Like most electronics systems, power supplies continue to get smaller. These systems leave room for added functions and processing power, and the supply fits existing physical formats to avoid the cost of redesign. We're at the point where smaller designs are achieved by combining a range of techniques, rather than through dramatic technology breakthroughs. It's the skill of the designer that separates a very good power supply from an average one. Let's look at AC/DC power supply design in the 100- to 200-watt range to see how to minimize the size and cost of a power unit while maximizing efficiency and application flexibility.

In most applications we'll want to generate as little heat as possible. For a 100-watt to 200-watt power supply, we can most often reasonably expect to achieve an efficiency of 90 percent. An improvement in efficiency by 1 percent represents 10 percent less heat dissipation at the upper end of the power range, and this can make a significant difference in the measures we need to take to cool the supply. Simpler designs are best, consistent with the optimum degree of functionality desired. Control and alarm signals, current sharing with similar units, and the ability of the power supply to maintain its performance over a wide range of AC input conditions are all important. Let's look at the supply section by section, from input to output, to see how to achieve these objectives at the lowest cost.

Input filter
Use a two-stage filter with high permeability cores to minimize size while providing high common-mode and differential noise reduction. Stacking some components vertically can save board space and improve cooling.

Power factor correction circuit (PFC)
Silicon carbide diodes are now economically feasible. Their reverse-current characteristics mean that they don't require a snubber circuit and thus they will save you five or six components. In addition, they typically provide a 1 percent boost in efficiency. A stepped-gap inductor provides high inductance at high input line and supports maximum flux density at low line. If you operate the supply in continuous conduction mode (CCM) operation throughout the input range you will keep the peak switching current and input filter requirements to a minimum.

Main converter
If you use a resonant topology, you can virtually eliminate switching losses. This topology improves power supply efficiency and enables smaller heat sinks to be used. In fact, compact ceramic heatsinks, versus metal ones, can sometimes be applied to power transistors. They'll reduce noise and simplify filtering because ceramic heatsinks do not have any capacitive coupling to the drain connections of the switching MOSFETS. In addition, creepage distances can be smaller, which in turn implies a further savings in board space.

(Click on Image to Enlarge)

The main design blocks of a switching power supply

Output rectifier
Opt for synchronous rectification using switched MOSFETS rather than output rectifier diodes. This circuitry improves efficiency through a significant reduction in power dissipation. For example, the power dissipated by a diode with a forward voltage drop of 0.5 volt carrying 20 amps will be 10 watts. On the other hand, a MOSFET with an on-resistance of, say, 14 milliohms at +100°C dissipates just 5.6 watts—a 44 percent improvement. Once again, a ceramic heatsink can replace a metal one.

Control circuit
More highly integrated control circuits are becoming available; take advantage of them to save component count, manufacturing costs, and board space, even where the integrated circuits themselves may be more expensive than a discrete component approach. One example is the IR1150, which is a PFC chip that operates as a one-cycle control (OCC) device. It cuts component count drastically without reducing power system performance. Similar application-specific chips can provide the control circuitry plus over-current, over-voltage, and over-temperature protection for the main converter block. The chips can also control the output rectifier switching.

Other desirable control options for increased application flexibility include power sharing with synchronous monotonic start-up, an inhibit circuit to shut down the power supply via logic control, a 'power good' signal, and the control functionality needed for a standby converter. The standby converter provides an independent 5-volt output whenever AC power is present.

Such functionality is typified by supplies like XP Power's EMA212, which delivers more than 200 watts from a 3-by-5 inch footprint and a profile of just 1.34 inches. The unit needs just 12 CFM of forced-air cooling, which is easily achieved using standard 40-by-40 mm fans. Efficiency is 91 percent at full rated load.

About the author
Gary Bocock is a qualified electronics engineer (HNC) and Member of the Institute of Electrical Engineers (MIEE). He has worked in the power supply industry for 25 years in design, development, applications and management roles. He has been with XP for 14 years and has held a variety of engineering and management jobs, culminating in his present position as Technical Director.