Critical-conduction-mode interleaved PFC IC and low-loss MOSFET provide high conversion efficiency in power supplies for various types of equipment.

Power supplies are the ubiquitous workhorses that convert AC or time-varying DC power into the stable, low-noise constant-voltage DC power required by the microcomputers and other semiconductor devices in electronic circuits. Over the years their conversion efficiency has steadily risen, yet further improvement is still possible. Today that's an ongoing goal because even a 1-percent gain in conversion efficiency will directly reduce system power consumption, providing important energy savings. Renesas has long contributed to progress in this area, having been one of the first semiconductor producers to supply products such as critical-conduction-mode interleaved PFC ICs and also battery-controller ICs that can determine remaining battery power with a high level of precision. Among our current R&D projects, we are working to develop improved power MOSFETs for power supply designs that are smaller and more efficient. For instance, our product range now includes a 6mm x 6mm DrMOS device that uses SiP technology. These and other developments are highlighted in this article.

Manufacturers of all types of electronic equipment are being constantly challenged by their customers to produce new products offering higher performance and advanced functions. Simultaneously, they are increasingly requested to produce systems that are more energy efficient. Thus, the role of the power supply in these products is steadily becoming a more important design issue, whether the supply is producing stable DC (direct current) power from electricity from the AC (alternating current) mains or from a battery.

Major efforts are being made to boost the efficiency of the power conversion process, which is already quite high in many cases. Achieving greater efficiency in the power conversion process decreases power loss, so less heat has to be dissipated. It also decreases the load on the electricity generating station, battery, or other power source. Additionally, of course, it lengthens the operating time of systems that run on batteries.

One of the factors driving the need for power supplies that are more efficient is the effect on end users of international energy efficiency indices such as the Energy Star program, which require extremely high electrical power conversion efficiency. Equipment manufacturers have discovered that better index numbers help boost sales. Thus PC and home appliance manufacturers in particular strive to achieve excellent index scores. With power supply conversion efficiencies already in the range of 80 percent to 90 percent or more, obtaining even a 1-percent improvement is a significant result. The latest power MOSFETs and future products in development are key solutions for making valuable gains in this area.

Another trend affecting power supply design is the move to “intelligent batteries,” sometimes also called “smart batteries.” Rechargeable batteries with advanced functions have become essential features of some types of products, offering, for example, the ability to perform detailed management of remaining battery capacity and provide status information to the system CPU. Protection circuits that prevent fires and other problems by monitoring the current during charging and discharging are other key features of intelligent batteries.

Power supplies with an AC input convert that AC to DC outputs. Typically such power supplies consist of a rectifier circuit, power-factor-correction (PFC) circuit, pulse-width-modulation (PWM) circuit, and a transformer. Of these, the PFC circuit has been the focus of considerable R&D activity. This component passes the electric current through an external inductor to shape the current into a smooth sinusoidal waveform in order to improve the supply's efficiency and minimize the higher-order harmonic noise being generated.

In a power supply design that doesn't have a PFC circuit the current flowing from the rectifier circuit to the PWM circuit has a pulse waveform with a very high peak current that generates significant harmonic noise. If the electric utility company and electronic equipment manufacturer do not design their equipment and circuits to handle this peak current, failures will occur. Achieving better power factor correction is an important way to enhance the reliability of infrastructure equipment and decrease energy consumption.

Renesas has addressed this issue directly, becoming the first company to produce a PFC IC that uses the critical-conduction-mode interleave method (see Figure 1). Critical-conduction-mode interleave operation works by alternating between two inductor circuits. This design approach delivers twice the power of methods that use a single inductor circuit continuously (critical-conduction-single method). Critical-conduction-mode interleave also helps make power supplies thinner by limiting the ripple on the output current to a low level, allowing the use of a smaller smoothing capacitor (see Figure 2). For example, a circuit board that uses the R2A20112 critical-conduction-mode interleave PFC IC can be made 33 percent smaller in area and 40 percent lower in height than designs that use previous-generation PFC ICs.

PFC efficiency typically falls away at light loads of 40 percent or less, so Renesas is developing technology to minimize this problem. We have also developed a "slave drop" technique that disconnects one of the inductors and a "load tracing boost" technique that reduces the voltage step-up in the PFC. Testing has demonstrated that these methods can provide a PFC efficiency improvement of 2 to 3 percent.

