High Performance Motor Applications are Driving the Automotive Industry

The adoption of the electric motor
We have to go back many years to discover a time when vehicles did not incorporate an electric motor. Then, the cranking was manual and the engine cooling fan and wipers were mechanically coupled to the engine. Electric motors quickly became associated with the combustion engine and initial adoption was mainly driven by comfort. These motors are low power (<100W), commonly require a simple relay to drive the load and are the best choice for efficiency and performance in such systems. As motors began to be adopted for safety reasons such as anti-lock braking systems and traction control, they required more reliable drive systems.

However, more recently, the automotive industry has turned its focus towards reducing fuel consumption. The pressure for green transportation has challenged engineers to find smart and optimized solutions wherever possible within the vehicle. Electric motors can achieve impressive performance when driven by smart electronics. An electronic solution is particularly useful for high power motors (>100W). In modern cars both engine cooling and blowers now use electronic power control but the range for electric motors is wide. Many functions in the car still use mechanical systems coupled to the combustion engine. Water and oil pumps are good examples where electronic can bring a significant improvement in terms of efficiency. With electrical control, the power is effectively delivered to the motor to provide exact power requirement at any time.

Variable speed presents major opportunities in the automotive world
Applying variable speed motor control for engine cooling and blowers in vehicles is a recent innovation. Older cars feature a speed control system for both engine cooling and blowers using resistors and relays. With such a system, the speed of the motors is limited to a few discrete values. Any speed value requires a resistor placed in sequence with the motor. Such a solution has very poor performance because the speed of the motor cannot optimized for the power requirement. This leads to typical efficiency of less than 50 percent in most of cases.

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Figure 1. Simple motor drive using ballast resistors

The latest advancements in power electronics enable variable speed electronic motor control to become the solution of choice for many applications. Using variable speed control, typical system efficiency of over 90% can be achieved over the entire load range. Taking a typical 400W engine cooling fan as an example, the electronic controller dissipates 100W less power than the resistive fan controller over a typical load cycle. This 100W of saved power is equivalent to about 0.1l/100km of fuel.

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Figure 2. Typical application using the AUIR3330S

The challenge of driving motors in PWM is to meet the EMI requirement. At 20kHz, the system will generate noise on the battery side. The di/dt during the turn-on and turn-off phase is the major contributor of EMI. To meet EMI requirements, a passive filter must be connected between the battery and the inverter stage. This filter is usually built with 2 large capacitors and an inductor. The cost of the filter is an important cost for the whole system. In a simple system using a MOSFET, the only way to reduce the di/dt is to insert a resistor on the gate to slow down the switching. By doing this, the switching losses will increase drastically reducing system efficiency and requiring an increase in the size of the heat sink. In such systems there is a trade-off between the size of the EMI filter and the heat sink.

The AUIR3330S features proprietary di/dt control on the output to reduce the conducted emission on the battery board net. This active di/dt control provides optimization for both EMI and switching losses performance breaking away from the paradigm of trading off EMI filter size and heat sink size. This special feature requires a specific gate shaping in the MOSFET which cannot be implemented with discrete components. In normal applications using a MOSFET with a driver, the switching times are controlled with by controlling the drive current using a gate resistor. Furthermore, the AUIR3330S offers a solution to drive any kind of motor with full speed range capability. The high level of integration allows the designer to develop a compact solution. With a few external components, a full speed range design can quickly be realized.

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Active DI/DT
During turn-on, the driver applies a high current to reach the threshold of the MOSFET as fast as possible. When the current begins flowing in the MOSFET, the gate current is lower to limit the di/dt. When the drain-to-source voltage begins decreasing, the gate current increases to limit the switching losses. Compared to a MOSFET driven by a resistor, the switching losses will be the same during the di/dt phase but significantly lower during the dv/dt. Therefore, with the same level of EMI, the power dissipation is much lower in the AUIR3330S, requiring a smaller heat sink.
The active di/dt requires a complex driver able to apply different gate currents during the different phases of the switching. The AUIR3330S also includes smart circuitry in order to detect the di/dt and the dv/dt phases.

Modern motor drive applications also require additional features such as protection and diagnostics. The AUIR3330S integrates features which prevent any failure of the system in abnormal mode including over-temperature condition, short circuit on the output and loss of ground connection or bootstrap capacitor. In any of the above fault conditions, the AUIR3330S is protected and will report a diagnostic to the micro. The diagnostic is a digital value which can be read directly by the micro.

In addition, the AUIR3330S has a current feedback function in order to read the load current by measuring the voltage across the Rifb resistor. The system can monitor the load current to control the power delivered to the load. Stall motor conditions can also be detected.

The current sense feedback is used to set the threshold of the over current protection. When the voltage across the Rifb resistor exceed 4.5V the output is automatically switch off. This feature prevents any failure on the wiring or in the motor in stall condition and can be adjusted to each system need.

Summary
Featuring full speed electronic control, electric motors can now be adopted in many new applications. In cars, some loads are still driven directly by the engine: for example, the water pump, oil pump and power steering pump. Adopting electric motors for those loads significantly simplifies the mechanical design by removing belts and wheels, and by offering many other optional locations to implement them in the engine compartment. The AUIR3330S offers a solution to drive any kind of motor with full speed range capability and its active di/dt control feature provides optimization for both EMI and switching losses performance.