Power Modules and Devices Advance Motor Controls
The core function of power modules is to perform motor power switching and amplification in electric variable-speed and servo drives. Some power modules integrate capabilities and intelligence well beyond these basic functions. They simplify motor drive design via low parts count, compactness, and reliability.
The core function of power modules is to perform motor power switching and amplification in electric variable-speed and servo drives. Some power modules integrate capabilities and intelligence well beyond these basic functions. They simplify motor drive design via low parts count, compactness, and reliability. They also add to the drive’s bottom line.
Power module developments are moving forward on several different fronts and encompass a variety of product types (see box on module classes). Closely related technologies, such as gate-commutated thyristors (GCTs) and gate turn-off (GTO) thyristors are also covered here, though strictly speaking, they’re discrete power devices rather than modules.
Today’s workhorse technology for power switching devices in most motor control power sections is the insulated-gate bipolar transistor (IGBT). And innovations continue for IGBTs. Historically, IGBT process improvements came by way of finer lithography capability. Now such advances are reaching a point of diminishing returns. Trench gate technology—currently used in lower power MOSFETs (metal-oxide semiconductor field-effect transistors)—is under development for IGBTs. Trench gate methods consider the device’s internal architecture and bury the gate below the surface in contrast to conventional IGBT gates which must be formed on the surface of the power chip.
Eric Motto, applications engineer with Powerex (Youngwood, Pa.) notes that 600 and 1,200 V trench gate IGBTs are under development. He estimates that “V CEsat [on-state voltage], which affects device efficiency, can be lowered as much as 30%.” In lower voltage IGBTs (250 V), trenched gates are already being implemented in a few applications, for example, motor drives on fork lift trucks. (Powerex, exclusive marketer of Mitsubishi semiconductors in North America, is co-owned by GE and Mitsubishi Electric.)
Another technology innovation is a form of IGBT called the non-punch through (NPT) type device. Developed by Siemens for higher blocking voltages (1,200 V or more)—this design approach improves on the efficiency of the original IGBT device by substantially reducing conduction losses. NPT technology is also extremely rugged. Recently Siemens (and others) have brought NPT-IGBTs down to 600 V rating and its wider markets. Advances in production were necessary to make the ultrathin wafers required at these voltages.
According to Larry Rinehart, vice president of R&D at Semipower Systems (San Jose, Calif.), “During 1997 conduction loss [in NPT-IGBTs] dropped from 2.5 to 2 volts, a 20% improvement over standard IGBTs.” He continues, noting that “conventional IGBTs employ punch-through design which has more losses when operating at high switching rates. NPT now can compete with higher performance traditional IGBTs and should begin cutting into market share of those devices. We use punch-through IGBTs today, but are evaluating NPT devices for 460-V, 15 kW modules.” Despite NPT advances, punch-through IGBTs continue to be attractive as lower product costs are achieved through increased sales volumes, says Mr. Rinehart.
Other device technology is entering the motor power module scene, especially in higher voltage applications. “Two revolutionary power semiconductor technologies, high-voltage IGBTs and GCTs are improving performance, simplifying designs, and raising the reliability of applications ranging from 100s of kVA to many MVAs,” explains Mr. Motto.
GCTs are a new wrinkle on the venerable GTO thyristor employed in higher voltage drives for a long time. However GTOs have had some fundamental drawbacks which include low switching speeds and higher circuit cost (e.g., a need for snubber components). Gate-commutated thyristors represent an important improvement over GTOs. GCTs can switch at 1-2 kHz versus 200 Hz for GTOs, and they do not require snubber circuitry.
How much intelligence?
A central issue for power modules is the “right” amount of functions to place on the device. Module designers always struggle with how much intelligence to integrate into their modules. The more functions, features, and intelligence on-board, the greater the value added to a module. However, each additional function takes away from the customer’s design prerogative. Making modules too specific for an application, cuts into production volumes and raises cost. Products range from a simple h-bridge function to a complete drive in a module at the other extreme.
Ken Jones, sales engineer at Collmer Semiconductor (Dallas, Tex.) notes clear signals from his customer base. “We plan to push our technology into higher levels of control, creating more intelligent modules. Our customers plead for a more complete solution,” he says. (Collmer is exclusive North American distributor of Fuji Semiconductor Div. products.) Some features Mr. Jones’ customers are looking for include:
Temperature and current sensing that can distinguish recoverable failures from catastrophic ones; and
More performance and environmental information to allow intelligent decisions in real time.
