Why Are You Still Using A Relay To Drive A Car Motor?

- Dec 07, 2018-

Why are you still using a relay to drive a car motor?

With the advent of smaller and smarter integrated circuits (ICs) in automotive electrical systems, it’s time to start looking at the “elephant” in the room: why do we still use relays to control car sunroofs, window lifts, electric locks, and rear Luggage cover lifts, memory seats, compressors and various pumps on the car? Although relays are affordable and easy to design, their functionality is somewhat cumbersome for modern motor applications due to their limited life and large size. For a quiet, small, and secure solution, solid state ICs are the best choice for automotive motor control applications.

Solution size

Let's compare the two solutions. Figure 1 shows a typical relay solution with the same voltage and current rating and an equivalent solid-state solution.

For solution size only, solid-state 8mm x 8mm quad flat no-lead (QFN) and two dual-package N-channel MOSFETs account for approximately one-third of the board area of the relay solution. Looking at the z-axis, the entire solid-state solution is about 0.9 mm, or 0.035 inches high. If you want to build a motor driver printed circuit board (PCB) that fits just right on the back of the motor housing, TI's solid state solutions are perfect for this application.

In addition to size, the solid-state gate driver integrates a complete set of protection features that must otherwise be discretely built in the relay solution. These features include:

Motor current measurement

For any type of current regulation, both relay and solid state systems require a shunt resistor. 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. But solid-state motor drives typically integrate a low-side shunt amplifier, so the only discrete component you need is a single current-sense resistor. Figure 2 shows the difference between the integrated motor drive IC and the discrete current measurement circuit topology.

Motor speed curve

Motor speed curves with relays are extremely inefficient. Designers can use a different size resistor placed in series with the motor or a multi-winding motor with different speeds to achieve a multi-speed control scheme for power windows, lift gates, sunroofs, sliding doors or pumps with relays. If you want to choose a different speed, both solutions require more relays, which means 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 PH/EN 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-stage pumps, custom trajectories for sliding glass sunroofs, soft-closed power windows, economical variable speed windshield wipers or any other type of simple motion control motor application.

Related reference design

The small-size 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 with integrated shunt amplifier and two dual-package automotive grade package MOSFETs to create a very small power stage layout compared to typical relay solutions. The design also includes two TI's DRV5013-Q1 digital latching Hall effect sensors for encoding the motor position.

Designing a motor control system using TI's solid-state motor drive will help reduce the size of the PCB solution, allowing more and more motors to be controlled from the same module. Thanks to TI's highly integrated and simple control scheme for motor drives, designers can quickly and easily redesign most modern brushed DC motor controller circuits that are currently using relays.

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