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Automotive Product Finder Magazine | DC motors trends in automotive body electronics
DC motors trends in automotive body electronics
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The content of electronics in automotive systems is constantly growing, sustained by an increasing request of automation, enhanced safety, power optimisation and quality feelings. In this context the number of application using DC motors are facing up a constant boost. Leonardo Agatino MICCOLI, analyzes market trends for automotive DC motors, and shows how Solid State Drivers (SSD) represent the preferred design architecture in terms of diagnostic capability, optimised switching time, weight saving and (on top of all) improved reliability.
The annual growth rate demand of automotive DC motor systems is estimated to be stable in the surroundings of 3.1 per cent for the next 5 years. The body perimeter is sustained by classical application like door lock, electric mirror, seat adjustment, washer pumps, wiper, window lift, sun roof, and sliding doors. Moreover, new emerging applications are facing the market, for example, head up display, deployable hand door, power trunk lift, e shifter, EV-lock charger, etc. Considering this scenario, the worldwide demand for automotive DC motors in body domain is estimated to reach 2 billion units in 2020. The share for each application is represented in Figure 1, and all applications described are in the range from 30W up to 200W.
Share of DC motors actuated by relay vs silicon in body applications
Historically, the automotive industry has seen relays as an easy and cheap solution to drive DC motors, but the feeling is changing and nowadays car makers are considering the SSD as the more appropriate option in new application designs. Thanks to high reliability quality and enhanced diagnostic features, the SSDs keep easy to implement innovative features like driving variable load profiles (for example, Power trunk Lift Gate) or implementing controlled and smooth movements (for example, window lift or seat adjustment), getting rid of relay switching noise and increasing luxury.
On top of all, world-wide local legislations are introducing new limitation for vehicles emission of both pollutant substances and CO2, modifying cars’ architecture, especially with regards to the supply of power loads and requiring the adoption of more efficient electronic devices. Although the new standards impact most power-train systems, some relevant contribution comes also from car Body Control Modules (BCM). As a result, the forecast of DC motors actuated by SSD will grow with a 6.7 per cent average growth rate from 2020-25, gaining market share vs relays adoption.
In this scenario, ST VIPower M0-7 H-bridges family represent the best in class devices for motor control in automotive applications. The M0-7 H-bridges are based on the integration in one single package of logic functions and power structures, allowing an intelligence inside the chip that goes beyond simple driving to protection and fault, providing advanced diagnostic and protection features, reduced component count, improving reliability and PCB area saving.
Improved reliability leads to 10x longer operating life
Relays contacts are electrically conductive pieces of metal which touch together allowing the circuit current to flow. Typical issues of mechanical switching contact are the audible noise and the mechanical vibration perceived by final customer as an unpleasant feeling (especially in switching frequency driven application). Moreover, during relay switching are generated arc noises that produce electromagnetic interference (EMI). To reduce relays’ switching noise, extra components are needed such as RC snubber and flywheel diode. These extra components will have a negative impact on final architecture complexity. The mid and long-term effect of electro-mechanical stress generated during switching will be a reduced contact resistance and performance, making relay unusable and shorten life. Degradation of relays’ performance will lead to low reliability.
Solid State Switches have no moving parts since mechanical contacts have been replaced by power transistors: no issue of arcing contacts, magnetic fields or audible noise. The input controls are compatible with most IC logic families and needs no extra buffers, drivers or amplifiers, drastically reducing PCB complexity and area. This results in increased reliability level – up to 10x switching times.
Application area saving due to tiny power packages
The evolution of the automotive market in the direction of autonomous driving requires the utilisation of a wider number of sensors as well as actuators. Considered this larger number of devices must always be housed inside the same compartments, it’s easy to understand how constraints coming from space occupation are becoming more and more stringent..
The H-bridge configuration is the typical topology used to drive bidirectional DC motor: turning on alternatively the bridge switches, it is possible to control motor direction or to brake the motor. Even if H-Bridge architecture can be easily implemented using relays, the amount of board space will be significantly reduced adopting the SSD.
Considering a typical relay footprint area approximately of 250 mm2, the board area needed to implement an H-bridge architecture by relays will be at least 500 mm2. In addition, the implementation of high-voltage transient suppression, system diagnostic and protection features will require additional discrete circuitry like buffers, operational amplifier and sensors. The extra components will significantly increase the final board dimension and complexity, and will have an adverse impact on application reliability. Finally, the design of the board covers and enclosures must also take care of the height of the relays leading to a vertical keep-out distance of 17 mm.
Switching time and PWM contro
By guiding an H-Bridge architecture, special care must be taken to avoid unwanted short-circuits between the battery line and the ground, especially during switching phases; this event is commonly defined as dynamic shoot trough. When a shoot trough event occurs, it will generate an extra noise on battery line and an extra power dissipation that will reduce the system efficiency. This phenomena becomes more critical if the H-Bridge is driven with fast switching control like PWM signal. PWM input signal is commonly used to control H-bridge architecture, and varying duty cycle it is possible to modulate motor speed and torque implementing advanced features like anti-pinch function, smooth movement at start and stop to increase quality feeling, stall condition control, motor speed regulation independently from battery voltage and reduced start up inrush current.
The typical DC motors profile has a startup phase with an inrush current 10-12 times bigger than normal current. All electric parts must be dimensioned to sustain this high current for a short time and this will have a consistent impact on final application in terms of cable sizing, PCB area and driver capability. Indeed, relay data sheets give maximum contact ratings for resistive DC loads only, but this rating is greatly reduced for highly inductive or capacitive loads.
Driving DC motor with a PWM signal, it is possible to achieve a smooth motor start-up limiting the torque. The inrush current will be reduced prolonging the motor activation phase. Driving the DC motor with a PWM signal will allow to optimise the power dissipation and to reduce cable sizing, contributing overall to weight saving. Relays are not a suitable choice for systems requiring fast output switching, indeed the switching times are limited by mechanical tips movements that typically goes from 5 ms up to 15 ms. Moreover, the MCU have to implement proper logic protections to avoid unwanted cross conduction events.
Leonardo Agatino MICCOLI is Senior Engineer-Technical Marketing for Audio & Body Division, VIPower & Lighting Business Unit at STMicroelectronics.
Automotive Body Electronics
Leonardo Agatino MICCOLI
Vipower And Lighting Business Unit
Solid State Drivers
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