Better Power Packages Make Better Circuits

May 1, 2007
Advanced power packages are breaking barriers that affect dc, ac and thermal performance in switch-mode power supply circuits for a broad range of applications.

Packaging design has become central to significant advances in the power electronics sector. More than a decade ago, the on-resistance of a packaged power MOSFET was on the order of an ohm; the package's contribution was comparatively small — on the order of several milliohms. However, as power semiconductor process- and device-design engineering have improved power silicon, fabricators are now able to make MOSFET die with on-resistances as low as about a milliohm.

As a consequence of this evolution in power semiconductor technology, it is now often the package resistance and not the die's dc performance that limits primary parameters such as maximum sustainable operating current in most modern plastic-packaged power devices. Additionally, packaging technologies are central to thermal and high-speed electrical performance. Despite the focus of many applications on dc performance, these latter two considerations often distinguish a superior design from marginal alternatives.

Basic Anatomy

The semiconductor's package serves three basic functions: it connects the die to the external circuit, removes heat that the die dissipates, and protects the die from contaminants and mechanical abuse. The challenge for packaging engineers has always been to make a notable improvement in at least one of these functions, while neither weakening the other functions nor driving up the OEM's net-installed costs.

Since the TO-220 and its near relatives replaced the older TO-3 and TO-5 metal-can packages, traditional plastic packages have comprised three basic components: the lead frame, bond wires and molding compound (Fig. 1). The TO-220 offers a comparatively inexpensive assembly process, yet provides good thermal performance and good mechanical robustness in the presence of the large temperature swings that power devices experience.

During recent decades, the SO-8 and its descendents have dominated miniature power packaging. Thermally enhanced SO-8 packages improve thermal performance by providing a shorter thermal path. Instead of using thin wire bonds, thermally enhanced SO-8s mount the die on a copper pad with connections to the leads. As a result, a thermally enhanced quad flat no-lead (QFN) package, for example, provides lower thermal and electrical impedances to the pc board than a standard SO-8 with which it shares a footprint.

Thermal Limitations

Packages provide several heat-dissipation paths for the semiconductors they contain. In plastic packages, the heat that dissipative mechanisms generate both radiates and conducts into the molding compound. The plastic compound's low thermal conductivity limits “topside” heat flow.

The packaging material's thermal conductivity (k) is expressed in watts per meter per degree Kelvin. Common power semiconductors use epoxy-molding compound, with k in the range of 0.9 W/mK to 1 W/mK. As a result, the package's thermal conductivity often limits the device's current rating in high-power applications. The OEM designer's available methods to improve the thermal performance of a power device is either to add a heatsink, which increases the bill of materials and assembly costs as well as product volume and weight, or to choose a device packaged in a more thermally conductive material.

Packaging engineers focus significant effort on improving components' thermal performance. Many have considered replacing epoxy packages with metal, which provides greater thermal conductivity, but this approach requires a complete revision of semiconductor manufacturers' back-end processes — packaging and post-package testing. The benefit of such work is great, however. For example, International Rectifier's DirectFET package, one of a few advanced power packages that feature a metallic envelope, provides a thermal conductivity of 385 W/mK (Fig. 2). Furthermore, an epoxy-free advanced power package provides low mechanical stress.

Metal packages also allow efficient topside cooling and readily accommodate external heatsinks. To take advantage of dual-sided cooling, use a standard thermal interface material (TIM) to fill voids between the heatsink and the metal can's top surface. Air is an effective insulator and eliminating air gaps with TIM can improve thermal conductivity at the device-to-heatsink interface by as much as a factor of 400.

In addition to providing superior thermal performance, the metal can causes a lower heat rise to begin with in circuits that have high-average currents. Modern-day plastic packages provide 1.5 mΩ to 1.9 mΩ of die-free package resistance (DFPR). The metal-can alternative reduces the DFPR to about 0.1 mΩ. This improvement in package dissipation simplifies the thermal design of high-current circuits, allowing OEM designers to take full advantage of the power-density advantages that advanced power packages bring.

