Electronic Design

AC-DC Switcher IC Yanks The Plug On Linears

A primary-based CC/CV design slashes parts count to achieve cost parity with 3- to 4-W linear power supplies.

Over the past few decades, switching power supplies have become an increasingly popular alternative to linear power supplies. Also commonly called switchers, they offer higher efficiency, smaller size, and lighter weight than linears. These advantages were initially exploited to build power supplies in the higher-power ranges, where the size and efficiency benefits of the switchers were most dramatic.

But over time, improvements in switching power-supply design have boosted their performance even while reducing their cost. Those gains enabled switching power supplies to make steady inroads against linears at lower and lower power levels. Today, switchers dominate most of the power-supply spectrum. Only at the very low-power end of the range (up to a few watts of output) are very inexpensive linear transformers still popular. Despite a bulkiness from which their nickname "bricks" comes, these unregulated linears are still widely used as ac-dc adapters and battery chargers, as well as standby and auxiliary power supplies.

With the introduction of its new ac-dc switching power-supply IC, Power Integrations hopes to change that situation by knocking linear power supplies out of that last low-power niche. The LinkSwitch family of constant-current/constant-voltage (CC/CV) switching power-supply ICs lets designers construct switchers that are cost-competitive with linear supplies and have outputs up to about 3 W. Such linears are commonly used to power applications like cellular and cordless phones, MP3 players, appliances, and even industrial systems.

Within a monolithic device, the LinkSwitch IC (LNK501) integrates a 700-V power MOSFET, pulse-width modulation (PWM) control, high-voltage startup, current limit, thermal shutdown, and fault protection circuitry. This chip makes it possible to build a switching power supply with just 14 components (Fig. 1). In comparison, existing switching power-supply designs may require two to four times that number of parts.

Of course, the comparison that counts is against linears, which typically contain eight to 10 parts and cost less than $1 (parts and assembly) to manufacture. For example, when LinkSwitch is used to build a 2.75-W, 5.5-V output switcher, the price for all external components excluding the IC comes to just under 32 cents when parts are purchased in million-piece volumes.

Although the LinkSwitch design requires a few more pieces than a linear, the cost of manufacturing an adapter or charger via LinkSwitch falls within a few cents of a linear. While this achievement alone is remarkable, the price of a LinkSwitch-based power supply will be under a linear if the costs of packaging and shipping the power supply are considered. That's because a LinkSwitch-based supply will be smaller and up to 75% lighter than a linear.

With cost removed as a barrier, LinkSwitch promises to bring the benefits of switching power supplies to the numerous applications still reliant on bricks. In many cases, the size reduction of the switcher will be its greatest attraction. Some linears are larger than the electronic devices they power. Also, LinkSwitch allows the switcher to operate off of a universal input voltage range (85 to 265 V ac), a feature not possible with linears.

Universal input voltage allows equipment makers to stock one adapter (not counting the ac plug in some instances) for use anywhere in the world. A LinkSwitch switcher is rated for 3-W output with operation over the universal input voltage range. This value can be increased to 4 W, though, when the input voltage is restricted to 230 V ac ±15%.

LinkSwitch also offers higher efficiency. When measured over the full, 85- to 265-V ac input range, the full-load efficiency of a LinkSwitch-based adapter is typically greater than 70% (Fig. 2). That's more than double the efficiency of a linear. Under the no-load condition, the LinkSwitch switcher would consume less than 300 mW, in accordance with the pending requirements of the European Union No-Load Consumption Standards for external power supplies. No-load power for LinkSwitch is 196 mW at 115 V ac and 240 mW at 230 V ac. Even at the high-line condition of 265 V ac, the LinkSwitch draws only 260 mW. In contrast, no-load power consumption for a 3- to 4-W linear is normally 500 to 1000 mW at 230 V ac.

Meanwhile, typical efficiency under standby operating conditions (10% load) is 60%, putting LinkSwitch-based adapters in harmony with the EPA's Energy Star requirements for cordless telephones and answering machines. By 2004, these consumer products must lower standby power consumption to levels ranging from less than 1 W to below 2 W, depending on product category, to garner the Energy Star rating. LinkSwitch also satisfies Blue Angel requirements.

In terms of regulation, comparisons between a LinkSwitch design and a linear are difficult because the differences are so dramatic. While the switcher operates from 85 to 265 V ac with acceptable line regulation, the linear simply can't maintain any semblance of regulation over such a wide range. Even over a limited, upper-voltage range such as 190 to 265 V ac, several models of linears may be required to satisfy regulation requirements.

