Putting Power in the Palm of Your Hand... Without Burning It!Imagine an unlimited power source for handheld electronic devices. How about an integrated circuit chip with all the power you would ever need to power the device? This technology is not here yet but power supply on a chip is increasingly the focus of researchers worldwide.
SOC stands for Supply on Chip. And while current technology is promising, there are still many limitations. Take a cell-phone for example. With today's power-management schemes, a cell phone would quickly become too hot to handle. But Power SOC is about efficient power, not available power. The ultimate desire is to create an integrated circuit that would provide unlimited power without the residual heating.
Designing the right power scheme to make SOC viable is where the challenge lies. But there are a number of variables that designers must take in to consideration in their quest for a viable Power SOC, including electronic components, power conversion, regulation, management, and passive components.
First, let's look at where we've been. Integrated circuits with external inductors are already much smaller than their predecessors. For example, Analog Devices' 6-MHz ADP2121 switcher that relies on an external inductor, takes up only 5 mm2, including the inductor. And that pursuit of smaller, more efficient circuitry drives the current wave of design fervor, and a real need of further reducing power-control circuitry into a Power SOC. But there are limitations. The common cell-phone illustrates the current state of Power SOC development. A majority of cell-phones have four or more antennas, including Bluetooth, GSM (global-system-for-mobile)-communication, CDMA (code-division/multiple-access), and 3G (third-generation) units, as well as video and baseband-RF sections. To ensure power efficiency, each core must quickly turn on or off. When the system is out of balance, power is wasted on a core that's not in use. For the most efficient use of power, each core needs its own DC-voltage source. Yet, four or more power management units hanging off a multicore chip would dwarf the chip, even if they each measure only 5 mm2. How do you solve for that? Placing several power-supply modules on the board and then using semiconductor switches to turn the cores and subblocks on and off makes sense. But, research has found that it's difficult for that type of power supply to remain efficient over its entire load range. This type of approach ends in a master power supply that spends most of its time in a low-current-load range, yielding less power efficiency and more system heat.
There are two main challenges to creating a power supply on a chip. First the need for efficient, cost-effective switching devices and second the need for quiet, nonradiating magnetics. While high-speed switching devices exist in the RF realm, they are typically based on exotic semiconductors and thus cost prohibitive.
But that's changing. Cian O' Mathuna, PhD, a researcher in power magnetics at University College Cork's Tyndall National Institute, predicts that complete power SOCs, including an integrated inductor, will soon be able to fit into a footprint of only 1 mm2.
Others, like Vicor, Linear Technology, and Enpirion are looking at PSIP devices as the essential model that could one day lead to a viable Power SOC. While PSIP devices don't take advantage of the cost reduction and added reliability of wafer-level fabrication, they do meet applications' needs for small parts, simple design, reduced costs, and simpler assembly.
For example, Enpirion has designs as small as 22 mm2. This includes two external capacitors. They separate the switching and control circuitry from the inductor and place the two die in one package. According to Michael Laflin, director of product marketing for Enpirion, the company can fabricate the inductor with a silicon wafer or with more traditional multilayer spirals, depending on the application's requirements. Those requirements include such factors as load current, inductance, loss budget, and saturation current. The different loss characteristics and behaviors at different frequencies of the specific magnetic material must also be taken into consideration. Enpirion has been fairly tight-lipped about the technology behind its inductors because as they say, "magnetics technology is the secret sauce in their devices."
Enpirion detailed the lessons it learned in manufacturability, yields, reliability, and cost recently at the Power SOC Workshop. These prosaic determinants are the keys to developing successful products. According to Laflin, when it comes to power management, "cost drives everything."
Tyndall National Institute at Ireland's University College Cork is renowned for their research in wafer-scale magnetics. Their inductors have a "racetrack" geometry of electroplated copper windings. The efficiency of their buck-converter SMPS (switched-mode power supply) was 80% at 20 MHz using a commercial chip inductor from Coilcraft. When researchers substituted the Tyndall 2.5mm2 microinductor, the efficiency decreased to 76%. Then again, the Tyndall inductor was not designed for the circuit.
According to O' Mathuna, the researchers can tweak both copper and core losses. "We're electroplating copper windings into a mold to a thickness of, say, 35 to 50 microns," he says. "If we go to a higher thickness of copper, we would be able to emulate to a large extent the Coilcraft inductor. In addition, the magnetic-core material has its own inherent losses. In thin-film-plated magnetic materials, that resistivity is going to be very low-that is, those materials are going to be quite conductive. For example, a nickel-iron material has a resistivity of about 45 µΩ-cm. If we move to another magnetic material with closer to 100 to 150 µΩ-cm, we could reduce the eddy-current losses."
High-speed switching devices present another challenge to power SOC development. In the most common DC/DC SMPS topology, there is a negative relationship between efficiency and switching frequency. Namely, with each increase in switching frequency, efficiency goes down. "If your design is switching at 5MHz and you increase it to 20MHz, you'll have at least four times more switching loss," says Ted Thomas, product-line manager for Texas Instruments' power-management products. "Switching loss ends up dominating the efficiency of the power supply."
There are several topologies that have shown promise when applied to switching frequency and efficiency. A (ZVS) zero-voltage-switching topology for example, allows the device to turn on and off only when there is zero voltage running across the circuit. For years, ZVS technology has been used in AC/DC-power supplies, because of their ability to accommodate large currents. Low-switching-loss topologies, however, are more complex. If manufacturers could package the appropriate magnetic components for the design, however, they could be used as building blocks for future circuit designers.
Nontraditional technologies are quickly becoming viable options for designers searching for faster switching devices. A good example is silicon carbide, which can support high switching rates with relatively low losses of power. Its high cost however, makes it impractical for mainstream devices. This past September, International Rectifier introduced a gallium-nitride platform that the company touts as having game-changing power density. The advantage of such a platform lies in the fact that it uses inexpensive and readily available silicon wafers to build circuits-lower cost without sacrificing performance. The devices can also switch at much higher voltages. International Rectifier plans to begin shipping samples by year's end.
Power supply on a chip technology is constantly advancing. For the short-term, it applications is being explored as a way to bring greater efficiency to hand-held devices. But eventually the benefits for larger power using electronics like laptop computers and servers will be immeasurable. Until then, look for 10- and 100- MHz PSIPs on standard silicon to emerge in the coming months.
Power SOCs: A "crazy" idea that just might work is written by Margery Conner, Technical Editor at EDN. For the complete article and many more visit EDN Magazine.