Embedded power is used in a wide range of distributed power architectures, with much of the design focus going to the dc output side. Challenges on the ac input side are equally demanding, however, including power factor correction (PFC), hotswappability, and pulse width modulation (PWM). Semiconductor companies are providing products for this side of the house that range from digital PWM (DPWM) to One Cycle Control (OCC) for PFC. Power factor correction is gaining attention as new harmonic emission standards become mandatory in Europe and Japan.
Other countries are at various stages in accepting these standards, which means power supplies could be subject to more stringent requirements. High minimum power factors are no longer limited to highpower applications, for instance. Newer norms and standards apply to products dissipating as little as 75W or 26W for lighting devices. Low PF results in poor electrical efficiency, creating the need for larger generators, transformers and other distribution devices that would otherwise not be necessary. Other PF conditions include overloads, voltage distortions, noise, parasitic harmonics in the network and reciprocal interference in the system.
Analog-based PFC control methods have long been used in power supplies. One PFC controlmethod gaining in popularity is to use an analog controller with a discontinuous conduction mode, such as Fairchild Semiconductor¡¯s(www.fairchildsemi.com) FAN7527B and FAN4812. The company also introduced its new FSAB20PH60 PFC smart power module, which implements the partial PFC switching convertercircuit topology, switching at twice the line frequency of the traditional PFC topology. The partial-switching method is popular in for 1 to 3kW air conditioners.
International Rectifier (www.irf.com) introduced the IR1150 family of µPFC for ac-dc PFC circuits. IR utilizes its licensed One Cycle Control approach in the IR1150 to deliver the performance of continuous conduction mode (CCM) PFC with the simplicity, reliability and low component count of a discontinuous current mode circuit for PFC applications. The company claims that the patented OCC technique ¡°radically alters traditional thinking about PFC solutions. It¡¯s a form of CCM PFC using a new ¡®integrator with reset¡¯ control block.¡± Companies are also looking at full digital power factor correction, although these solutions can have higher costs associated with them. Fairchild offers an approach that features a control method based on a modified sinusoidal pulse width modulation. Fairchild¡¯s proposed method does not need to sense a line current or voltage, and can be implemented by using one of their low-speed and low-priced conventional microcontrollers.
Power Factor One Inc.(www.powerfactorone.com) uses its patented control methods including Constant Pulse Proportional Current, Analog Reset System, Start Enforcer and Non-Linearity Correction to significantly simplify the converter¡¯s circuit and improve the controller¡¯s performance. The company states that their PF484x PWM controller circuits include near-unity PFC circuits that meet all international conditions. It provides near-unity power factor for universal input line (85 to 265Vac) with no need for sensing the input voltage. GPT is also finalizing development of a digital power management module that incorporates a PFC stage, which is expected to be commercially available within the next year.
A new PFC/PWM controller from ON Semiconductor (www.onsemi.com) with integrated standby and high-voltage start-up capabilities. The NCP1603 combines a PFC controller chip and a PWM control chip in a single, 16-pin SOIC package. Under light load conditions, the NCP1603 PWM controller disables the PFC function, saving in the range of 200mW of power.
Another company looking at digital implementation is C&D Technologies (www.cdtechno. com). They recently presented a paper at the Digital Power Forum that introduced a digital/analog power-controlled 5500W rectifier with three-phase ac input and high power factor. The topology uses three FET PFC boost switches and tolerates a wide input range. Fault detection and control is monitored by a microprocessor. The generation of the control reference for PFC correction is handled both on an analog level and a microprocessor level.
Zilker Labs (www.zilkerlabs.com) has introduced the ZL2005, a mixed-signal, power management and digital PWM IC that integrates a compact, efficient buck controller, high-current adaptive drivers and full power and thermal management functions in a single package, eliminating the need for complicated power supply manager chips and numerous discrete components.
Digital PWM control is also being slowly introduced as a way of enabling high-frequency PWM converter applications. Texas Instruments (www.ti.com) introduced the UCD9501 high resolution DPWM controller, which is part of its Fusion Digital Power family of products and can achieve up to 150 picoseconds DPWM resolution.
Silicon Labs (www.silabs.com) debuted the Si8250 DK, a development kit that includes tools for designing digital power supply controllers. The kit includes a digital power supply and PMBus adapter, along with software that the designer uses to describe the desired functions of the controller. The code is synthesized, compiled, then put into the chip.
Companies like Roal Electronics (www.roalelectronics.com) are looking at front- end power supplies with combined digital and analog control. The company has a single output, Digital Power Front End with a patent-pending multiphase digital PFC. A single 8-bit microcontroller is used to manage the PFC, Vbulk regulation, alarms and signal monitoring. Interleaved two-transistor forward converter PWM control is achieved by using a standard, low-cost, 10-pin analog IC with direct digital integration for power management.
Ac powering may not be the primary focus of many power supply design engineers, but the demands of distributed power architectures will make it a challenging area in the next few years. As regulations change, and as digital power moves into the commercial mainstream, opportunities will emerge for power supply manufacturers to increase efficiency and reduce costs.
