Optimised IPM Spawns Energy-Efficient Motors

May 9, 2012
Traditional interior permanent magnet (IPM) solutions for the 30W to 200W inverterised motor-drive market require either a large construction or use of an external heatsink. International Rectifier's alternative approach uses PCB copper traces to dissipate heat from the module. As a result, it has a smaller package design and can eliminate an external heatsink.

More than 200 million motors currently feature a power range below 750W. As such, the opportunity is ripe for energy savings via electronically controlled variable-speed solutions. In fact, 55% of the total electric energy produced today is used to run motors of every size, shape, and efficiency.

Variable-speed drives can save as much as 70% of the energy costs by varying the load speed. Inevitably, then, new energy-efficiency standards for fans and pumps demand the adoption of electronically controlled motors.

Traditional interior permanent magnet (IPM) solutions for the 30W to 200W inverterised motor-drive market require either a large construction or use of an external heatsink to achieve satisfactory thermal performance.

Such an approach makes it cumbersome, and often uneconomical, to completely integrate the motor controller within the motor system.

Addressing those issues, International Rectifier's (IR) developed the µIPM family. The new alternative approach (patent pending) uses printed-circuit-board (PCB) copper traces to dissipate heat from the module. As a result, it saves cost due to a smaller package design and, in some applications, the elimination of an external heatsink.

The µIPM device makes it possible to design energy-saving solutions that comply with recent energy standards for fans (heating and ventilation) or pumps (water circulation) up to 200W. Housed in QFN-like packages, the µIPM family comprises a series of integrated three- or single-phase (half-bridge) motor-control circuit solutions. By adopting the high-voltage FredFet MOSFET switches (specifically optimised for variable frequency drives) and IR’s HV driver ICs, dc ratings range from 1A to 10A with voltages of 250V and 500V.

Measuring 12mm by 12mm by 0.9mm, IR claims the µIPM is the smallest IPM available in the target market. Typical package solutions include lead-frame-based single-in-line for through-hole application, or gull-wing leaded lead-frame-based SOP28 (or similar type) packages.

Gull-wing lead and DIP packages tend to have poor die-to-PCB dissipation. They usually need an external heatsink, which add costs and mechanical stress problems (Fig. 1).

1. Shown is a traditional DIP-IPM gull-wing lead package requiring a heatsink (left), versus IR’s µIPM PQFN design (right) that uses the PCB as a heatsink.

According to IR, the µIPM PQFN is the first fully integrated inverter solution to use the PCB as a heatsink. It closely resembles module solutions in point-of-load and VRM applications, but for the first time isn’t limited to low-voltage products.

Like point-of-load or VRM QFN based packages, the µIPM’s power semiconductors (500V FredFets) and HVIC die are also bonded to the lead-frame, which is exposed and soldered to the PCB. However, smaller dimensions and power dissipation through the PCB present challenges that must be overcome to fully optimise the motor drive design.

In general, IPM current capability depends on the dc bus voltage, the ambient temperature, and the switching frequency. As these specs climb higher, the losses become greater.

In the case of a surface-mounted solution (e.g., the µIPM family), the current capability also depends on the PCB design. Specifically, that means the copper thickness, the copper pad areas, the number of layers, and, ultimately, the maximum allowable PCB temperature. In other words, the maximum junction temperature of the power semiconductors is less of a concern than the maximum PCB temperature.

By increasing the PCB copper thickness, it lowers the overall thermal resistance of junction to ambient and, thus, the PCB temperature. A lower temperature leads to higher current capability.

Output current capability increases with either higher ΔTCA or when using a two-phase modulation versus a three-phase modulation scheme. Similarly, a lower switching frequency allows for higher output current due to lower switching losses.

Other cost-effective methods can further improve the µIPM’s performance. Examples include solder streams on thermally conductive traces, or copper wires/jumpers on thermally conductive traces.

However, other more traditional packages may see little current improvement when using these simple methods. Under the same application and load condition, IR says that its µIPM devices deliver greater current capability and higher efficiency versus traditionally packaged IPM solutions.

On another front, the µIPM compares favorably in terms of heat dissipation through the PCB versus competing products. For example, a fan controller was built with a 1oz. PCB operating at 100mA and 15 kHz, using two-phase modulation at a 320V dc bus. The competing device, housed in a traditional package, measured 34.8°C hotter than the IRSM636-015MB at room temperature (Fig. 2). The former’s hot spot lies at the center of the power IC (SOI), while the latter’s highest temperature is localised at high-side common drains point—far from control IC die.

2. In identical fan controller designs, a gull-wing lead SSOP IPM ran 34.8°C hotter (left) than the IRSM636-015MB µIPM (right).

Under more demanding load conditions, most of today’s traditional products show even more critical limitations versus the µIPM’s temperature or power-dissipation capabilities.

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