Another issue for power module designers is to add or not to add pulse-width modulated (PWM) functions on the module. Powerex’s Mr. Motto believes that PWM is too application-specific. “It’s a touchy area because many of our customers would not be happy if we were to put PWM generation in our power module. Where do we cross over from being a semiconductor manufacturer to becoming a motor drive manufacturer?” he asks.
Mr. Motto sees high-voltage ICs as critical to integrating more functions for motor controls up to about 2.2 kW. “The high-voltage IC is used as a gate driver and eliminates the need for optoisolators. It also provides a level-shift function, converting from to low to high voltages in the power module,” he explains. As a result, inverter size can be drastically reduced because wiring and parts spacing requirements on the module decrease at lower voltages. “Of course, there is always a downside.” He describes HVIC devices as “finicky about the circuit layout.”
And what of the “ultimate” drive in a module approach? Semipower Systems continues to lead the way in integration and packaging density by putting an essentially complete ac drive in a module, for rating up to about 20 kW (see sidebar).
Evolving module packages
Packaging design provides good opportunities for module developments. One trend noted at Motorola Semiconductor Products Sector (SPS, Phoenix, Ariz.) is a “conspicuous movement away from traditional bolt-down packages over the last five years,” says Jacques Lederrey, marketing manager. “This change in design philosophy—to printed circuit board (PCB) mounted packages in electronic drives—has been fueled by drive companies eager to make use of existing PCB automation tools and processes to reduce assembly costs of their product.” The approach applies a growing range of drive sizes.
Package standardization is a seemingly simple issue, but a vexing one for drive manufacturers. Modules in standard form—if available from different suppliers—would let users pick the best module for a given drive application, reducing development efforts. “Yet, most power module vendors have not effectively addressed this issue, frequently offering several incompatible packaging solutions,” remarks Mr. Lederrey.
Standard packaging takes a step forward with Motorola’s recent VersaPower line of “molded-in-lead design” modules. VersaPower uses automated wire bonders instead of soldered joints to make all connections. This lowers production costs while making the modules more mechanically and electrically robust. The user benefits from simpler, less costly assembly.
VersaPower comes in symmetrical dual in-line pin (DIP) packages. The 16-pin version offers a choice of a simple sixpack, 3-phase inverter; or the 3&3 Series—which is a complete power stage with 3-phase bridge and inverter in a single package. A larger physical package in the 24-pin Integrated Power Stage (IPS Series) allows more pins and power for added functions such as temperature sensor and brake IGBT.
“Ease of circuit customization within an unchanging form factor” is a key benefit cited by Motorola. “With VersaPower, we can customize the power electronics with the exact features required by the application and deliver prototypes in five to eight weeks,” adds Mr. Lederrey. This “versatility” means easily adding an extra IGBT or providing control for other motor technologies (e.g., switched-reluctance or steppers). And further feedback from the customer can be quickly adapted before going to high-volume production.
“eupec” GmbH (Warstein, Germany) stresses the concept of separate but specially designed driver and power circuit packages that fit together well mechanically and electrically. eupec (European Powersemiconductor and Electronics Co.)—now one of the companies of Siemens—enjoys substantial independence to manufacture and market a wide range of power semiconductors to Europe and, through its eupec Inc. (Lebanon, N.J.) subsidiary, to North America.
Products from eupec include low-cost IGBT and diode packages wave-solderable to printed circuit boards. Named Econopack, these modules include rectifier stages, inverter stages, and special functions like brake circuits and soft starters. “Econopack reduces circuit size and cost, while raising module reliability,” states Thomas Spreen, eupec marketing manager.
Introduced by eupec, the “Econo concept” began with low-current sixpacks, but today’s larger (Econo 3) modules offer a lot more capability: up to 1,700 V; up to 600 A. Higher current can be obtained by paralleling modules on a board. Some Econo modules use NPT-IGBTs. A pin-compatible version of Econopack is now made by Motorola and Toshiba, according to Mr. Spreen.
He adds that Econopack will be expanded in the second half of 1998 with a new line of PIMs (see module classes box) that integrates an inverter, converter, brake chopper, and temperature sensor in one package.
