Electronic Design

Power Supply Controller Keeps Efficiency High Across All Loads

Using digital regulation, this power controller yields high performance at low loads with unconditional stability.

Traditional analog and pulse-width-modulated (PWM) power-conversion controllers have adequately served the needs of power-supply designers for decades. Lately, though, they seem to be approaching a "brick" wall. To gain just a 1% improvement in conversion efficiency requires substantial improvement in topology, associated components, and circuit design. Pressured by the demands of the upcoming systems and market needs, power-supply designers now seek more flexibility, robustness, and superior performance across all line and load conditions—and they want all of these enhancements without any additional cost.

Thanks to digital technology from fabless semiconductor startup iWatt Inc., a new breed of power-conversion controllers may accomplish exactly that. Plus, they bring a new level of flexibility and programmability to power supplies not feasible with traditional PWM and other analog methods. These controllers promise to dramatically improve the overall cost and performance of future ac-dc and dc-dc power-converter solutions, both isolated and direct coupled.

The company's developers crafted a proprietary regulation technique known as pulseTrain. Applying this novel way of controlling power, iWatt designers have readied a new line of digital switch-mode power-supply (SMPS) controllers. The company expects them to overcome the drawbacks of the older technology, while paving the path for a new digital trend in power-supply designs.

To demonstrate the efficiency of the new digital controller, measurements have shown an efficiency of 80% to 89% over a full range of line and load conditions, for a 60-W single-stage, single-switch ac-dc converter with active power-factor-correction (PFC) circuitry (Fig. 1). "Unlike conventional PWM, the digital controller can achieve in excess of 85% efficiency across all line and load variations," says Deepak Savadatti, iWatt's senior marketing manager.

This digital controller offers many advantages. Some key benefits include no optocoupler or feedback-loop compensation required due to primary-only feedback control, and the ability to drive any power-supply topology.

With a conventional PWM controller power supply, efficiency drops dramatically at low loads, as does stability. By eliminating the costly optocoupler and associated components, the new pulseTrain technique provides cheaper and slimmer supplies. Unlike an analog approach, the digital approach also offers a built-in active PFC capability in ac-dc applications of up to 250 W. "Only a few binary comparators, a few gates, and a few pins are needed with this digital approach to realize low-cost energy-compliant converters of all popular topologies," asserts Savadatti.

The pulseTrain technique features a pair of pulse generators—a power pulse generator and a sense pulse generator. While the power pulses in this scheme are deployed to affect the transfer of energy across the transformer to the load, sense pulses are employed to monitor the voltage at the load. The optimizer sets the on- and off-time of these pulses. Using the primary side feedback, the pulse-rate controller (PRC) regulates the voltage by gating fixed-width and fixed-period power pulses on a pulse-by-pulse basis (Fig. 2).

In contrast to traditional analog PWM or pulse-frequency-modulated (PFM) converters, wherein each power pulse is sized so as to drive the output voltage precisely onto the target, the pulseTrain converters behave like digital "bang-bang" servomechanisms, explains Mark Telefus, iWatt's chief technology officer. At each cycle, the pulseTrain samples a binary error signal to determine whether or not to pulse the output voltage in the direction of the target. As a result, the output voltage limits cycles within a narrow range around the target voltage. The controller parameters determine this limit cycle envelope. For all practical applications, it's smaller than the ripple obtained with traditional analog regulators that employ filtered error signals.

In essence, the inherently digital pulseTrain controls the output voltage through the presence or absence of power pulses. For instance, if the output voltage goes below the set level, power pulses are emitted continuously until the desired level is achieved. Likewise, when the output voltage is higher than the desired level, sense pulses are sent instead of power pulses. A sense pulse has a much shorter on-time and transfers less energy than a power pulse (Fig. 3).

Interestingly, this technique also decouples the shape of the pulses from the process of regulation. Thus, the on-time and off-time can be set independently to achieve a variety of system optimizations. Consequently, the off-time can be set to ensure that the switch in a flyback converter is turned on when the secondary current drops to zero, assuring critically discontinuous operation. On the other hand, the on-time can be set to ensure a constant peak primary current. Together, these optimizations yield higher efficiency response over the entire range of line and load variations.

Digital Controller Chips
By implementing this technique on silicon and integrating it with other functions like control logic, a waveform analyzer, a multiplexer, a voltage regulator, and a driver, iWatt has readied three different digital controller chips for a variety of ac-dc and dc-dc converter applications. The iW2101 is an isolated dc-dc controller chip, the iW2201 is an ac-dc controller IC, and the iW2202 is a monolithic ac-dc controller with built-in active PFC circuitry.

Unlike other PFC solutions, which use complex circuitry along with a conventional PWM controller, the iW2202 offers both PFC and voltage regulation through primary-only feedback in a single-stage, single-switch topology (Fig. 4). While other PFC methods result in excess of 1000 V across the bulk capacitors, the iW2202 solution uses less than 400 V in the same application. That translates into lower stress and higher reliability for the components, or higher system performance at lower cost.

Internal Waveform Analysis
The controller's internal waveform analyzer is a novel way to determine crucial circuit parameters. In this scheme, the reflected voltage of the flyback transformer is measured at selected times to reveal the secondary voltage, the transformer reset time, and the zero-voltage switching time. These results are applied immediately on either the current cycle or the next cycle to achieve the desired performance. Therefore, the controller permits the voltage, current, and PFC to be controlled independently.

Measurements indicate that a single-stage active PFC power converter built with the iW2202 achieves a PFC of 0.98 for an input of 120 V ac, and a dc output of 19.0 V at 3.0 A of output current. The combination of high efficiency and power-factor performance will enable a designer to meet the harmonic distortion specifications set by EN61000-3-2 and no-load power consumption standard set by Blue Angel.

Output regulation for the new design is less than ±1% over line and load variations. Also, the dynamic response is nearly five times faster than the conventional approach, claims iWatt. The iW2202 is rated to support universal line voltages from 85 to 270 V ac, 50 to 60 Hz.

The company also is working on extending its new technology to programmable switching regulators (PSRs). The multi-output PSRs that iWatt is working on will incorporate all of the power-conversion and management functions to power mobile devices like PDAs, digital cameras, and other handheld products. In addition, they will be able to talk to the host processor in the system. The PSRs should be unwrapped sometime next year.

Price & Availability
Implemented in 0.6-µm CMOS, the first three integrated digital SMPS controllers to be released for sampling include the iW2101 isolated dc-dc controller, the iW2201 ac-dc controller, and the iW2202 ac-dc controller with PFC. All three come in eight-pin small-outline IC packages. The iW2202 is slated to go into production in January, with the other two expected to follow soon after. In 10,000-unit quantities, the iW2202 is priced at $3.50 each.

iWatt Inc., 90 Albright Way, Los Gatos, CA 95032; (408) 341-0487; fax (408) 341-0455; www.iwatt.com.

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