Increasing Power Density

Small surface-mount packages increase power density and reduce costs. In recent years, surface-mountable packages such as the TO-263 (D2PAK) and SO-8 have largely displaced through-hole packages such as the TO-220, TO-247 and TO-251 (I-PAK) in a broad range of applications including servers, communications infrastructure, automotive and consumer electronics. More recent advanced power packages economically provide industry-leading electrical and thermal performance at previously unattainable power densities.

As an example, the small-can Direct-FET's footprint is about 60% of the SO-8's and 27% of the D-PAK's. The medium-can version fits in an SO-8 footprint, but its lower profile — 0.7 mm compared to the SO-8's 1.7-mm profile — provides 40% volumetric savings. The advanced package's lower thermal and electrical impedances allow the package to manage current levels that would otherwise demand a significantly larger package.

In contrast to traditional packages, the DirectFET houses its die in a copper can with die pads directly solderable to the circuit board. The large die-to-circuit-board contact area minimizes the thermal conduction path. The result is a die-to-circuit-board thermal impedance as low as 1°C/W, which compares favorably with the DPAK, D2PAK and SO-8. Additionally, the DirectFET's compact copper envelope yields a junction-to-case thermal impedance of only 3°C/W (1.4°C/W with heatsink), compared to 18°C/W for an SO-8. The package's low electrical resistance also contributes to the thermal performance by reducing the power dissipation for a given operating current.

AC Performance

As power circuits' switching speeds have increased, so has the need for fast and clean switching edges. Slow rise and fall times and large ringing waveforms on the converter's output waste energy and exacerbate an already challenging thermal-design environment. In this context, advanced power packages' short conduction paths between the die and application circuit reduces the packaged device's parasitic inductance compared to that of traditional packages. When circuit-board designs use good high-frequency layout techniques, the package improves high-frequency performance and minimizes ringing. Conversely, if pc-board design has not been carefully done, use of advanced power packages may expose excessive board-level parasitics as the source of high-frequency performance issues.

The metallic envelope also provides a low-inductance off-board conduction path for the power device's backside contact — often the MOSFET drain. Connecting the external elements on the drain node through the package can reduce routing density on the board and provide an opportunity to tighten a layout further. Taking advantage of such layout techniques allows an OEM designer to shrink power loops, which helps reduce radiated EMI.

Form, Fit and Function

The various attributes of advanced power packages — high thermal conduction, low package resistance and low package inductance — have individually and collectively inspired new, more-efficient product designs. Yet some of the most eye-opening developments would be unattainable were it not for the degree of miniaturization these packages offer as well. In laptop-computer power adapters, for example, advanced power packages shrink the circuit-board area for the converter's power stage. They also reduce the volume that internal heatsinks require. Overall, the adapters are smaller, lighter and less expensive to manufacture, ship and inventory.

Hand tools are also benefiting from the performance improvements and size reductions that advanced power packages bring to a variety of low-voltage motor-control applications. For example, a three-phase half-bridge power converter using six power-stage MOSFETs fits comfortably in a hand-drill shell. The converter delivers 50 ARMS (without current sharing) and a peak power of 1.2 kW.

Advanced power packages help tool designers reach a design power density and peak torque unattainable with traditionally packaged parts. Additionally, such power converters exhibit greater efficiencies for longer battery life in portable tools.

Another application example is the Torqeedo electric boat motor (Fig. 3), which is available in several models ranging in rated power output from 400 W to 2 kW. In an 800-W model, the motor operates on a nominal 29.6-V manganese Li-ion battery and delivers 27 A to a three-phase brushless dc motor.

In this product, the Torqeedo's designers take advantage of the FET package's thermal and electrical properties in this space-efficient design (Fig. 3). In applications like this one, traditional SO-8s do not provide a sufficient sustainable operating current rating, forcing designers to parallel multiple devices. Larger packages such as the D2PAK can provide the current but are less efficient and have higher thermal resistance.

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