Packaging: realities and desires
Users continue to demand smaller packages as well as more functionality in the module. In the lower power range of motor controls, mounting power components on a board helps with both greater density and increased functionality. Powerex has developed five ASIPM (application-specific intelligent power module) lines for this arena. ASIPMs integrate more sophisticated diagnostics (coded fault signals), current sensors, and use HVICs for level shifting.
Its latest ASIPM is a DIP module in a low-cost, transfer-molded package resembling an overgrown discrete device (second photo). It combines an HVIC with IGBTs and freewheeling diodes in a single module. No question, the target is the low-cost inverter market. The DIP will find usage in embedded applications (motor and drive in the machine) like air conditioning and pump/fan controls. Cost targets are aggressive; power requirements are in the 0.37 to 1.5 kW range.
New from Toshiba America Electronic Components Inc. (Deerfield, Ill.) are two Power Integrated Module families that round out the company’s IGBT offerings. Called the Transfer Mold (TF) and Direct Bond Copper (DBC) families, they’re intended for smaller, low-cost general-purpose (600-1,200 V) industrial drive applications.
TF modules consist of a rectifier and six inverter IGBTs in one unit; their current range is 10-20 A at 600 V (5-10 A at 1,200 V). DBC modules add a brake IGBT to the above package, but are available without it. Current range here is 15-25 A at 1,200 V, with twice the amps at 600 V. These PIMs also use the newer NPT chip structure, said to be the third generation at Toshiba. TF and DBC modules are in full production now; pricing ranges from $20 to $40 each.
Motorola SPS sees an emerging market for embedded drives, where production flexibility will be paramount. Very sophisticated large appliances, with extra features and much more efficient motors, represent one such potential sector. “Appliance manufacturers, unable to use standard drives due to highly cost-driven goals, will rely on special drives customized to their applications. This new market is likely to more than double the growth rate of small intelligent motor controls,” says Mr. Lederrey. Motorola’s standard package approach and ability to rapidly customize modules is in line to serve this emerging market.
Other specific market forces impact motor power module usage. The “drive on a motor” concept is one of them. Most electric motor companies are offering or experimenting with such integral motor and controller units ( CE , Dec. ’97, pp. 36-42). The idea is to reduce size, setup, and installation costs associated with variable-speed motors and drives. Users and suppliers have some reliability concerns when mounting the drive on a source of significant heat and vibration—the motor. Nevertheless manufacturers are pushing ahead, expecting benefits to overshadow concerns and make motor-drive packages an important future market. A sizable portion of that market is expected to rely on power modules.
Another trend is the heating up of the medium-voltage (MV) motor drive market via new drives using new power devices. Powerex’s Mr. Motto says, “In the medium-voltage (2,100 and 4,160 V) drives market a technology battle is shaping up between key power switching methods: HV-IGBTs (available to 3,300 V) and gate-commutated thyristors.”
ABB Semiconductors (Lenzburg, Switzerland; Pittsburgh, Pa.) is very much in the picture with its new technology that co-locates the gate driver with the GCT in a low-inductance package (see CE , July ’97, p. 17). ABB calls this innovation the integrated gate-commutated thyristor (IGCT)—photo above.
Mr. Jones of Collmer/Fuji notes that “customers in the MV drive market are looking for power modules with 3,300-volt and higher capability at 2,000 amps. At those levels self-protection on the module is also desired. The module has to be able to withstand a number of high-voltage spikes and not destroy the control circuitry and main board.”
IGBTs vs. IGCTs
One view of the technology battle lines around the major power devices is shown in the diagram provided by Powerex (second page of article). Standard IGBTs are clearly the device of choice up to 600 volts and under 2,000 amps. Above the 1,000-volt level, HV-IGBTs will battle with GCTs for device supremacy. But the dividing line between technologies is far from crisp. Direct comparison of device applicability is complex. Choice often depends on application specifics and the device supplier.
Powerex sets the HV-IGBT line at 2,000 A and down, and with ac line voltage between 1,000 to 4,160 V. In this area, it believes IGBTs are better. If the voltage is 4,160 and higher and the current exceeds 2,000 A, then a GCT is more likely to be better, according to Powerex.
