Many regulatory agencies such as the Department of Energy and the California Energy Commission (CEC) have issued guidelines or mandates for power factor correction (PFC). Two significant motivations for doing this are increased efficiency and improved power quality. PFC addresses the former by synthesizing a resistive load for each half cycle of the ac mains. This aligns the phases of the current and voltage supplied by the mains to the load, maximizing the ratio of load power to transmission-loss power for the given load current.
This same property of PFC also improves power quality by eliminating non-linearity in loads, which can be caused by electronic components such as bridge rectifiers. A linear load reduces the injection of current harmonics, and therefore EMI, back onto the mains.
Along with the emergence of PFC controller ICs that contain both the sensing and control functions required for PFC are ICs that integrate a switching regulator controller with a complete PFC controller stage. PFC controller ICs continue to evolve, with recently developed solutions offered by several vendors.
For example, two new devices from ON Semiconductor are the NCP1605 and the NCP1653/53A. According to Dhaval Dalal, systems engineering director for ON Semiconductor, the NCP1653 is a fixed-frequency current-mode PFC controller. The part has only eight pins and requires few external components, the values for which are derived from a simple procedure.
The NCP1605 is designed for topologies that cannot cope with a wide input voltage range and have no-load or light-load efficiency requirements. The NCP1605 simplifies the interface between the downstream converter and the PFC converter in all operating modes, including startup, transients and standby. Both devices also feature a programmable current-limit threshold that allows end users to achieve the appropriate tradeoff between efficiency and noise immunity for their applications.
According to Dalal, the most important initiatives for external power supplies are Energy Star and those from the CEC, which are applicable in the United States and with other countries. The European Code of Conduct is also important. Various standards for standby power for television sets are listed in the table.
Vipin Bothra, application manager for STMicroelectronics, stated the L6563 is the company's latest PFC product. It uses fixed off-time control, which allows the controller to be used in both transition-mode and continuous-mode topologies. The L6563 also allows the on-time to be increased during zero crossing so that the total harmonic distortion (THD) can be improved. It is envisioned the part will mainly target high-volume applications such as PC power supplies.
According to Bothra, the biggest challenges designers face are dealing with abnormal conditions and dynamic situations such as startup and shutdown. Hence, it is important to have two-level overvoltage protection and two-level current sensing. To deal with startup and shutdown, an interface is needed between the PFC and the dc-dc converter. In the future, new efficiency standards requiring better efficiencies at less than full load will require some innovative design and open the door for future PFC controllers.
Another challenge with PFC controllers has been the ac mains voltage sensing, typically provided from the output of a voltage divider. One alternative to this method is to internally emulate the ac voltage by measuring the inductor current using a shunt resistor. This proprietary technique is used by International Rectifier's IRF1150/50S. This device is a continuous-current mode PFC controller with an input range of 75 W to greater than 4 kW. According to International Rectifier's Mario Battello, marketing manager, SMPS, while conventional controllers of this type use a multiplier circuit architecture that requires the ac mains voltage to be sampled, the IRF1150 uses a proprietary technique called One-Cycle Control to measure the inductor current. It then combines this information with an internal integrator to control the PWM signal to the gate driver. The gate driver has a drive current of 1.5 A, which enables the IR1150 to support a wide range of input power.
According to Battello, this device was specifically designed to comply with green energy mandates. It is equipped with a feature allowing it to be switched off. The device also has a maximum THD of 4%. However, while improved efficiency is an obvious benefit of the device, the resulting reduction in EMI is another benefit. For this reason, digital flat-screen TV sets represent a major emerging application for PFC controllers.
According to Jaekuk Ryu, marketing manager for PFC at Fairchild, the FAN7528 (see the figure) voltage-mode, critical-conduction mode (CRM) PFC controller also does not require the rectified ac line voltage information, so it can save the power loss of the input voltage-sensing network that is necessary for the current-mode CRM PFC controller. Instead, it uses voltage-mode PWM, which compares an internal ramp signal with the error amplifier output to generate the MOSFET turn-off signal.
Ryu added that UL standard UL60950 imposes some new safety requirements on all controllers by the “adjacent pins short” test, which will require attention from designers. These new constraints will open up opportunities for new controller designs, which are now designed specifically with such requirements in mind. Some of the old controllers won't pass these tests, and thus won't be viable for new power supplies.
When developing alternative PFC control methods, microcontrollers can be extremely useful for power designers. Justin Milks, an applications engineer with Microchip's Security, Microcontroller and Technology Development division, stated that Microchip does not manufacture any products that are specifically designed for PFC. However, there are several features found within Microchip's general-purpose microcontrollers that enable them to perform PFC and act as switching power-supply controllers.
Milks stated that two particular products of interest are the PIC16HV616 and the PIC16HV785. Both of these mixed-signal microcontrollers feature integrated shunt regulators. The PIC16HV616 features an SR-latch tied with two onboard comparators, basically forming a current-mode power-supply peripheral. The PIC16HV785 features an analog PWM generation module, complete with onboard comparators and op amps. The power of the microcontroller allows the user to perform other tasks in addition to PFC. The microcontroller can be used to implement the final regulation stage of the power supply or to perform intelligent monitoring.
Furthermore, the microcontroller hardware can be reconfigured on the fly. This may allow the system to reconfigure itself for a low-power mode, possibly helping it to meet Energy Star and other standards. An emerging application for these and other microcontrollers has been intelligent power-supply design. The numerous onboard analog features allow Microchip's PIC microcontrollers to play a role in both the control and monitoring of power supplies. The benefits of reducing EMI on the grid and in the load will probably create a wide range of opportunities for PFC controllers in increasingly sensitive electronics systems. However, present development seems to be mainly driven by compliance with mandates and guidelines focused on reducing electrical energy consumption.
Given the benefits of industrial-scale PFC and the inability of emerging PFC manufacturing mandates to address deployed legacy equipment, it is not unreasonable to anticipate PFC may one day be applied at the residence level. Nor is it unreasonable to believe that power designers would rise to this potential challenge, especially after reviewing these recently developed components.
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