Worldwide, the move to replace simple fixed-speed motor drives with more sophisticated Variable-Speed Drives (VSDs) is rapidly gathering pace. Driven by new regulations, such as the European Commission’s Energy-related Products (ErP) directive, manufacturers of electric motors are having to equip their products with the ability to match their speed and power output to the load, providing for a huge reduction in average power consumption.
Indeed, increasing the efficiency of electric motors provides great scope to reduce both the operating costs and the environmental impact of energy generation: by installing inverters and high-efficiency motors, some 43TWh of energy could be saved annually in the European Union alone (source: Siemens research).
At the heart of all VSDs is a common type of power circuit: an inverter. A motor’s inverter may be implemented as a discrete circuit made up of as many as 20 components. Power IC manufacturers, however, have recently invested heavily in the expansion of their range of Intelligent Power Modules (IPMs), a component in a single package which integrates most of the components in an inverter circuit.
In the domains of both RF and power electronics design, the module is a well known and understood concept. In general, modules offer the advantages of small size, good performance and ease of use, which are balanced by the drawbacks of higher unit cost and lack of flexibility.
In their latest generation of products, IPM suppliers are attempting to swing this balance firmly in their favour by improving the choice of options available to the designers of VSDs. This article examines the case for and against the IPM in the light of the latest product developments.
Why energy efficiency has come to the fore
As part of the European Union’s 20/20 initiative (setting the goal of reducing CO2 emissions by 20% by 2020), the provisions of the 2009 ErP directive are being steadily tightened. For standard motors with a power output in the range 0.75kW-375kW, there are four classification levels: IE1-4, with IE4 requiring the highest efficiency.
Since 2011, all electric motors have been required to be classified at least at level IE2. From the start of 2015, electric motors in the power range from 7.5kW-375kW must be at the higher IE3 level, or at IE2 only if they are a VSD type. This same rule will apply from 2017 onwards additionally to motors at the lower 0.75kW to 7.5kW power range.
For many OEMs, the best way to meet the requirements of the ErP directive is to replace legacy fixed-speed motors with a new VSD design. Many hundreds of new products to be introduced in the next two years will therefore include a new VSD. Such products include:
2kW-20kW: heating-system pumps, air-conditioning equipment, conveyor systems, universal and servo drives >20kW: a wide variety of industrial equipment
An inverter will be found in every one of these VSD motors, as shown in Figure 1. And most designers will focus on the following requirements for the inverter design:
• High efficiency, which helps the motor manufacturer to comply with regulations such as the ErP directive
• Small size – cutting the motor’s size and weight helps to reduce Bill-of-Materials (BoM) costs, improves operating efficiency and saves fuel in transportation equipment, and releases space for other uses. It also means the motor-control system may be placed closer to the motor, resulting in reduced EMI and parasitic effects in the PCB.
• High reliability, for long and predictable service life and low recall and repair costs
So how well does an IPM, which is also known as a Smart Power Module (SPM) or Small Low Losses Intelligent Moulded Module (SLLIMM), meet these requirements? In fact, all IPMs use the same basic ‘recipe’ of components, as shown in Figure 2. An IPM is comprised of:
• Six power transistors, either IGBTs or MOSFETs
• Six fast-recovery diodes
• Gate-drive ICs
• Gate resistors
• Optional thermistors
• Bootstrap diodes
Some modules also integrate an internal shunt resistor for output-current measurement whilst others rely on external shunt resistors for this function. In addition, some module manufacturers integrate noise-absorbing capacitors or comparators.
When integrated into a module’s single package, an inverter gives the designer the benefits of reduced size and improved reliability. The board footprint occupied by a modular solution is around 50% smaller than that of the equivalent discrete solution, because the module eliminates the board-level interconnections required between discrete components.
Because the module solution reduces the inverter component count from around 20 to one, reliability is also improved. The risk of failures or mistakes in the assembly process falls dramatically when the component count falls. In addition, the number and length of the interconnecting traces is massively reduced, cutting the inverter system’s susceptibility to EMI and noise, and providing for more stable, predictable and reliable performance. The module also eases the design team’s compliance burden, since it provides a fully pre-tested sub-system, typically including UL-recognised isolation characteristics.
Other advantages are also available to the user of an IPM. Crucially, the implementation of an inverter design is far easier, and provides for a shorter time to market, when using a module. The module will be supported by full documentation, with verified performance data. In addition, IPM suppliers provide easy-to-use, free PC-based development software which helps to accelerate and simplify the configuration of all the inverter’s parameters.
And of course, the matching of the internal components, and handling of the parasitics, are dealt with by the module manufacturer. In a discrete design, the OEM designer must manage these issues.
