Intelligent Power-Management Peripheral for Motor Control

The design of modern motor control systems is quite complex and requires a deep understanding of control system algorithms, microcontroller and digital signal processors, sensor signal measurement and analog-to-digital converters, high voltage interface and gate drivers, and the output inverter power stage.

Traditional methodologies do not efficiently partition the design tasks into well-defined architectural elements with standardized interface protocols. This results in complex and customized designs, high initial product cost, as well as high lifetime ownership cost.

International Rectifier is introducing Accelerator, a new architecture for motor-drive inverters. This power management architecture uses a new chipset positioned between the microcontroller or DSP (digital signal processor) and the motor and high-level, a well-defined protocol for communicating with the chipset.

The guiding principles behind this design are:

• Intelligent partitioning
• Standardized interface protocol
• Performance enhancement of the power management functions.

The fundamental concept behind the Accelerator is to treat the inverter as an intelligent power management peripheral for the microcontroller or DSP. With the Accelerator architecture, control system software can be written at a higher hierarchical level (for example in C) without the need to program motor control power management functions at the bit level.

Motor control power management functions might include pulse-width modulation (PWM), dead-time generation and compensation, diagnostic and protection, or voltage and current measurement interfaces. Accelerator integrates the high-voltage analog and power circuits required to turn the software algorithms into motor-drive waveforms into the chipset in concert with the other power management functions.

A suite of technologies is used to implement the chipset, including mixed signal 0.5-µm CMOS and high-voltage integrated circuits (HVIC) with ratings from 600 V to 1200 V. The intelligent power management peripheral can be applied to a variety of applications, including:

• Driving AC or brushless DC motors for industrial AC drives
• Industrial servo drives
• Appliance drives (air conditioners, washing machines, refrigerator compressors)
• Robotics
• Electronic power steering (12 V, 42 V)
• Integrated starter alternator (42 V)
• High reliability drives (aviation, space).

Figure 1: A simplified block diagram for the Accelerator intelligent power management peripheral.

You can program the PWM waveform generator by sending input coordinates via a serial interface.

Figure 2: A simplified circuit schematic for the first series in the Accelerator chipset family.

Sampling frequencies, including high frequencies capable of replacing analog PWM generators, range between 3.6-kHz and 58.6-kHz with 12 bit to 8 bit voltage command resolution. The main timer maintains the same pulse resolution at all frequencies.

The dead time generator's resolution is 266 ns and the maximum dead time is about 4.2µs. The fault diagnostic-snapshot feature uses four FIFO registers. These registers capture and store the voltage vector, the fault-pin status, and two phases' current-feedback readings—up to the last four consecutive PWM states—as a fault history.

Figure 3: The overall block diagram. Programming of the space vector modulator, dead time generator, and current sensing interface is done through a dual-port memory. The memory is periodically read or written by the controller at a high level through a well-defined serial interface.

High voltage IC technology integrates a 3-phase gate driver in a single chip such as the 600 V IR213x and 1200 V IR223x product families.

Figure 4: The first series of the International Rectifier motor control chipsets integrates the 600 V IR2137 or 1200 V IR2237 in the gate driver. Added features include IGBT de-saturation protection and synchronized soft shutdown. The protection against short circuit conditions, such as line-to-line short, ground fault and shoot through, results in controlled di/dt and no voltage spike across the IGBT during short circuit turn-off as shown.

Linear opto-couplers suffer from linearity drift over temperature range and operating life because of current transfer ratio (CTR) degradation over time. The International Rectifier motor control chipset uses high voltage ICs such as the 600-V IR217x and 1200-V IR227x product family. Differential voltage in the range of ±200-mV, that sits on top of fast switching common mode voltage up to 600 V or 1200 V, is converted to a ground-based PWM output from the IR217x chip.

The PWM signal is then processed using a 12-bit counter with a clock frequency of 120-MHz. The function also contains auto-offset cancellation with initial offset calibration capability. A fast 13-bit hardware division block calculates the duty ratio of the incoming PWM signal, canceling any temperature dependence of the slope, and canceling the temperature drift to enable temperature-independent data acquisition.

Overall, the motor current sensing interface provides a high-speed, high-resolution measurement system whose performance cannot be attained by the conventional capture/compare unit found in the motion control DSP or microcontroller.

Figure 5a: A linearity plot of the direct and compensated current measurement outputs from the IR2171 or IR2172 compared to open loop and closed loop Hall effect sensors over the temperature range.

