Regulators transition from linear to switch-mode

Regulators transition from linear to switch-mode

A new breed of switchers solves the objections to using switch-mode converters and paves the way for replacement of LDOs

BY DOUG LOPATA
Enpirion
Bridgewater, NJ
http://www.enpirion.com

The combination of today’s energy prices and an ethical obligation are driving businesses to implement “green” initiatives aimed at reducing energy consumption. Companies who make electronic devices have an additional challenge to develop products that deliver longer battery life and improved efficiency for today’s budget conscious and conservation-savvy customers. Whether designing a hand-held device or a rack of servers, engineers need to find ways to reduce power consumption without sacrificing performance.

Power supply — small part, big impact

The technologies used to implement a product’s power supply can have a significant impact on its overall efficiency. According to Ecos Consulting, more than 3.6 billion power supplies are now in use in the United States. The firm estimates that between 1% and 2% of all U.S. electricity consumption use could be saved by more efficient power supplies. That translates to 35 billion to 70 billion kWh per year, worth $3 billion to $6 billion annually.

Dc/dc power distribution techniques — as opposed to ac/dc or dc/ac conversion models — provide power efficiencies of 80% to 90%. But not all direct dc power systems are created equal. Linear regulators have long been the mainstay of dc/dc power supplies for electronics developers because they are inexpensive and offer a small footprint, low part count, low noise, and high ripple rejection. But linear low-drop-out (LDO) regulators dissipate power in the form of heat, and therefore are not an efficient option for designers looking to maximize battery life or develop products that meet the strict requirements of Energy Star certification.

Switch-mode dc/dc converters, on the other hand, are far more energy efficient, but have classically required a bigger footprint and higher part count, are more costly, and are prone to conducted and radiated EMI.

Linear power conversion (see Fig. 1 ) is inherently lossy, using resistive elements to affect regulated output: η (efficiency) Vout/Vin. Conversely, switch mode conversion (see Fig. 2 ) leverages energy storage elements to affect a low-loss conversion: η » 1 – (conduction + switching + inductor losses).

VBAT = 3.7 V nom

BIN_BB = 1.2 V

Load Current = 600 mA

Power delivered to load =

600 mA * 1.2 V = 720 mW

Power converted to heat =

720 mW * ((3.7/1.2) 1) =

1,500 mW

Total power consumed =

720 mW + 1,500 mW = 2,200 mW

32% goes to work, 68% goes to heating user hand and ear

Fig. 1. Linear power conversion is shown example for a mobile device.

VBAT = 3.7 V nom

BIN_BB = 1.2 V

Load Current = 600 mA

Converter efficiency = 90%

Power delivered to load =

600 mA * 1.2 V = 720 mW

Power converted to heat =

720 mW * ((1/0.9) 1) =

80 mW

Total power consumed =

720 mW + 80 mW = 800 mW

90% goes to work, 10% goes to heating user hand and ear

Fig. 2. Switch-mode power supply conversion example for a mobile device.

The aggregate energy wasted by linear conversion has crossed a threshold of intolerability for electronics developers, fueling a drive to replace linear with switch-mode conversion in key power conversion functions. Fortunately, a new breed of switchers has emerged that solve the noise, size, cost and other historical objections to the use of switch-mode converters, paving the way for across-the-board replacement of inherently inefficient LDOs.

Enpirion’s PwrSoC (power supply on a chip), for example, is an ultra-integrated dc/dc converter, and the first to feature an integrated inductor. Enpirion’s approach shrinks the converter’s footprint by capitalizing on semiconductor geometry advances that enable a 10x increase in switching-frequency, as well as advanced packaging approaches. The device integrates the necessary PWM controller, power FETs, compensation circuitry, plus the inductor itself into a single IC package. The integration also reduces the propagation of switching noise often associated with switch-mode power conversion.

Fig. 3. Typical switch-mode dc/dc converter shows controller and protection circuitry.

Engineers now have a switch-mode power supply option that offers a small footprint, low parts count, low noise, and ease of design, making it a near drop-in replacement for inefficient linear converters. In many markets, the improved efficiency achieved by “switching” to a switch-mode converter can help a company realize significant financial and energy savings.

Battery-draining mobile devices

The most pervasive digital device in the world, the cellular telephone, has evolved to become the most pervasive computing platform. The 50 mA power demands of early cell phones have given way to feature-rich devices that consume an order of magnitude more current, arguably making power conservation today’s most crucial handset-design issue. Increasingly complex RF modulation schemes, high-speed data protocols, fine-line silicon geometries, and decreasing core voltages also drive an obvious need for improved power efficiencies in mobile devices.

To support today’s higher performance data standards and applications, mobile device processors have undergone a remarkable transformation in clock speeds and silicon integration. While processors running at 26 to 52 MHz in 0.2 µm geometries once sufficed for GSM phones, today’s 130-, 90-, and 65-nm baseband processors are clocked at 150 to 300 MHz — faster clock speeds and higher gate counts equal more power draw as dictated by P = CV2f/2 and mA/MHz/Mgate metrics.

Shrinking silicon geometries are accompanied by lower core voltages that increase the voltage drop between power source and powered circuitry. Lithium-ion batteries, one of the most popular types of battery for cellular phones, supply 4.2 V max and 3.7 V on average, but core processors in the majority of portable devices operate off much lower voltages, ranging from 1.6 to 1.1 V. The power discarded as heat in that resistive drop is a waste of precious battery power.

Lightweight, easy to design-in, and relatively cheap, LDOs have been a long-standing choice for battery powered devices. But their poor linear conversion efficiency is increasingly incongruous with the needs of mobile device developers. These devices literally throw away power and virtually demand that designers turn to switch-mode power converters.

Switch-mode converters are highly efficient, using nonresistive (ideally) inductive or capacitive elements to decrease wasted power. While switch-mode converters have been historically more costly, the power output to cost differential between switch-mode converters and linear regulators has become nearly negligible.

Power-hungry data centers

Data centers are critical in facilitating the modern economy, with public and private sector organizations increasingly relying on data management to fulfill their missions. As our society shifts from a paper-based economy to a digital one, demands for information storage have increased significantly. But, like the paper mills that preceded them, data centers come at a high cost in terms of energy dependence and environmental damage.

The EPA reports that U.S. data centers used 61 billion kWh of electricity in 2006, representing 1.5% of all U.S. electricity consumption, and double the amount consumed in 2000. Based on current trends, the EPA projects that energy consumed by data centers will continue to grow by 12% per year.

This is a problem of national concern and a challenge for data center managers trying to maintain a budget and fulfill corporate green initiatives. A typical rack of servers draws about 20 kW of electricity power – equivalent to a fleet of 70 hybrid cars, and costing more than $17,000 annually (Lawrence Berkeley National Laboratory).

In a data center, less than 50% of the electricity goes to the servers’ CPUs, and some of that is converted to heat. The rest of the electricity is used to run other components of the servers, the center’s cooling system, or is lost in power supplies, uninterruptible power supplies and switches.

Linear regulators, which dissipate heat and waste energy, are a step backward for today’s forward-thinking engineers. Switch-mode power supplies could save thousands of dollars in annual electricity costs for a typical data center.

Any device that dissipates power as heat is unacceptable for today’s energy-efficient ethos. Economics and eco-ethics are the catalysts for finding the most-energy-efficient power conversion techniques possible.

New switch-mode converters on the market today provide designers with the performance of a high-efficiency switching dc/dc converter with the size, simplicity, low noise and low part-count normally associated with a linear regulator. These innovative advances in power management pave the way for across-the-board replacement of linear regulators with switch-mode converters. ■