Today's engineers must learn to prosper in an environment that emphasizes a plug-and-play design strategy. Most major appliance manufacturers, for example, have adopted the practice of considering even critical electronic subsystems (such as a motor controller) simply as a component.

This strategy keeps costs under control. It also allows the OEM to focus its internal design team’s attention on value-added features instead of spending time designing various functions that can be outsourced to the OEM's suppliers.

Plug-and-play designs can place additional burdens on suppliers. For the circuit designer, it means creating flexible designs that are frequently a combination of subsystems. Typically, a main circuit assembly implements the basic functions and one or more smaller assemblies provide the customization for optional interfaces, capabilities and even applications.

In motor control applications, the choice of processor is important to ensure enough flexibility to deliver plenty of performance at a low cost. Digital signal processors are increasingly being used for precisely this reason. At the same time, the designer must use flexibility to achieve lower manufacturing costs over a wide range of scenarios. Designs are most cost effective in terms of engineering time expended if they can ramp from trial production runs up to 250,000 or more with basically the same design.

Control a la carte

Fig. 1. IPM for flexible motor control design with built-in protection circuits.

The following application examines an appliance motor control design that takes flexibility to the next level. The same architecture can be used for a variety of motor control applications. It can also be adapted to different applications such as Uninterruptible Power Supplies, frequency converters and variable power sources.

For this particular white goods motor control application, the cost target was about $15 for a 1 HP motor (washing machine application) when production was ramped to 250,000 units or more. The design had to be flexible enough so variations could handle the requirements of a 5 HP motor (for a home air conditioner) or a half-horsepower (garbage disposal) motor.

Module requirements

The key components in the design are the digital signal processor and the Intelligent Power Module. Together, they deliver almost all of the design’s functionality. IPM requirements include voltage and current options in the range of 3 A to 50 A and 600 VDC to 1,200 VDC. Cost targets dictate DIP or SIP packaging.

A more subtle requirement for the IPM is that its drivers match directly to the IGBT used in the design. This simplifies the design effort by reducing component count, switching losses, and improving reliability as compared to designs that require external driver circuits.

Some IPMs on the market today offer an array of built-in protection modes including under voltage, over voltage, over current, over temperature and shoot through. Choosing one of these IPMs reduces part count, engineering design time, and therefore final product cost. A diagram of a typical IPM is shown in Fig. 1.

Processor requirements

Fig. 2. Comparative parts count.

The processor has stringent requirements. To handle field-oriented control of brushless DC motors, it needs the signal processing capabilities of a DSP, cope with the normal level of function integration of an MCU and be inexpensive enough to be used with an induction motor. Digital signal controllers, like those found in the Texas Instruments TMS320C2000 platform, are specifically designed for these applications. The choice of controller is determined by cost, performance, and manufacturing requirements.

The control optimized high speed DSP-based devices are capable of reading current and voltage signals during predetermined PWM patterns. This allows for simple sensors to be used to calculate individual phase leg currents and voltages by coordinating the internal A/Ds and PWM forced states. Functionality that is created by the DSP’s speed allows the design team to use less expensive non-isolated sensors to replace costly isolated sensors.

Integrated features like fast A/Ds, multiple communication channels, PWM modulators, and high speed operation is key to adding value. The DSP that has these features can be used to reduce design time, unit cost and manufacturing time. For example, one of the communication channels, Controlled Area Network, could be used for test and calibration data exchange during manufacturing to automate this phase of the product.

An RS-232 interface offers unsuspected value in contactless control. Safety regulations prefer contactless (which primarily means infrared) connections between the appliance's internal parts and its customer interface medium. RS-232 is a prime candidate for this type of communications due to its wide range of speed and control options. Contactless control also mitigates EMI concerns since the coupling capacitance is essentially eliminated.

Topology and packaging

Fig. 3. Fewer parts enhance reliability.

A three-phase design with a single low-side current sensing resistor, an NTC temperature sensor, IPM power device, and DSP controller is the basic topology. This can be modified for two-phase and one-phase motors by simple re-programming of the DSP or through multiple resident programs.

The fast A/D converters make it unnecessary to use a differential amplifier for voltage feedback measurements. The analog-to-digital converters and the DSP’s accuracy and speed allow the channels to be read sequentially without a differential amplifier. The signal error caused by the DSP can be very low since the samples can be as close as 500ns.

Careful system partitioning is just as important in packaging. When major appliance manufacturers view power controllers as plug-and-play components, there is usually little consistency between their specifications. In particular, different applications and OEMs specify quite different frame sizes for the PCB. But to be cost effective, one design should accommodate all of them with a minimum of design changes.

By treating the DSP and its support components like a single component and assembling these functions on a mini PCB and the IPM module with its related power components on a main PCB, a design that achieves a great deal of flexibility is accomplished. The main PCB, which contains the IPM and other devices that change according to manufacturer requirements for voltage and power level, it is a straightforward design that uses through-hole components that, along with the DSP, can be changed easily without complicating the manufacturing processes.

Since each converter application uses the same DSP PCB assembly, it can be manufactured in large volume. It is then simply installed into any of the different forms of the main PCB, which has been configured with the appropriate type of IPM.

Parts count and reliability

Fig. 2 shows a comparison for the conventional MCU/discrete design approach versus the new methodology. The DSP/IPM design can cut the parts count in half.

MTBF (mean time between failures) metrics are improved by reducing the number of components. Additionally, mechanical stresses on the main PCB are reduced by using the DSP PCB assembly approach because thermal stresses normally associated with greatly dissimilar components on a common PCB are eliminated. Circuit protection is enhanced by the utilization of specific IPMs and three sensors are replaced by one as compared to the MCU/discrete-based design. The differential amplifier also can be eliminated by using the fast A/Ds of the DSP. Fig. 3 shows a few metrics related to reliability.

Lower cost, more functionality

By opting for more powerful and highly integrated components, the number of parts, manufacturing costs, and engineering costs can all be reduced. The plug-and-play approach of the motor controller with the two-board system produces manufacturing flexibility that keeps unit costs low.

This approach also allows production scaling from low volume to hundreds of thousands of units while keeping a design that is highly cost effective. On the performance side, using advanced DSP and IPM components provides up to 30 percent better efficiency compared to conventional designs based on MCUs and discrete components. Finally, the reduction in parts count improves reliability.

Jeffrey Reichard is CEO, Tier Electronics, Menomonee Falls, Wis., and Andrew Soukup is worldwide TMS320C2000 marketing manager, Texas Instruments, Houston.