ABB Semiconductors gives wider scope to IGCTs, stressing their generally lower inductance. “They’re hard to beat for losses when the dc link voltage is greater than 1.5 kV or switching of more than 1,500 A is involved,” says John Marous, ABB’s marketing manager. “Another advantage for IGCTs is in high on-off thermal cycling applications such as heavy transportation vehicles.”
Inability of IGCTs to control di/dt directly—they use a reactor in series—is an apparent disadvantage versus IGBTs. This is largely offset by other factors when IGBTs operate at higher voltages, but for 1.2 kV or less, IGBTs have a definite advantage.
Below 1.5 kV, and for turn off currents under 1.2 kA, ABB sees IGBTs as cost-effective. Still, the deciding factor may come down to user preference. “Familiarity with an isolated module package can override the effort needed to build a hockey-puck-style IGCT product,” comments Mr. Marous.
Higher power, medium voltage
ABB Semiconductors views the scene from the markets it serves—mainly high-power applications such as MV equipment, traction drives, and transmission/distribution. ABB generally regards any switching device that requires more than 600 A turn-off current and can block 1,200 V or more as “high power.”
The important point is the more stringent service environment. “High-power applications place a premium on package inductance, effects of catastrophic explosions, and electromagnetic interference (EMI),” explains Mr. Marous. Unclamped inductance, either internal to the module or associated with a bus, requires higher voltage silicon to block the resulting voltage spikes. “And high-voltage silicon involves necessarily higher losses. Module pinouts become more important as bus inductance depends on the bus layout,” he says. Module design and dimensions can also be affected.
Another complication is the possibility of an explosion if a module can’t handle a large fault current. “Any charged particles being projected could short the external bus, cause mechanical damage to other parts of the system, or cause more severe faults, ” he adds.
Fusing, while not simple, is often advisable. In some cases IGBT modules can be designed to release pressure before it rises to a critical level. More robust explosion management can be offered with hockey-puck IGCT devices. IGCTs are not isolated, and, in case of failure, always fail in the safer short-circuit state (as opposed to possible open-circuit failure for IGBTs).
Powerex’s Mr. Motto agrees it’s an advantage for GCTs to fail shorted. “In high-power applications, if the device fails open it could cause an arc or, worse, an explosion. HV-IGBTs tend to fail open, lessening their attraction versus GCTs.”
eupec’s product range also extends into high-power/voltage modules. Product naming is not simple. Its IHV (IGBT High Voltage) modules range up to 3.3 kV (see photo), while the IHM (IGBT High Power Modules) have ratings of up to 2,400 A and 1.2-1.7 kV.
“These modules have single, dual, and bridge configurations, using several chips in parallel. All paralleling of the chips is done inside the module, says eupec’s Mr. Spreen. For proper parallel operation current paths inside the module must be symmetric, and switching characteristics and V CEsat variation of the IGBTs must be tightly controlled.
Medium voltage applications (loosely defined as 2-9 kV) add to the above issues of higher power, lower voltage. Switching and conduction losses with IGBTs become much higher at 1.8 kV (and above) than in the 600 to 1,200-V range. Other problems for MV power modules cited by ABB include more complex voltage isolation requirements, more difficult cooling, more losses when short-circuit protection is performed via IGBT gate control, and the need to lower di/dt conditions due to diode limitations.
Higher voltage diodes (>1,200 V) can get severely limited in terms of di/dt. Using IGBTs to control di/dt adds to the losses. Compensating for losses is possible by:
Using bigger or paralleled modules;
Using costlier cooling methods; or
Changing the module design (e.g., lower frequency, limiting IGBT functions), or going to another technology.
ABB Semiconductors notes a need to improve MV and high-power modules in some areas. Among them are more effective management of voltages, currents, and their rates of change (dv/dt, di/dt); also the standardizing of such functions as the gate drive and protection methods. At the same time, “there may be less desire to put extra functionality inside the module as with lower voltage power modules,” says Mr. Marous.
Issues of di/dt and dv/dt are crucial for newer high-voltage IGBTs and IGCTs that eliminate the need for snubbers. In prior HV-IGBTs and GCTs, snubber circuits protected the freewheel diode, which is a limiting factor in higher power/voltage inverters.
Development of new products is slower in the MV and high-power arena, but work is ongoing in such areas as lowering switching losses; better module packaging to manage inductance; isolation of voltage; and control of explosion and thermal impedance. For diodes (used with both IGBTs and IGCTs), the direction is to increase snubberless di/dt ratings and the softness of recovery.
