Power-Management Chip Set Builds Smarter Motor-Control Systems

Motor-control systems contain a complex combination of functions that must be carefully pieced together to obtain the desired performance. In a given system, designers have to combine a microcontroller or DSP with sensors, analog-to-digital converters (ADCs), high-voltage interfaces, gate drivers, and an inverter power stage. To build a motor-control system, designers must know how to fit all of these elements together and how to implement the complex control-system algorithms governing their operation. Very often, the design solution is implemented with a customer-specific ASIC and a number of discrete components.

Motor-control system design can be both costly and time-consuming. According to International Rectifier, El Segundo, Calif., that's because traditional approaches don't efficiently partition the design tasks into well-defined architectural elements with standard interfaces. The company intends to remedy the situation by introducing a new architecture for the power management of the motor-drive inverter. This power-management architecture, which will be implemented as a highly integrated chip set, represents a major break from existing conventions.

In current designs, the focus is typically on coding the DSP (or microcontroller) to do a series of computationally intensive tasks that determine the motor's operation. With International Rectifier's approach, a dedicated chip set performs these tasks and simplifies coding of the DSP to the point where only high-level programming is required.

From the DSP's perspective, the chip set appears as a "peripheral" that's accessed through a standard digital interface and controlled with a universal software driver supplied by International Rectifier. As with a standard computer printer, the driver is set up as a dual-port memory. In accordance with this methodology, the company's power-management chip set is referred to as an "intelligent power-management peripheral." Included in the chip set are all of the high-voltage analog and power circuits necessary to drive the power inverter (Fig. 1).

The intelligent power-management peripheral not only simplifies the control-system design, but also improves performance. It can be used to build smaller, more highly integrated motor-control systems with less power dissipation and greater accuracy. Along with these benefits comes lower system cost. International Rectifier's chip set reduces the number of components required, while also allowing the use of a less-expensive microcontroller or DSP.

The intelligent power-management peripheral can be used in a variety of ac or brushless dc motor applications. Examples range from industrial ac and servo drives to home appliances to emerging automotive functions, such as electronic power steering and the integrated starter/alternator. Although intended primarily for applications at 3.75 kW and below, the chip set can be applied in designs that go up to 20 kW.

One of the chip set's benefits is integration. It applies high-voltage IC technology for gate drive and current sensing—functions that presently require as many as six optocouplers and three Hall-effect sensors. In International Rectifier's solution, the optocouplers are replaced by a gate-driver IC (the IR2137) and the Hall-effect sensors are dropped in favor of current-sensing ICs (the IR2171s) (Fig. 2).

This reduction in parts simplifies and shrinks the layout by as much as 75% when compared to existing solutions. In fact, International Rectifier's chip-set solution fits on a pc board that's small enough to mount on top of an Econopak 2, a standard package outline for IGBT power modules. Future versions of the chip set should reduce the layout even further as the company plans to integrate the gate-driver and current-sensing ICs into one device.

In terms of performance, the intelligent power-management peripheral offloads the complex calculations for pulse-width modulation, deadtime generation, and deadtime compensation from the microcontroller or DSP. As a result, the microcontroller or DSP only needs to execute high-level commands to determine whether the motor should move forward or reverse, as well as how much torque should be applied. Coding of the controller is greatly simplified because designers don't have to spend time coding the interpolation routines for reconstructing the PWM signal. Instead, they will download a software driver for their particular DSP or microcontroller from the International Rectifier web site.

The reduction in demands on the controller gives designers greater flexibility. They can opt for a less-expensive microcontroller or DSP to lower system costs. Or, they might use a more powerful existing controller to accomplish performance enhancing tasks like motion profiling, filtering, or Internet-based communications.

Additionally, the chip set affords a major benefit in terms of deadtime generation and compensation. Deadtime is the time allowed for switching current between the high- and low-side switches so that both switches aren't on simultaneously. While longer deadtime provides a greater margin of safety, deadtime represents a distortion of the pulse-width modulation. For ideal motor performance, therefore, it's desirable to have no deadtime and a perfect sinewave driving the motor.

Deadtime affects torque precision and starts to impact the motor's operation when the motor is at zero speed. That condition occurs either at startup or when the motor is changing directions. A minimum deadtime is required, though. For solutions that employ optocouplers, that time is typically 1 to 2 µs. The high-voltage ICs found in International Rectifier's chip set reduce this parameter to 100 ns. Designers can take advantage of the shortened deadtime by building their designs with more-efficient IGBTs in the power stage. This benefit follows from the inherent tradeoffs in ruggedness and efficiency associated with IGBTs.

The company projects significant cost savings with its chip set. Consider how only a few years ago, the price tag on a 3.75-kW ac industrial drive could range between $500 and $1000. Contrast that with a similar drive built using International Rectifier's gate-driver and current-sensing IC. This drive costs just $200. Over time, as the company develops versions of the chip set that are more tightly integrated, system-level costs should come down further while performance goes up.

The initial implementation of the Intelligent Power-Management Peripheral chip set will include the IR2171 current-sensing IC, the IR2137 gate driver, and the IR4021 bias-supply regulator. These devices are currently available. In addition, the chip set will include a yet-to-be numbered power-management peripheral and controller IC.

International Rectifier Corp., 100 North Sepulveda Blvd., 8th Floor, El Segundo, CA 90245; (310) 252-7105; www.irf.com.