Power-Supply ICs Propel Switchers Into Non-Isolated Applications

Offline switching ICs have reached such high levels of integration that switching power supplies can now supplant low-cost linear supplies, even in low-power applications requiring just a few watts. However, most of these chips were developed with isolated power supplies in mind, leaving many low-power non-isolated power-supply applications to rely on linear or passive designs with all of their attendant drawbacks.

Power Integrations now addresses the non-isolated applications with its LinkSwitch-TN family of offline switcher ICs. Switching supplies built around LinkSwitch-TN ICs will be the first to achieve system costs on par with linear and cap-dropper power supplies. Yet the cost reduction won't diminish the LinkSwitch-TN-based supplies' capabilities, providing all of the performance advantages that switchers offer over linears and capacitive (cap) droppers.

Higher efficiency and output-current capability are two of the more compelling reasons for equipment makers to migrate to switchers. These benefits will help OEMs meet regulatory requirements for reduced power consumption while satisfying demands for greater current. But other switcher advantages, such as smaller size, wider input-voltage range, and fault protection, are notable as well.

Designed to implement buck, buck-boost, and flyback topologies, LinkSwitch-TN chips integrate a 700-V power MOSFET with the required power-supply controller functions. These include an oscillator, on-off control circuitry, a high-voltage switched current source, frequency jittering for electromagnetic-interference (EMI) reduction, cycle-by-cycle current limiting, and thermal shutdown.

The chips derive startup and operating power from the input voltage on the drain, eliminating the need for a separate bias supply in buck or flyback converters. As a result, a buck converter can be built with fewer than 10 external passives (see the figure).

LinkSwitch-TN's internal fixed switching frequency was set at 66 kHz. As a result, up to 120 mA of output current is possible using a standard, off-the-shelf 1-mH inductor. The choice of switching frequency balances the need for a low-cost inductor (lower frequencies demand a larger, more expensive inductor) versus the desire to minimize EMI (which increases at higher frequencies).

The LinkSwitch-TN ICs target a variety of applications, including white good appliances, utility meters, industrial controls, and LED lighting systems. In such applications, low-cost, non-isolated cap droppers and linear supplies typically are employed to power equipment-control circuitry.

The simplest supply design, the cap dropper, steps down a line input such as 120 V ac to a board-level voltage such as 12 V. The cap dropper's output is then rectified and may be regulated using a simple zener diode or a linear-regulator IC. Cap droppers are most practical when the current output is limited to about 100 mA. Above this level, cap droppers cease to be cost-effective.

This output current limitation becomes a concern as equipment designers add microcontrollers, displays, and other functions to their products, increasing their power-supply needs beyond what cap droppers can easily deliver. By delivering as much as 360 mA of output, LinkSwitch-TN-based supplies pave the way for added functionality in the target applications. Because the LinkSwitch-TN generates a regulated output, no zeners or linear regulators are required as with cap droppers.

Switchers built around the new chips will also improve efficiency. While a cap-dropper supply typically achieves about 20% efficiency, LinkSwitch-TN switchers will achieve efficiencies ranging from 65% to 85%. The higher efficiency will enable equipment makers to reduce standby current to the levels required for Energy Star and Blue Angel certifications.

In a self-powered buck topology with optocoupler feedback, the chips consume just 50 mW of no-load power when operating from a 115-V ac source. That value rises to 80 mW when the source is 230 V ac. Even better performance is achieved in a flyback topology with external bias circuitry. In this case, no-load power consumption is 7 or 12 mW, respectively, with a 115- or 230-V ac source.

As with other switching power supplies, those based on LinkSwitch-TN can operate from a universal input voltage range (85 to 265 V ac). So unlike cap-dropper circuits and linear supplies, which require multiple versions to satisfy different input voltage requirements, LinkSwitch-TN supplies will be suitable—using a single design—for all of the line voltages encountered globally.

In terms of overall supply size, the new chips also have an edge. In cap droppers, X2 safety-rated capacitors are required for 230-V ac input systems. These are large through-hole parts. Yet the LinkSwitch-TN chips, which come in DIP or surface-mount packages, can form an all-surface-mount power-supply design (see the table, "LinkSwitch-TN-Based Switcher Versus Cap-Dropper Power Supply").

The LinkSwitch-TN-based switching power-supply designs resemble existing solutions like Power Integrations' TinySwitch. Like TinySwitch, LinkSwitch-TN employs an on-off control scheme in which the on-time for each switching cycle is set by the value of the external inductor, the chip's current limit, and the value of the high-voltage dc input bus. Output regulation is achieved by skipping switching cycles in response to an on/off signal applied to the Feedback pin (Enable on TinySwitch). Contrast this with the standard pulse-width-modulation (PWM) control, where the duty-cycle of the switching frequency is altered in response to load changes.

But unlike TinySwitch, the logic of the Feedback pin has been inverted in the LinkSwitch-TN. And, this pin also functions as the voltage reference. Consequently, a direct feedback scheme can be employed in the buck or buck-boost topologies. This innovation eliminates the external optocoupler and zener voltage reference required by TinySwitch while still achieving ±10% voltage regulation at the rated output current.

Eliminating the optocoupler and zener saves cost, making the LinkSwitch-TN switcher design affordable in new applications. At the same time, it overcomes the high-temperature and humidity problems associated with optocouplers.

LinkSwitch-TN's auto-restart mode protects the chips against output overloads and short-circuits, as well as open-loop conditions. In addition, jittering of the switching frequency reduces EMI by approximately 10 dB, reducing external filtering requirements to a simple resistor in many cases.

The LinkSwitch-TN family includes three models—LNK304, LNK305, and LNK306—which are rated for output currents of 170, 280, and 360 mA, respectively, when operated in a continuous conduction mode. Each model is offered in an eight-pin DIP (LNK30xP) or surface-mount package (LNK30xG). In lots of 10,000, the LNK304P costs $0.42.