However, through-hole starts to limit power density because through-hole packages aren't as efficient in heat transfer as newer designs. Only five years ago, a power density of 5 W/in.3 was state of the art for an ac-dc supply. Now, densities typically hover at the 20-W/in.3 level, with a handful of prototype supplies achieving 25 W/in.3
New semiconductor packages include IR's DirectFET, a proprietary SO-8-size surface-mount package in which a copper "can" forms the drain connection from the other side of the die from the board (Fig. 3). ON Semiconductor and Diodes Inc. announced products in a different SO-8 flat-lead package—it has the conventional tab on the bottom and a clip on top.
Supply power density is a critical element in products ranging from blade servers to laptops. Blade-server supplies, which are about the size of a carton of cigarettes and put out around 2 kW, often take much design effort.
Even more challenging are laptop adapters, along with external supplies for LCD monitors and PCs that incorporate a processor and peripherals into the display unit. Adapters are essentially small, closed plastic boxes with no provision for fans or external heatsinks. Yet some laptop adapters now put out as much as 150 W. Adapters for PCs with displays incorporating the electronics and drives may double that number.
It gets really interesting when the trend intersects newly mandated energy-efficiency standards such as Blue Angel, Energy Star, and One-Watt Standby. Blue Angel and Energy Star require certain efficiencies when a product is running. The One-Watt Standby rule adds that when the product goes to sleep, it can't consume more than a watt. The power supply may only dissipate half a watt in standby mode.
For cell phones, this is no problem. Their efficiency is great over the whole load range. But a 150-W laptop adapter, which generally achieves its greatest efficiency (circa 85% to 92%) at three-quarters of full load, must still operate at no less than 50% efficiency down at 1 W.
That's a tough challenge. It requires some kind of intelligence in the power supply to turn off everything unessential during standby. Since no PFC limitations exist for products operating in standby mode, it turns off the PFC stage. Then on the secondary side, it can start to pulse-skip so that the device only wakes up every few milliseconds to see if the load needs more power. Ultimately, the semi industry must come up with new ideas and technologies to become more efficient.