More energy efficiency challenges have cropped up in the last few years than existed in the previous few decades combined. The forces driving this include rising energy costs, better options in component technology, rising copper costs leading to a cost-reduction differential between switched-mode power supplies (SMPS) and low-frequency step-down transformers, and legislation to bring about crucial changes in standards setting. The plethora of U.S. standards includes federal and voluntary, as well as some individual state laws. The California Energy Commission (CEC), for instance, mandates all federal standards, and on top of that, requires tougher efficiency standards for most equipment. Of all the energy efficiency mandates of CEC, the primary focus has been the reduction of power consumption in standby mode.
The 2007 Appliance Efficiency Regulations of CEC on compact audio equipment, effective July 2008, require a maximum 2 W of standby power consumption. This basically eliminates using low-frequency step-down transformers, which are almost universal in consumer audio electronics. Specifications on external power-supply adaptors are even more stringent. For a 10–250-W external power supply, CEC mandates standby power of less than 750 mW. Meeting these requirements not only requires a better power supply topology selection, but also a system-level scheme whereby load in standby mode needs to be minimized. Given the cost constraints of consumer applications, most systems rely on a single power supply within the system rather than a separate power supply for standby mode, such as used in the PC.
Meeting standby power standards requires saving every possible milliwatt. Some useful techniques relate to controller and power-supply topology, while others relate to design choices in general. For example, R-C snubbers across MOSFETs or diodes to suppress switching noise consume power every cycle, depending on the voltage swing across them and irrespective of the output power level.
For most power supplies under 100 W, which have output currents less than 5 amps, flyback topology’s simplicity and cost benefits make it the popular choice. However, traditional flyback power supplies running at fixed frequencies simply cannot meet these stringent requirements. The usual technique has been to operate the PWM in a burst mode, producing a dead time in between switching cycles to minimize switching losses. With the advent of new techniques, the burst-mode technique can now be implemented in quasi-resonant (QR)-flyback topology as well as in resonant topologies.
QR-flyback topology has lower turn-on losses of the main primary switch, making it a topology of choice for adaptor applications like laptop chargers. QR-flyback topology also has lower turn-off losses of the output diode, making it the preferred topology for high-output-voltage applications, such as those requiring 200-V or higher diodes. However, QR-flyback topology has some disadvantages, such as its variable frequency and difficulty in maintaining the zero-voltage turn-on feature when the input voltage is a wide-range input with no power factor correction (PFC) front end.
QR controllers have been around for many years and have evolved to meet the toughest regulatory requirements. The practical implementation of burst mode is critical to meet various load characteristics and ensure that output voltage does not go out of regulation during load transients. When QR controllers are used in conjunction with a PFC stage, reducing the quiescent losses of PFC circuitry during standby mode is vital. Startup circuitry presents one of biggest sources of power loss in standby power mode. New controllers with a built-in high-voltage startup feature eliminate these losses.
Resonant topologies provide an even higher efficiency compared to the standard flyback and QR-flyback topologies in most applications. The many flavors of resonant topologies include LLC resonant topology, widely used in applications where full-load efficiency needs to be more than 90%. LLC resonant topology achieves low losses due to its zero-voltage switching characteristics. However, this efficiency advantage comes at a price: more components, tight transformer tolerances, and the drawback of working over a restricted range of input. Controllers for this topology must sense the standby mode and be able to operate well out of resonance to reduce switching losses and operate in burst mode to improve standby efficiency.
As energy standards continue to evolve, so will the challenges. However, it also provides opportunities for industry to use technology in new ways, fostering innovation and design creativity.
About the Author
Vipin Bothra is application manager for power applications at STMicroelectronics in Schaumburg, IL, U.S. If you would like to contact Bothra, please e-mail firstname.lastname@example.org.