Relay drive car motor advantage
Some people ask, there are so many methods that can be used to drive automotive motors. Why are they still using relays for driving? This article will explain this problem and understand the advantages of relays.
With the increasing adoption of smaller and smarter integrated circuits (ICs) in automotive electrical systems, it is time to start solving the problem of blindness: why are we still in the sunroof module, window regulator, power lock, rear gear? Control the motor with a relay in the board lift, memory seat, compressor and pump? Of course, the use of relays is cheap and simple to design, but their function seems to be cumbersome for modern motor applications, given their limited lifetime and large solution size. For quiet, small and safe solutions, solid state ICs are the best choice for automotive motor control applications.
Solution size
Let's compare the two solutions, as shown in Figure 1: A typical relay solution and an equivalent solid-state solution with the same rated voltage and current.
Motor current measurement
For any type of current regulation, both relays and solid state systems require shunt resistors. The relay solution requires a separate discrete amplifier circuit to increase the voltage measured across the sense resistor. The increased voltage is then sent to a microcontroller (MCU) analog-to-digital converter (ADC) so that the digital logic in the MCU can determine when to turn off the motor or limit the current. However, solid-state motor drivers typically integrate a low-side current shunt amplifier, so the only discrete component required is a single current-sense resistor. Figure 2 shows the difference between the integrated motor drive IC and the discrete current measurement circuit topology.
Interface to MCU
When connecting relays and solid-state solutions to an MCU, solid-state ICs typically implement a direct connection between the MCU's general-purpose input and output (GPIO) and analog-to-digital converter (ADC) pins. These ICs are typically flexible enough to connect a 1.8, 3.3, or 5V logic level voltage to a ground-based, high-impedance pull-down resistor. For relay solutions, some similar current gain is required to control the solenoid coils in the relay in order to achieve similar input control. Figure 3 shows the difference in circuit topology connecting the relay and solid state drive to the MCU.
Motor speed curve
The motor speed curve with relay is very inefficient. Designers can use relays to achieve multi-speed control schemes for power windows, lift doors, sunroofs, sliding doors or pumps through different sized resistors placed in series with the motor or multi-winding motors with different speeds. Both solutions require more relays if different speeds are chosen, and more relays require more board space and discrete components.
With a solid-state solution, you only need to provide two pulse width modulation (PWM) signals from the MCU for TI's motor drivers to control motor speed. On the DRV8702-Q1 and DRV8703-Q1, TI provides a phase/enable mode in which only one PWM signal is applied to the enable pin, while a simple logic high or low phase pin controls the direction of the motor. The logic level PWM signal is directly converted to the MOSFET gate with the correct voltage to fully enhance the high side or low side MOSFET. With this type of interface, you can quickly design multi-speed pumps, custom motion curves for sliding glass sunroofs, soft-closed power windows, inexpensive variable speed windshield wipers or any other type of simple motion control motor application.
Instance
The small footprint sunroof motor module reference design is a solid state motor control module for skylight and window lift applications. The TI reference design uses the DRV8703-Q1 gate driver, which integrates a current shunt amplifier and two dual-row package automotive grade package MOSFETs to create a very small power stage layout compared to a typical relay solution. The design also includes two TI's DRV5013-Q1 latching Hall sensors for encoding the motor position.
Designing a motor control system using TI's solid-state motor drives will help reduce the size of the PCB solution, enabling more motors to be controlled from the same module. Thanks to the high integration of our motor drives and simple control schemes, designers can quickly and easily redesign most modern brushed DC motor controller circuits that are currently using relays.
