Intelligent power modules: the latest product developments, and how designers can take advantage of them

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 ErP (Energyrelated Products) 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.

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. 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 (see Figure 1). 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 specifying an inverter circuit, most designers will focus on the following requirements: • High efficiency • Small size • High reliability

How well does an IPM or ‘Smart Power Module’ meet these requirements? In fact, all IPMs use the same basic ‘recipe’ of components (see 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, as well as 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 improved as well. 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 greatly 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.

The efficiency of an IPM will typically be on a par with that of a discrete circuit, since both may use advanced IGBTs or super-junction MOSFETs, which benefit from the latest reductions in both switching and conduction losses.

Other advantages also help 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.

IPMs also provide a comprehensive set of integrated protection features as standard, including a high (typically 2.5kV) isolation rating. 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 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-3.75kW 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); and with various housing styles to meet a range of thermal specifications.

Indeed, 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, and that meets their system cost budget. Experienced users of modules 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, offering lower development, assembly and board costs, and reducing the need for expensive EMI shielding.

How to make module choices

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. In all cases, however, it is important to remember the advantage of modules as described above: unlike a yet-to-bedesigned 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.

MGJ series from Murata Power Solutions: ideal DC-DC converters for IGBT gate driving

At high power, inverters or converters typically use bridge configurations to generate line-frequency alternating current, or to provide bi-directional PWM drive to motors. These bridge circuits include IGBTs, the emitters of which are switching nodes operating at high voltage and high frequency.

This calls for a drive circuit with a particular combination of characteristics:

• The gate-drive PWM signal and associated drive power rails, which use the emitter as a reference, have to be ‘floating’ with respect to system ground • High immunity to the high transient voltages at the switch node • Very low coupling capacitance • Agency-rated and robust safety isolation from the control circuitry

A new line of DC-DC converter modules, the MGJ series from Murata Power Solutions, provides gate drive power rails satisfying all these requirements and offering a choice of asymmetric output voltages for the best system efficiency and lowest EMI. In addition, modules in the MGJ series provide the gate drive voltages required by silicon, Silicon Carbide (SiC) and even Gallium Nitride (GaN) MOSFETs, giving system designers the flexibility to migrate power designs in the future to the newer semiconductor technologies without needing to change their gate drive circuit.