Power MOSFET are key components in power supplies of all types, and Renesas has had great success in manufacturing these critical discrete components for world markets. We primarily offer devices with voltage ratings of 100V or less for DC/DC power supplies. Our products are also used as charge/discharge switches in battery packs, as switches for voltage regulator modules (VRMs), and as synchronous rectifiers in AC/DC and DC/DC power supplies. We have performed a extensive work in finding ways to reduce losses in power MOSFETs and thereby save energy.

Losses in power MOSFETs can be broadly divided into three categories: on-state losses, drive losses, and switching losses. On-state losses have been improved by reducing the on resistance, drive losses have been decreased by reducing the gate capacitance, and switching losses have been cut by reducing the capacitance between the gate and drain. As a measure of our success, the newest ? tenth-generation ? Renesas power MOSFETs are now among the best in the industry in terms of the FOM performance indicator, a design factor determined by the on resistance (R _on ) times the total gate charge (Q _g ); that is, FOM = R _on x Q _g .

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The small R2J20651NP DrMOS (integrated driver-MOSFET) device is an industry-leading product. It's the first integrated component that combines two power MOSFETs with a driver IC in a single small package: 6mm x 6mm. DrMOS is a technical specification proposed by Intel for the ICs used in the power supplies (VRM, DC/DC converters, and similar circuits) for CPU cores, DDR (double-data-rate) memory, and other components.

The R2J20651NP complies with the latest revision of the DrMOS specification (Revision 3.0). It uses our latest power chips for both the high-side and low-side MOSFETs. These advanced switches and the optimized circuit design of the device allows high efficiency to be achieved. Maximum efficiency is 96.5 percent in a DC/DC converter (5V input, 1.8V output) for DDR memory, for example (see Figure 3).

By building on the leadership demonstrated in our very successful first- and second-generation of DrMOS- compliant products, the new model inherits high heat dissipation characteristics of proven effectiveness. Moreover, it integrates temperature-detection circuit in the driver IC that aids the development of power supplies with enhanced safety features.

Power supplies that get their input from the DC output of a battery require an IC to manage the battery cell. The output voltage of that cell varies over time and excessive charging or discharging current can cause severe cell degradation that can lead in some cases to the risk of fire. A standard called the Smart Battery System (SBS) applies to the ICs used to manage battery cells used in notebook PCs. Solutions that comply with SBS embed an electronic circuit in the battery pack so that the notebook PC and battery pack can exchange various types of information. Protection functions keep the battery safe.

SBS ICs typically consist of an analog-front- end (AFE) chip plus a microcomputer, and Renesas has developed two generations of these devices. The latest of these ? the recently introduced devices in the R2J24020F group (see Figure 4) ? use SiP (System-in-Package) technology to combine both these functions in a single package, lowering the component count and shrinking the required circuit board area. (See the related story, Noise-reduction and Cost-saving Benefits Give SiP Technology a Growing Role in New Semiconductor Devices. )

The battery monitoring ICs in the R2J24020F group were developed specifically for incorporation into the lithium-ion battery packs used in notebook PCs. These devices comply with the SBS standard and support the exchange of information between the battery pack and notebook PC ? data that includes the battery type and charging capacity, remaining power, and number of charge and discharge cycles to which the battery has been subjected.

Inside a device in the R2J24020F group, one of the chips is an enhanced analog-front-end (AFE) battery-protection IC that monitors the status of the lithium-ion battery cells and controls the external MOSFET used for charging and discharging. The other chip in the device is a 16-bit microcomputer that handles the communications with the notebook PC (see Figure 4).

Here again, this latest generation of battery monitoring ICs builds on and benefits from the power-saving technology proven in a previous generation of products ? in this case, the devices in the R2J24010F group. Those older, widely used models combined an AFE IC and 8-bit microcomputer in a SiP package. By comparison, the newer-generation R2J24020F devices benefit from the faster processing speed of a 16-bit R8C CPU core that provides both faster instruction execution times and power savings. Also, the resolution of the internal A/D converter has been increased, improving the accuracy of the remaining-battery-power indication function and significantly reducing the processing involved in initial calibration.

A related product, but one that's different from the solutions in the R2J24020F group, is also available. It's a charger IC for use as the charging circuit in lithium-ion battery packs.

Renesas maintains active R&D projects aimed at developing better MOSFETs, battery monitoring ICs and other power supply components. Advances in device design, chip manufacture and packaging will contribute to new products scheduled for future introduction. We will continue to make steady progress on producing products that allow the implementation of reliable power supplies with high levels of conversion efficiency, solutions that contribute to greater energy efficiency in electronic products and the electric power infrastructure. Our researchers are also working to develop the devices needed to obtain compliance with new laws and regulations, like the upcoming law on electrical product safety due to come into effect in Japan in 2011.

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