Conducted (EMI) and radiated (RFI) emmissions are important factors in the development of motor drives. But what’s the impact on the power module designer? Opinions vary widely, with more than one vendor saying the issue falls on the drive manufacturer, not the power module supplier. Here’s a sampling of comments.
Ken Jones, Collmer/Fuji: “EMI/RFI emissions are more control dependent than power module dependent…generated by the switching technique used…Our customers compensate by using filters.”
Eric Motto, Powerex: “Conducted EMI is a big issue in isolated-base modules, affected by capacitance between power devices and the metal base plate…Freewheel diode recovery also contributes to RFI noise and we have improved it…The customer can add filtering along with our module, but it adds cost.”
Semipower Systems’ Mr. Rinehart has a different challenge than the other module suppliers since his product contains all of the control elements. “When you make a drive in a module you can tradeoff switching losses for lower EMI/RFI.” He likewise thinks that “choice of the diode and its characteristics affects EMI. IGBTs have made great improvements in performance, but fast recovery diodes continue to pose EMI/RFI problems.”
Mr. Motto’s perspective is that regulatory requirements are not at the module level, but at the inverter stage. He concludes, “In North America the rules are lax. Europe and Japan have stricter requirements.” However, “global inverter manufacturers need to be concerned about all regulations anywhere in the world.”
|George Gulalo is president of Motion Tech Trends (Inglewood, Calif.), an electric motor and motion control consultantcy company.|
Power Module Classes
Power modules come with different levels of intelligence. Here is an informal classification of modules on the market.
Level 1 , Standard Power Module— Has a power bridge (inverter) containing two to seven power transistors, but no intelligent functions.
Level 2 , Intelligent Power Module (IPM)— The main feature that converts a module to intelligent status is adding a gate driver. Six transistors form the bridge and an optional IGBT provides dynamic braking. Protection features such as die temperature, current, and voltage sensing also distinguish IPMs from the basic power module.
Level 3 , Power Integrated Module— Besides level 2 functions, PIMs add a power supply that runs the gate drive and other digital circuits.
Level 4 , Modular Power Subsystems— They have the same functions as IPMs, but integrate added features either in the module or on an adjacent circuit board. Module and board form a compact unit.
Level 5 , Integrated Drive Module (IDM)— This is a complete drive with power section, control board, and I/O board wrapped into a one-module unit (see next sidebar). IDM includes all functions in Levels 1-4, plus microprocessor control; PWM signal generation; full current, torque, and voltage control/protection; analog and digital I/Os; operator interface; and digital communications.
‘Drive in module’ adds capacitor functions
Semipower Systems (San Jose, Calif), a leader in providing a complete motor drive in a module, has made another significant advance in module/drive packaging. Two new modules—DriveBlok and CapBlok—make up SSI’s “building block” approach to create motor drives targeted at OEM machine builders, system integrators, and as an extremely flexible solution to the “drive on a motor” concept. DriveBlok is a complete ac sensorless, flux vector drive with the functionality and I/O subsystem of high-performance packaged drives.
Its companion CapBlok is a revolutionary package containing the necessary capacitors, and plugs into DriveBlok (see photo). The combination creates a complete integrated drive in a module—said to be the smallest configuration for 1.5-20 kW sizes—and only needs a heatsink. CapBlok’s design, capacitor technology, and new thermal transfer concept increases the reliability of the capacitor bank by fivefold, according to Larry Rinehart, vp of R&D. This allows Semipower to offer a drive that has warranty levels similar to some of the highest reliability ac motors.
Semipower’s modular drive solution contains a sensorless flux vector software-based control system providing 4-quadrant speed and torque control. It can be configured to run either ac induction or brushless permanent magnet motors. There is an impressive array of programmable control features and performance such as 250% starting torque; 0.2% speed regulation; 1.5 msec torque response, 100:1 constant torque-speed range, and an RS-485 communication link.
DriveBloks are available in two module sizes: Series 1000—0.75 to 3.7 kW and Series 2000—5.6 to 20 kW. Each series comes in 230- or 460-volt versions. DriveBloks come with an easy to use Windows 95/NT-based software package called wInControl. This allows the set up of all motor parameters, I/O points, and can be used to run the motor and monitor system performance.