Over the last decade, electronic equipment has taken a great step ahead by offering smaller solutions and higher performances. These dramatic improvements have been mainly driven by microprocessors that quickly moved from the MHz clock-speed range to GHz and to memory-storage units that now contain several Gigabytes. This has been made possible by the advances in silicon-technology geometries, which have moved from a few microns to sub-micron today. While the 1.0micron technology is used to operate at 5V, the 0.1µm technology operates at only 1V. As the power remains almost the same, the voltages have come down and currents have gone up. This new technology trend forced the designers of power-supply solutions - and therefore of the latter's discrete components - to follow the trend of offering more efficient solutions and miniaturised systems that fulfill these requirements.
All power supplies are a combination of power-switching devices, diodes, capacitors, transformers, inductors and integrated ICs, each of which has contributed to this revolution. Switching MOSFET device technology has improved efficiency and increased power density. Typical drain-source on-resistance has fallen by 75% in the past five years, offering typical RdsON of a few mOhms for a 30V MOSFET, plus innovative packaging technology. Such a technology offers high thermal dissipation and lower internal capacitance, allowing operation at several MHz with minimal switching losses. Higher switching frequency allows the use of smaller inductors and capacitors because less energy needs to be stored during each cycle. Aluminum electrolytic and tantalum capacitors have been replaced by organic OSCOM and POSCAP capacitors offering lower ESR (20 to 80mOhms), higher capacitance and lower risk of destructive failure. Low-voltage application and high-switching frequency allow the use of small-size ceramic capacitors with very low ESR (1 to 20mOhm). Ceramic capacitors have made huge advances in the past five years: the development of dielectric material has allowed thinner, more uniform layers with higher dielectric constants. Latest power magnetics use powered core or gapped ferrite materials with flat planar transformer technology to achieve higher current density, lower DC resistance and lower leakage inductance. The deployment of copper traces in the multi-layer PCB board to form windings for the transformer allows automatic production and interconnection and avoids the added labor costs associated with traditional wire-wound transformers.
Isolated power supplies
All these technology improvements have obviously influenced power architectures and topology trends. Forward-active-clamp topology - and its derivatives - is the benchmark for high-current-isolated DC/DC applications. Demands for higher conversion efficiency, power density, and reduced cost have encouraged the use of the forward converter with active-clamp reset. The active-clamp capacitor recycles energy stored in magnetised transformer, as well as the leakage inductance, to improve overall efficiency. Utilising active clamp techniques improves overall efficiency easily to as high as 92% with output currents from 15 to 30A. Higher output-current applications may benefit from a dual forward-active-clamp solution. Active-clamp controllers enable the forward topology to be more efficient and more flexible in regards to the possible duty cycles. The dual interleaved version of an active-clamp controller combines both of these topologies in a very clever way: power is well distributed within two separate transformers, with much better efficiency and heat dissipation compared to one high-power planar transformer. Figure 1 shows a dual interleaved forward-active-clamp solution .
Non-isolated synchronous buck converters, generally called point-of-load (POL) converters, are widely used in power supplies. A synchronous buck converter consists of a high-side and a low-side MOSFET, which replaces the conventional buck converter's catch diode to provide a lower loss path for the load current. Taking advantage of higher MOSFET efficiency and higher switching frequency, POL follows this trend, offering IC controllers switching up to several MHz with enhanced gate-driver capabilities to reduce losses. Various pulse-width-modulation (PWM) techniques have been adopted to provide faster output-voltage regulation whenever line voltage and load current change. The new generation of PWM control logic is the emulated-peak-current-mode controller for low output voltages. Unlike traditional peak-current-mode controllers, which sense the current while the high-side FET is on, the current here is measured while the low-side FET is on; following this step, the controller emulates the peak-current waveform and uses that information to regulate the output voltage. A typical POL application with emulated peak-current mode is illustrated in Figure 2. Emulated peak-current mode offers all the intrinsic advantage of a classic current-mode controller without the noise-susceptibility problem often encountered from diode-reverse-recovery current, ringing on the switch node and current-measurement-propagation delays.