power MOSFET output stage. The pre-driver IC provides isolation between the microcontroller’s low voltage domain (typically less than 5.5 volts) and the maximum 40 volts on the car battery connection system.
2. Provide gate overdrive for high-side stage to enable use of N-channel MOSFETs.
3. Monitor and protect external MOSFETs and connected loads to improve system robustness and diagnostic capabilities.
4. Provide specific AC characteristics for turn-on and turn-off curves to minimize electromagnetic radiation and loss of energy efficiency for switching applications.
Applications that require bidirectional motor rotation use an external MOSFET H-bridge configuration – the H-bridge is integrated to simplify operation. In contrast, applications with higher thermal management and load current requirements use pre-driver and external MOSFET solutions. Since the higher the number of channels, the more manageable the power consumption of the pre-driver is, the global system response improves significantly as the number of high-current motors in the system increases. A single gate driver IC such as the NCV754x monitors all half-bridge nodes and connected loads and prevents unintended motor motion due to fault conditions.
The H-bridge architecture allows engineers to direct current flow in the motor, as shown in Figure 1. The direction of the current determines the direction of rotation of the motor.
Figure 1: H-Bridge Motor Control
The amount of current flowing through the motor is directly related to the motor speed and available torque. If the current in the motor increases, the speed of the motor increases if the torque requirement of the external load remains the same. Alternatively, the increased current can translate into higher torque for heavy loads while maintaining a constant speed.
ON semiconductor‘s FLEXMOS pre-drivers include programmable current sources and calibration technology, allowing system designers to choose from our broad portfolio of N-channel MOSFETs available in the Application Specific Standard Product (ASSP) market. The MOSFET Product Recommendation Tool can be used to select the right MOSFET for the right application.
The pre-driver topology provides faster design cycles to meet changing system requirements based on different automotive OEM specifications, regulation levels and model types.
Figure 2: NCV754x application diagram
Shorter design cycles are achievable because the system designer can adjust the MOS power stage for different loads while keeping the rest of the hardware components the same. While maintaining the overall software flow, minor software modifications are required to fine-tune the control of the new MOSFETs. As can be seen in Figure 3, the reuse of IP for software development is more pronounced when selecting a pre-driver solution from the same product family.
Figure 3: NCV754x Register Summary
System designers can optimize cost by using a variety of pre-driver options to achieve the number of motor-actuators required by OEM specifications through different combinations of channels. Due to the flexibility of the pre-driver, these devices can be used in a variety of end applications, including:
● Seat Control Module
● Door locks and latches
● Lift door actuator
● sunroof or sunroof actuator
● window regulators
The half-bridge driver block has multiple programmable pull-up and pull-down current sources for charging the gates of the external MOSFETs. Figure 4 shows a simplified view of a half-bridge pre-driver configured with an external MOSFET and motor load.
Figure 4: Simplified half-bridge pre-driver controlling external MOSFET and motor load
The programmable current source highlighted in orange in the image above enables advanced slew rate control. For faster turn-on, HS1’s pull-up current source can be increased in stages to charge the gate faster, providing predictable switching behavior for motor loads. The significant benefits of precise slew rate control are enabling higher pulse width modulation (PWM) frequencies with tighter minimum and maximum duty cycles, managing power loss and reducing electromagnetic emissions.
For motor control applications, a PWM above 20 kHz is highly desirable, above the audible frequency range. Better performance of duty cycle min and max provides advanced speed control for motor loads with varying torque requirements. For example, for heavy loads or motor start conditions, higher torque is required to keep the speed constant. System designers must increase the duty cycle to increase the motor current to achieve higher torque. Therefore, it is desirable to operate with a duty cycle close to the 100% value.
Due to the high current and high drain-source voltage (VDS) operating point, the MOSFETs experience maximum power loss when the channel starts to conduct. Typically, a quick transition from the high VDS state of the MOSFET to the fully enhanced VDS state is desirable. Therefore, switching losses are minimized during PWM periods of demanding loads. Switching too quickly can cause a shoot-through event, which can lead to module degradation or undesirable electromagnetic emissions from the system.
Selectable current sources for the gate driver stage allow system designers to modify switch profiles based on thermal or emissions requirements during design verification, end-of-line calibration during production, or even during software updates during the end product’s life cycle.
In addition to MOSFET control, the FLEXMOS half-bridge pre-driver monitors and protects the external MOSFET and connected load against short circuits by monitoring the VDS of the active MOSFET. Figure 5 shows an example of the output waveform and various fault conditions.
Selected FLEXMOS pre-drivers provide VDS monitoring schemes in two separate sections: dynamic VDS protection and static VDS protection. Dynamic VDS protection monitors the switching phase, while static VDS protection operates when the MOSFET is fully on. For a switching curve of known voltage versus time programmed on the factory line, the selected FLEXMOS half-bridge device monitors the progress of the output voltage (VHB) and determines if a fault condition exists.
Figure 5: Half-bridge output switching curve and VDS protection
For in-depth information on programmable current settings, PWM operation and device protection features, refer to our Application Notes – MOSFET Gate Drive Requirements and Initial Parameter Setting Schemes Using the NCV7544, NCV7546 and NCV7547 Half-Bridge Pre-Drivers (AND90124/D) and Reference tool. Please request an assessment tool through the ON Semiconductor sales team for a quick workbench assessment.
Source: ON Semiconductor, original Nixon Mathew
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