Finally, by increasing the internal operating frequency for these switching supplies, designers have been able to decrease overall size or, alternatively, pack a higher power rating into the same size unit. For example, a 125W supply from N2Power has a power factor correction (PFC) circuit operating at 87kHz and a half-bridge output at 43kHz. By moving both to 125kHz in the 160W design, the volume of the associated magnetics was reduced by one third, thus allowing the 160W unit to fit into the same 3-inch by 5-inch 1U footprint as the 125W supply.
Active components also are playing a major role in the shrinking of supplies in several ways. IC vendors are developing better analog components for power management, especially helpful for complex functions such as PFC. The result is better accuracy and performance at both high and low line voltages. Early generation PFC circuits could achieve power factor up to 0.99 (nearly a perfect unity value) at lower AC-line voltage of 90/110V AC, but the correction dropped to as low as 0.75 at 240V AC. Using the latest analog ICs, designs can now maintain high PFC across the full voltage range, resulting in fewer corrective components and thus smaller supply size.
The most dramatic change for supplies architecture is the inclusion of digital control circuitry in the supplys inner workings. In recent years, processors have been used in the secondary side of the supply to monitor key points and performance, to establish some operating parameters, and to manage a communications interface port. But the primary-side control loop remained analog.
With the availability of high-performance, low-cost DSPs, digital control now extends to the primary loop, which increases the flexibility in control and operating points, including on-the-fly adaptive control and dynamic operating changes. It also affects size, by putting more of the control functions in less space, due to fewer needed ICs and passive components to implement the hardware control-loop strategy. For example, a supply using DSP control can do three-phase AC line control with about the same footprint as a single-phase unit. The DSP can also provide the required PFC with no further footprint penalty.
Don Knowles joined N2Power as the vice-president of engineering 12 years ago after more than two decades in power electronics design and manufacturing sectors, spanning industrial, ICT, and medical electronics. He holds a degree in Electronics from American River College, Sacramento, Calif.