Some people ask, with so many ways to drive car motors now available, why are relays still being used? This article will elaborate on this question and explore the advantages of relays.
With the increasing adoption of smaller and smarter integrated circuits (ICs) in automotive electrical systems, it's time to address a problem we've often overlooked: why are we still using relays to control motors in sunroof modules, power window regulators, power locks, tailgate regulators, memory seats, compressors, and pumps? While relays are certainly inexpensive and simple to design with, their limited lifespan and relatively large solution size make their functionality seem cumbersome for modern motor applications. 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.
Figure 1: Relay Solutions vs. Solid State Solutions
For solution size, the solid-state 8mm × 8mm square flat no-lead (QFN) package plus two dual-row N-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) occupy approximately one-third of the board area of the relay solution. Looking at the z-axis, the entire solid-state solution is approximately 9mm high, or 0.035 inches. TI's solid-state solution is ideal for applications where a motor driver printed circuit board (PCB) can be easily mounted on the back of the motor housing.
In addition to size, solid-state gate drivers integrate a complete set of protection functions that must be built separately in relay solutions. These functions include:
Motor current measurement
For any type of current regulation, both relays and solid-state systems require a parallel resistor. Relay solutions require a separate discrete amplifier circuit to amplify the voltage measured across the sense resistor. The amplified 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 shut off the motor or limit the current. However, solid-state motor drivers typically integrate a low-side current parallel amplifier, so the only discrete component required is a single current sense resistor. Figure 2 illustrates the difference between integrated motor driver ICs and discrete current measurement circuit topologies.
Figure 2: Discrete and solid-state current measurements
TI's motor drivers take current regulation a step further by integrating a cycle-by-cycle current chopping method using an internal comparator connected to the output of an integrated current sense amplifier. All that's needed is an external reference voltage; the device handles the current limiting, freeing up resources that would otherwise be used in MCUs or discrete builds. The sense amplifier output is still connected to the package pins, but if you only require a certain level of current regulation, consider a fully integrated solution such as the DRV8702-Q1 or DRV8703-Q1.
Interface connection to MCU
When connecting relays and solid-state solutions to an MCU, solid-state ICs typically enable direct connections between the MCU's general purpose input/output (GPIO) and analog-to-digital converter (ADC) pins. These ICs are generally flexible enough to connect 1.8, 3.3, or 5V logic levels to ground-based high-impedance pull-down resistors. For relay solutions, some current gain is required to control the solenoid coil within the relay to achieve similar input control. Figure 3 illustrates the differences in circuit topologies for connecting relays and solid-state drivers to an MCU.
Figure 3: Interface connection to MCU
The relay solution in Figure 3 outlines the requirements for Darlington transistors in N-doped and P-doped (NPN) bipolar junction transistors (BJTs), with two resistors and a protection diode that would only directly interfere with the relay coils via the MCU GPIO pins. To create an H-bridge and drive a bidirectional motor, two dual-row package single-pole double-throw (SPDT) relays would be needed, meaning the aforementioned two circuit elements would be required to drive the two relay coils individually. Using one of TI's motor drivers, all these discrete components can be removed, resulting in a smaller and cleaner PCB solution.
Motor speed curve
Motor speed curves with relays are highly inefficient. Designers can use relays to implement multi-speed control schemes for powered windows, power doors, sunroofs, sliding doors, or pumps by using resistors of different sizes placed in series with the motor or by using multi-winding motors with different speeds. Both solutions require more relays to select different speeds, which in turn requires more board space and discrete components.
Using a solid-state solution, you only need to provide two pulse-width modulation (PWM) signals from the MCU to TI's motor driver to control the motor speed. On the DRV8702-Q1 and DRV8703-Q1, TI provides a phase/enable mode where only one PWM signal is applied to the enable pin, while a simple logic high or low phase pin controls the motor direction. The logic-level PWM signal is directly translated to the MOSFET gate with the correct voltage to fully enhance the high-side or low-side MOSFET. Using this type of interface, you can quickly design multi-speed pumps, custom motion curves for sliding sunroofs, soft-closing powered windows, inexpensive variable-speed windshield wipers, or any other type of simple motion control motor application.
Example
This compact sunroof motor module reference design is a solid-state motor control module for sunroof and window lift applications. This TI reference design utilizes the DRV8703-Q1 gate driver, integrates a current-parallel amplifier, and two dual-row automotive-grade MOSFETs, creating a very small power stage layout compared to typical relay solutions. The design also includes two TI DRV5013-Q1 latched Hall sensors for encoding motor position.
Using TI's solid-state motor drivers to design motor control systems will help reduce the size of PCB solutions, enabling the control of more motors from a single module. With the high integration and simple control scheme of our motor drivers, designers can quickly and easily redesign most modern brushed DC motor controller circuits that currently use relays.
