Switching power supply chips from firms such as Power Integrations Inc. (San Jose, Calif.), Fairchild Semiconductor International (San Jose, Calif.) and STMicroelectronics Inc. (Lexington, Mass.) offer significant reductions in power drain, especially when products are in standby or no-load mode. Vendors say such chips are increasingly significant because of the proliferation of energy-gulping electronic devices in homes and businesses.
Power Integrations' LinkSwitch series of switching power conversion ICs is intended to replace low-power linear transformer designs in adapters and battery chargers rated at 3 W and below (see the figure). LinkSwitch, along with Power Integrations' TinySwitch-II and TOPSwitch-GX product families, allows the firm to address power supplies from 0 W to 250 W, or 90% of all ac-dc power supplies currently manufactured, according to Rich Fassler, director of product marketing.
Compared with linear transformer-based power supply designs, LinkSwitch offers smaller size, lighter weight, better energy efficiency and superior performance, Fassler says. He adds, “The lighter weight can easily translate to a significant cost saving if, for example, a transformer manufacturer has to air freight products to meet delivery schedules.”
LinkSwitch chips combine a power MOSFET, PWM controller, high-voltage start-up, current limit and thermal shutdown circuitry on a single CMOS chip. With as few as 14 components, a LinkSwitch chip can implement a fault-protected, universal-input, constant voltage/constant current (CV/CC) switching power supply that meets worldwide energy-efficiency standards.
Leveraging technology developed for Power Integrations' TOPSwitch devices for higher-power applications, LinkSwitch's CV/CC design combines the primary clamp, feedback, IC supply and loop compensation functions, eliminating up to 20 external components typically found in alternative switcher designs. In all, the technology can eliminate as many as 50 external components, according to Fassler, and can be used without the opto-coupler typically required on the secondary side of the transformer.
Power Integrations' chips provide overload protection under output short-circuit conditions, and feature thermal and current limit protection, auto-restart for short-circuit and open-loop fault protection, and EcoSmart technology to reduce power supply energy consumption during standby and no-load conditions. These features enable engineers to meet all current or proposed government energy guidelines.
“Energy consumption is a growing problem,” notes Fassler. “Most homes in the United States have more than a dozen pieces of electronic equipment plugged into power strips, with power supplies based on linear transformer designs. It's estimated that as much as 10% of the energy we produce is wasted while products sit in standby or no-load conditions. LinkSwitch can reduce that energy waste by up to 90%.”
Fairchild's RS6X1220RT power switch is for dc-dc forward or flyback power supply applications as well as for VoIP (voice over Internet protocol) phones, digital feature phones, industrial power supplies and telecom central office and PBX power supplies.
Offered in TO-220F-5L and D2-PAK-5L packages, Fairchild's chip integrates an avalanche-rated SenseFET (200-V minimum breakdown rating) with a current-mode PWM IC in a design that minimizes the need for external components, simplifies designs and lowers costs. An integrated 300-kHz oscillator improves efficiency and allows the use of smaller transformers, according to Bob Conrad, senior vice president and co-general manager of Fairchild's IC group.
The power switch features overvoltage, overload, cycle-by-cycle current limit and thermal shutdown protection. It has temperature-compensated, precision current sources for loop compensation and fault protection circuitry, and also includes line-undervoltage detection, remote sleep on/off, undervoltage lock out, and an optimized blanking and gate turn-on/turn-off driver.
Conrad says the power switch can replace discrete MOSFET/controller or ringing choke converter (RCC) switching converter solutions, occupying less board space and offering better reliability.
STMicroelectronics offers two single-chip off-line flyback converters for ac-dc power conversion, the VIPer12A and VIP22A. Both combine PWM circuitry with a vertical power MOSFET. The VIPer12A is designed for off-line battery charger applications from 1 W to 10 W (1 W to 5 W for wide range 115-V to 230-V input). Its control circuit accepts auxiliary supply voltages from 8 V to 40 V. Features include the ability to start-up from zero auxiliary voltage using an automatically controlled current source from the line voltage, burst-mode operation for light loads and automatic undervoltage lockout with hysteresis.
The VIP22A features current-mode control, a fixed 60-KHz switching frequency and an 8-V to 40-V VDD voltage range. It can deliver up to 12 W (20 W for the European line voltage) in off-line battery charger designs, isolated or nonisolated standby power supplies for televisions and video monitors, and auxiliary switch-mode power supplies for motor controls and other applications. The chip provides an automatic burst mode under low-load conditions; auxiliary undervoltage lockout with hysteresis; a built-in high-voltage start-up current source; and overtemperature, overcurrent and overvoltage protection with autostart.
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