Mixed-Signal Chipsets Ease Motion Control Design

Nov. 1, 2003
The system architecture and the digital control technology of the iMOTION platform were introduced in Part I. In Part II, the mixed signal chipsets that are designed to support the two iMOTION platforms will be introduced.

To support two iMOTION platforms, namely encoder-based and sensorless motion control, integrated mixed-signal chipsets have been codesigned and codeveloped. As shown in Fig. 1, the chipsets provide all the hardware, along with digital control, high-voltage analog and power stages under a single design platform. While digital ICs provide a configurable digital control engine for motor control design, the analog ICs offer linear current sensing and gate driving, with power IGBT chips ranging from 7 A to 15 A ratings in isolated packages for the power stage. In fact, power modules integrating HVIC and IGBTs in 3-phase inverter stage with ratings from 6 A to 20 A also have been developed.

Digital Control

The digital control IC in the iMOTION platform is based on the concept of a dedicated control engine that integrates the complete closed-loop current and velocity control. Unlike a traditional microcontroller or DSP, the chip doesn't require any programming to complete complex motor control algorithm development. Combined with high-voltage gate drive and current sensing ICs, the user can implement a complete motor control with minimum component count and virtually no design effort. Although the chip contains dedicated logic to perform closed-loop control of ac current and velocity, it has a wide range of application coverage through flexible configuration ability. The drive can be easily configured for induction-machine, closed-loop vector control or permanent magnet motor servo drive.

Rich motion peripherals, analog and digital I/O also can be configured. Host communication logic contains asynchronous communication interface for RS232C or RS422, or RS485 communication interface, a fast slave SPI interface and an eight-bit wide host parallel interface. All communication ports have the same access to the host register set. The user can write, read the predefined register to configure and monitor the drive through these communication ports.

Two separate control engines are available. The IRMCx201 is designed for encoder-based servo drive control capable of closed-loop current control bandwidth of 5.5 kHz. The IRMCx203 engine is designed for sensorless sinusoidal current control of spindle. Appliance permanent magnet motor drives are capable of stable operation up to 100,000 rpm.

Using the IRMCK201 engine as an example, the application circuit is shown in Fig 2. To complete a high-performance servo drive control, all necessary components are shown in the connection. Although this is a typical hardware configuration, the user can customize the design. While IRMCK201 provides direct interface to IR2175 high-voltage current sensing ICs, the user can still interface to other current sensing devices, such as a hall-effect sensing device and/or low-side shunt resistors. A parallel interface is provided to allow connection to a microcontroller for embedded application layer programming. Alternatively, the IRMCK201 can operate in a stand-alone mode without the host controller. A serial EEPROM would be used to load motor-specific parameters into the chip.

All logic and algorithms are pre-designed, and the user doesn't need to make any effort to develop code, which alleviates the tedious design process. If needed, the user can configure the drive to tailor the control to meet the required specification. This configuration can be done easily by accessing the host register interface through the communication ports. Configurable parameters include an update rate for PWM carrier frequency and velocity control, all PI controller gains, PI output limit range, current feedback scaling, encoder/resolver feedback scaling plus enable/disable switch for slip gain to configure control structure for induction machine or ac permanent magnet machine.

Analog Control

The analog section of a motor drive largely determines the design complexity and the component count of the overall system. The collection of diverse functions include:

  • High-voltage gate drives for the 3-phase IGBT output stage.

  • Current-sensing circuits, either through phase leg or phase output.

  • Voltage-sensing circuits for the dc bus or sometimes the phase output.

  • Signal processing circuits for different types of sensors.

  • Analog-to-digital converters to interface with the digital controller.

  • Power supply controller, usually PWM-type for fly-back topology.

  • Power factor controller, usually PWM-type for boost or buck-boost topology.

  • Additional analog and digital circuitry for protection and interface functions.

The variety of circuit types presents a difficult challenge. Traditionally, the circuits are implemented using discrete components, which are produced without regard for optimized operation with one another in a specific application. The result is circuitry with variable performance levels, design time and cost.

High-voltage IC (HVIC) technology in 600-V or 1200-V ratings that can implement the integration of all the analog control functions is available. A half-bridge gate driver implemented in 600-V HVIC is now the de facto standard for IGBT gate drivers, largely replacing the optocouplers for appliance applications and certain lower power industrial applications. The added benefit of using an HVIC as opposed to optocouplers is that it is a simpler power supply design to bias the HVIC in a bootstrap configuration. Integration of the gate drivers into a single chip with a 3-phase format is the trend for space savings and cost reduction. Other functions such as current sensing, voltage sensing, power-supply controller and power factor controller are also implemented using HVIC in the iMOTION platform.

For example, the IR2136x family of 600-V 3-phase gate drivers integrates all the gate drive functions for a 3-phase inverter in a single chip, as shown in Fig. 3.