Figure 5b: The 1-kHz sinusoidal current waveform is shown in the top trace. The processed waveform (after hardware divide to eliminate temperature drift) is shown in the bottom trace to indicate resulting minimal phase shift (less than 10( phase shift). The middle trace shows the direct output from the IR2171."

Figure 5c: The Bode plot data supports the 10( phase shift at 1 kHz which implies that any closed current control with the IR2171 can achieve a total bandwidth of 1 kHz without any performance degradation.

International Rectifier's new power management system provides versatile, "intelligent" diagnostics. A fault condition in the power circuit might include:

• IGBT line-to-line short
• IGBT ground fault
• Over-current indication from the IR2172 OC (over-current) pin
• Over-temperature of the IGBT module.

In the event of any fault in the power circuit, the system latches the last moment of PWM pattern as a complete switching state and reports back to the host system through the fast serial interface communication. This snapshot of the IGBT switching state, along with OC status of the motor current, allows motor drive engineers to easily analyze the failure. When used with the IR2172, the drive engineer can also implement the fast current control through the OC status bit without tripping the motor because of an overcurrent.

All the detailed power management functions are performed within the chipset and the results are presented in memory locations that are updated periodically. The upper level control software is greatly simplified compared to motor-control DSP programs because it requires only the high-level data updates to and from the chipset.

The intelligent power management peripheral roadmap is evolutionary, allowing performance enhancements to be added in each new generation while maintaining the standard interface protocol to the system controller. The first series in the chipset family includes robust power stage protection, linear current measurement, SVPWM, dead time generation and system monitoring functions.

System performance is enhanced by the temperature-compensated current measurement technique such that torque precision can reach ±1% vs. the ±2 to 3% in existing motor drives. Another benefit of the intelligent power management peripheral is its relatively small size—it uses integrated technologies in mixed-mode CMOS and HVIC.

Mechanical integration of the power management peripheral with the power stage is greatly simplified. When comparing the intelligent power management peripheral approach to the older and bulkier method of using discrete opto-couplers, Hall-effect sensors and additional glue circuit components, typical reduction in PCB size can reach up to 55% and component count reduction can reach 25%.

The entire chipset and its support components fit easily within the dimensions of an industrystandard outline Econo2 package. Further developments will couple new functions in the chipset with the additional integration of sensor components inside the power module, while maintaining a well defined pin-out standard for the interface protocol.

The second stage of development will focus on the advanced dead time compensation, ripplefree motor current sensing, IGBT temperature sensing, tripfree overcurrent limit control, and histogrambased power diagnostics. Performance enhancements include up to a 10 times improvements in usable speed ranges because of accurate dead time compensation using voltage feedback. The additional intelligence in the power management peripheral feature set will allow applications to be extended to sensorless vector control of industrial drive and precision control of brushless DC servo drive.

Voltage feedback-based dead time compensation itself is not unique, with many attempts recorded in the industry over the years. However, because the implementation limit, they did not succeed. The traditional approach of sensing the motor phase voltage was based on either optically-isolated coupling devices or a high resistor divider network. Both methods suffer from inaccurate measurement because of a large time constant and accuracy limit associated with the hybrid component structure of opto-coupling devices. Using HVIC technology enables the accurate voltage feedback to solve the traditional problem.

The new motor current sensing IC will have a unique feature to eliminate erroneous sampling arising from motor ripple current caused by the fast PWM switching. The current-sensing IC is able to synchronize the sampling event relative to the motor PWM switching. Therefore the effect of motor current ripple, which varies depending on the motor inductance, will be eliminated to achieve the stable current feedback system to the closed loop motor control.

IGBT module temperature feedback and DC bus voltage feedback will be added to the next generation of the power management peripheral IC to facilitate the torque foldback control and current limit control which achieve a new level of trip-free operation.

New power diagnostics will have FIFO memory to store all IGBT switching states of the last ten consecutive PWM switching periods in the event of the drive fault condition (in other words, overcurrent and overtemperature). This will help to more effectively guide the application user to the root cause of the motor drive system fault and shorten the downtime on the factory floor.

Other applications will also benefit from the development of the Intelligent Power Management Peripheral Roadmap. Appliance drives will become lower cost and easier to design with application specific feature sets for compressors and for direct drive washers. Automotive drives for electric power steering (EPS) and for integrated starter alternators (ISA) will have higher torque and speed performance. High-Reliability drives for space applications will have robust protection and complete diagnostics.