Abstract

The content of electronics in automotive systems is constantly growing, sustained by an increasing request of automation, enhanced safety, power optimization and quality feelings. In this context, the number of applications using DC motors are facing a constant boost.

This article will analyze market trends for automotive DC motors and display how Solid-State Drivers (SSD) represent the preferred design architecture in terms of diagnostic capability, optimized switching time, weight saving, and (on top of all) improved reliability.

In particular, we will show how the new VIPower™ M0-7 H-bridge family are the best in class choice of full integrated circuits specifically designed for automotive DC motor control.

Market Trends

The annual growth rate demand of automotive DC Motor systems is estimated to be stable in the area of 3.1% for the next 5 years. The Body perimeter is sustained by classical applications like door locks, electric mirrors, seat adjustments, washer pumps, wipers, window lifts, sunroofs, and sliding doors. Moreover, new emerging and appealing applications are facing the market, some examples are: head-up displays, deployable door handles, power trunk lifts, e-shifters and ev-lock chargers.

ST Market Trend

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 the picture below, all applications described are in the range from 30W up to 200W.

ST Chart

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 SSD can keep easy to implement innovative features like driving variable load profiles (e.g. Power trunk Lift Gate) or implementing controlled and smooth movements (e.g. window lift or seat adjustment), getting rid of relay switching noise and increase the luxury feeling. On top of all that, legislators world-wide are introducing new limitations for vehicle emissions, for 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 for DC motors actuated by SSD will grow with a 6.7% average growth rate in 2020-25, gaining market share vs relays adoption.

ST Chart2

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

ST Diagram

Relay 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 customers as feeling unpleasant (especially in switching frequency driven applications). Moreover, during relay switching, more arc noises are generated that produce Electromagnetic interference (EMI). To reduce relay switching noise extra components are needed, like 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 shortening 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 additional buffers, drivers or amplifiers, drastically reducing PCB complexity and Area. The result is an increased reliability level: up to 10x switching times.

Application Area saving thanks to tiny power packages

ST Diagram 2

The evolution of the automotive market in the direction of autonomous driving requires the utilization of a wider and wider number of sensors as well as actuators. Considering that 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: by turning on the bridge switches alternatively, 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 by 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 adverse impact on application reliability.

Finally, the design of board covers and enclosures must also take care of the height of the relays leading to a typical vertical keep-out distance of 17 mm.

Thanks to the outstanding shrinking of VIPower™ M0-7 technology, the ST H-Bridge family is able to implement the complete motor drive architecture into advanced tiny power packages: SO-16N and PowerSSO-36. The reduced footprint respectively of 60mm2 and 106mm2 and the thickness under 2.5mm will allow PCB shrinkage and system weight reduction. Moreover, VIPower™ M0-7 H-bridges offer an eco-friendly product portfolio of lead-free packages ensuring outstanding thermal performance.

Switching Time and PWM control

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 through. When a shoot through event occurs, it will generate an extra noise on the battery line and an extra power dissipation that will reduce the system efficiency. This phenomenon 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, indeed 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
  • Reduced Start Up Inrush current

ST Diagram 3

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.

ST Diagram 4

By driving DC motors 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 optimize 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 5ms up to 15 ms. Moreover, the MCU have to implement proper logic protections to avoid unwanted cross conduction events.

The VIPower™ M0-7 H-bridges family guarantees fast switching times (1 μs typ.) that ensure a switching frequency up to 20 KHz. The Switching profiles are specifically designed to optimize EMI and to optimize switching losses. Moreover, the chip embeds special protections that avoid dynamic and static cross-conduction issue. As a result, the VNH7 family is designed to optimize the system efficiency.

- Miccoli Leonardo Agatino, STMicroelectronics