Line regulation for a LinkSwitch design would be about ±3.5% over the 85- to 265-V ac range, while a linear might exhibit ±20% regulation over 190 to 265 V ac. A look at load regulation shows even worse results for linears (Fig. 3). In addition to differences in load regulation, the LinkSwitch-based switcher has a built-in current limit that kicks in at around 500 mA.

When developing LinkSwitch, Power Integrations relied on experience gained with its TOPSwitch and Tiny-Switch series of switching power-supply ICs. These products use more-standard flyback circuit configurations. Although they provided low-component-count solutions compared to other switchers in the 0- to 250-W range, even their designs couldn't match the cost of a 3- to 4-W linear-transformer-based power supply. So the company made a simple but fundamental modification to the standard flyback circuit configuration. It further simplified the design and reduced the number of external components needed.

In a standard flyback, the control chip is positioned in the low-side section of the primary, where it derives its feedback signal from a third winding on the isolation transformer, or from an opto-coupler along with an associated current-sense resistor and other passives. Whether or not an optocoupler is used, the third transformer winding is needed to efficiently provide low-voltage supply current to the control chip.

With LinkSwitch, the IC was moved to a high-side position within the voltage-clamp circuit. Now the clamp circuit has the additional functions of supplying power to the chip and feedback from the reflected output voltage across the main transformer primary winding. Clamp energy that was normally wasted now supplies the IC operating current, helping to improve system efficiency. Eliminating the third winding on the transformer allows the use of a simpler, less-expensive transformer, while reducing the number of external parts. The change sacrifices some of the switcher's regulation, but that regulation still exceeds that of the unregulated linear. At the same time, the switcher retains the ability to operate from a universal supply voltage.

Conceptually, the change was very simple. Primarily it demanded acceptance of less voltage regulation than was provided by a typical switcher while still satisfying the performance requirements imposed on the targeted linear supplies. However, it also required some changes in the chip's internal control characteristics. A description of LinkSwitch's operation helps to explain the concept.

When VIN is applied at power-up, the control pin capacitor (C3 in Figure 1) is charged through a switched high-voltage current source connected internally between the chip's drain and control pins. The voltage between the control and source pins then rises until it reaches 5.6 V. At that point, the chip's control circuitry begins operating and starts to switch the internal high-voltage MOSFET. From then on, C3 provides power to the chip.

Following power-up, the chip operates in a CC mode. In this stage, the output voltage rises and with it the reflected voltage across the primary transformer winding. As the reflected voltage rises, the feedback control current, IC, in-creases. IC, in turn, determines the chip's internal current limit, ILIM (Fig. 4). That value is at a maximum when it reaches IDCT. However, before it hits that limit, it arrives at another value, IDCS. The internal current limit characteristic of the chip maintains an approximately constant power-supply-output current until the control current reaches the IDCS value.

At that point, the chip switches to a CV mode of operation. Once IC exceeds IDCS, the chip begins to control the duty cycle of its switching frequency to maintain an almost constant output voltage. When the control current reaches IDCS, duty cycle is about 77%. As the output voltage rises further in response to changes in the load, IC increases until ILIM equals IDCT (corresponding to a 30% duty cycle). While it does so, the chip reduces its duty cycle to maintain a CV output. When the duty cycle falls below 3.8 %, the switching frequency decreases from 42 kHz to 30 kHz to lower energy consumption under light loads.

The LinkSwitch design differs from the TOPSwitch implementation in a few ways. The current limit in LinkSwitch is adjusted as a function of IC to ensure that the power supply's output current remains approximately constant as the output voltage drops. Meanwhile, the gain of duty-cycle reduction (the slope of duty cycle versus IC) has been increased in LinkSwitch to boost the chip's line and load regulation.

In addition, the gain of current limit reduction has been chosen to provide an approximately constant output current as the output voltage drops. This ensures that the peak output current and the short-circuit current are well controlled, granting a further advantage over linears.

The LinkSwitch design eliminates the need for secondary-side voltage or current feedback, while maintaining regulation that's superior to linear power supplies. For applications that require tighter load regulation, LinkSwitch performance can be improved with optocoupler feedback. Other features include a low, 42-kHz switching frequency that simplifies the design of the external EMI filter (C1, L1, and C2 in Figure 1). The chips come in eight-pin DIP (LNK501P) and SMD (LNK501G) packages. The SMD version also is available in a tape-and-reel format.

Price & Availability

In quantities of 1000, unit pricing starts at $0.65. Samples are available from stock and production quantities are available within six weeks ARO.

Power Integrations Inc., 5245 Hellyer Ave., San Jose, CA 95138; Steve Micelli (408) 414-8821; www.powerint.com.

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