To simplify the interface to different digital controllers and IGBTs, the IR2136x family offers different versions with options for different UVLO (9 V or 11 V), logic level (3.3 V or 5 V) and input-to-output phasing.

True phase-current sensing is a desirable feature for closed-loop servo and vector control. The traditional approach uses Hall-effect sensors with or without closed-loop compensation. However, hall-effect sensors tend to have higher cost and are not compatible with automated PCB assembly because they require manual wire insertion. Measuring phase current using shunt resistor at the phase output is an acceptable alternative for lower power drive. However, the signal processing circuit is complicated because the small differential voltage measurement is sitting on top of high common-mode voltage that is also swinging from ground to the high-voltage bus, following the fast switching of the phase leg.

One solution is to use HVIC technology to integrate the differential measurement circuit, analog-to- digital converter, high-voltage level shift with common mode and dV/dt noise rejection, plus signal reconstruction to interface directly to digital controller. The IR2175 is an example of a phase-current sensing IC (Fig. 4). Input to the IR2175 is a differential voltage with ±260 mV linear range, while the common-mode offset can be as high as 600 V and dV/dt can be up to 50 V/ns. Output from the IR2175 is a low side PWM signal at 130-kHz carrier frequency with the duty-cycle scaling to the differential voltage level. An additional pin is provided to output an overcurrent signal when input exceeds the linear range limit. Overcurrent signal delay is only 2 µs and provides a fast shutdown path to protect against IGBT short circuit condition.

The roadmap for the HVIC product line includes integration of the three-phase gate driver and two to three channels of current sensing functions, either phase output, phase leg or ±dc bus current, in a single chip. Other functions to be integrated include the power supply and power factor controls plus EMI noise cancellation control. Additionally, tight integration and a common interface protocol between the analog and digital control ICs as part of the iMOTION platform result in higher performance at lower cost and lower parts count with less design time.

Power Si and Module

A new type of power module, called PlugNDrive, is used to integrate the Power Si with the analog control IC as part of the iMOTION platform. The module uses non-punch-through (NPT), IGBT technology matched with a hyperfast diode. In addition to the IGBT power switches, the modules contain a 3-phase monolithic gate driver IC, matched to the drive requirements of the IGBTs to generate the most efficient power switch consistent with minimum noise generation and maximum ruggedness. All of these components are mounted on insulated metal substrate (IMS) and encapsulated in an over-molded plastic package. The power rating can range to 3 kW for the larger package type.

Insulated metal substrate technology (IMST), which originally was developed as a low-cost method for mounting bare chips, has evolved into an excellent technology for achieving performance and reliability in high-density systems. The IMST substrate uses an aluminum plate as the base. The upper side of the substrate forms a sandwich of a high-voltage dielectric and a copper cladding on which the circuit is etched, similar to a conventional printed wiring board. This allows the creation of hybrid ICs that take advantage of two primary features of the aluminum substrate, namely high thermal conductivity and simple machining (Fig. 5).

The PlugNDrive module contains six IGBT dies, each with its own discrete gate resistor, six commutation diode dies, one 3-phase monolithic, gate driver chip, three bootstrap diodes with a current limiting resistor and an NTC thermistor/resistor pair for overtemperature protection as depicted in Fig. 6. The overcurrent trip circuit also responds to an input signal generated from an external sense element, such as a current transformer or sense resistor. The input pin for the T/Itrip circuit performs a dual function as an input pin for overcurrent trip voltage and an output pin for the module analog temperature sensing thermistor. The module schematic shows the thermistor and its associated components to facilitate the design of external circuitry. A small resistor (R3) is included in the bootstrap circuit to limit peak currents in the bootstrap diodes, especially when using large value bootstrap capacitors, which are necessary under certain operating conditions. Current ratings for available PlugNDrive modules range from 6 A (IRAMS06UP60 in SIP1 package) to 20 A (IRAMY20UP60 in SIP3 package).

Development System

To simplify the customers' design tasks, development systems based on the iMOTION platform have been readied. An example is the IRMCS20x 1-kW development system platform shown in Fig. 7. This development system uses the Accelerator digital control IC, IR2175 current sensing IC and the IRAMY20UP60 PlugNDrive module that integrates NPT IGBT with IR21365 gate driver IC. Using FPGA with downloadable code (IRMCO20x), a single hardware set can be used to adapt either the encoder-based or sensorless control. Alternatively, the dedicated digital control ICs (IRMCK20x) are used as the controller without changing the analog and power sections.

One advantage of using FPGA in the development system is the possibility to receive and download updates and improvements to the object code. A software tool, called ServoDesigner, is also provided that is loaded into host controller, such as a PC, and communicates with the development system via the RS232 port. The ServoDesigner tool is used to write/read to any bit/byte/word of FPGA register, create function definition, such as motion action, customize register names and sizes, interface to any register map of future iMOTION platform, and provide graphic waveform monitor/trigger of internal signal nodes.

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