The module also provides advantages at the production stage: a circuit with one component rather than 20 is easier and cheaper to assemble on the board, and the reduction in component count also streamlines inventory and vendor management.
The case in favour of using a module, then, seems remarkably strong. And yet many designs today continue to be implemented with discrete components. This is because OEMs have in the past found two main drawbacks in modules: inflexibility and cost.
Clearly, the specifications and performance of a module are determined by the module’s manufacturer – not by its user. And these specifications might, in theory, not match the user’s requirements exactly. A discrete design, by contrast, can be made to closely fit the application.
This argument, however, loses force when applied to inverter applications. IPMs are intended precisely for use as inverters in VSD designs, and the IPM suppliers have developed product ranges that meet the needs of nearly all users, from the 0.75kW to 7.5kW segment up to high-power motors using >20kW. These varied needs are met by devices covering a range of breakdown-voltage, peak- and average-current and thermal-dissipation requirements.
In part, the variation in user requirements is met by offering the same or similar circuits in different package types and sizes, including PQFN, surface-mount, Single In-Line (SIL) and Dual In-Line (DIP).
IPM manufacturers also tend to develop product variants with different housing styles, for different power-output ratings:
• Insulated Metal Substrate (IMS) technology is thermally efficient, very robust and offers an easy means to take temperature measurements for the protection circuits.
• Direct Bonded Copper (DBC) is a highly thermally efficient technology which supports higher power densities, as shown in Figure 3. Aluminium oxide or aluminium nitride ceramic baseplates also provide high thermal performance at a slightly lower cost.
• Fully moulded housings have a lower cost, but worse thermal performance, and so are restricted to modules with a relatively low power rating.
Manufacturers such as STMicroelectronics, Fairchild and ON Semiconductor see a great opportunity in the IPM market, and have been busy introducing new products at a rapid rate. So users now have a very good chance of finding a module that closely matches the requirements of their inverter. In addition, for most it is an advantage that the gate-drive IC is integrated in the module: when the gate driver is close to the gate of the IGBT or MOSFET, it is less affected by noise, and consequently operates more reliably and predictably.
Clearly the integration of the gate driver removes from the user the freedom to control the gate-drive voltage directly, and for a very small number of motor-control designers, this restriction might be of some importance. For most, it is an advantage not to have to match the gate-drive circuit with the power transistor.
The second commonly cited, and important, objection to the use of an IPM is on the grounds of cost: the unit BoM cost of a module is usually higher than the aggregated BoM cost of the discrete components that it replaces.
Experienced users of modules, however, tend to disregard comparisons of unit BoM costs. This is because the metric that matters is the total cost, not simply the aggregated BoM cost. And when that is taken into account the module will win more often than not.
As described above, the IPM offers size and reliability advantages because it is a single, highly integrated component that may be used in place of around 20 discrete components. This also provides great cost advantages:
• The circuit design is easier, quicker and therefore cheaper to implement
• Board assembly is quicker and easier, so production costs and wastage are lower
• The module’s board footprint is smaller, so the PCB is smaller and cheaper
• Lower component count results in higher reliability, for lower maintenance, repair and recall costs
• The module’s reduced susceptibility to EMI eliminates any requirement, in a discrete design, for costly EMI counter-measures and materials
When all costs over the lifetime of the motor – including design, BoM, production and after-sales costs – are taken into account, the modular solution can in many cases be the cheaper one. Perhaps more importantly, it is also the solution with the more predictable and lower financial risk across the project life-cycle.
How to make module choices
For all except high-volume applications in which BoM cost is by far the greatest contributor to total cost, then, an IPM is a highly attractive choice for designers implementing a motor drive’s inverter circuit.
The choice of IPMs and of IPM suppliers is wide. Since IPMs are produced in standard ratings and package styles, the differences between one similarly rated module and another are in most cases small. The choice of IPM supplier can then seem difficult to make.
For some applications, a particular feature or specification of one module over another – such as a small advantage in terms of efficiency at light loads, for instance, or a slightly superior thermal dissipation rating – might be important. For many, the specifications of rival modules will not be decisive.
In all cases, however, it is important to remember the advantage of modules as described above: unlike a yet-to-be-designed discrete circuit, a module offers clearly documented and verified performance. The user must be able to trust absolutely the quality and performance of the module he or she chooses. And this means that the only golden rule when choosing a module supplier is to select a company with an excellent track record in the power semiconductor market, and with a strong reputation to protect.
On these grounds, Future Power Solutions is happy to recommend ON Semiconductor, STMicroelectronics and Fairchild as proven and trusted IPM suppliers.