Leverage Latest Power Devices to Improve Data-center Efficiency

June 18, 2010

Written by Steven Sapp, Ritu Sodhi and Sampat Shekhawat

In a typical data center, less than half of the power consumption goes into the computing function. Consequently, data-center operators are looking at every opportunity to improve power conversion and distribution efficiency, including eliminating the number of conversion stages through the distribution of high voltage direct-current sources.

This article, featured in the April 16, 2010 issue of Green SupplyLine, provides an update on the great strides that have been made during the last decade in the development of power semiconductor devices and products to help reduce wasted energy through improved efficiency at every stage of the power delivery system. 

Read the complete article.

Tags: data center, MOSFET, power semiconductor

Using high-voltage backlight inverters for better LCD TV performance

April 2, 2010

The adaptation of digital broadcasting throughout the world is offering consumers the screen resolution that they have not experienced before with CRT TV.

Digital broadcasting offers higher resolution, no motion blurring, larger panel size and lower price - features that consumers are demanding today. And the LCD TV is the next generation of appliances to benefit from the features of digital broadcasting.

The adoption of high-voltage backlight inverters, instead of the existing low-voltage backlight inverter, is one way to enhance the performance of LCD TVs and reduce total system cost.

The advantage of using a high-voltage backlighting inverter is that it can be connected to the power factor correction (PFC) directly without a DC-DC converter, while the low-voltage backlight inverter requires a DC-DC converter after the PFC stage. In fact, the DC-DC converter used with high-voltage backlight solutions to power other loads needs to process only around 30% of the total LCD TV power, because the typical power consumption of the backlight unit in an LCD TV power is 70% of the total. As a result, the high voltage solution reduces the cost of the transformer and the MOSFET in the DC-DC converter.

A half-bridge topology is typically used in high-voltage inverters. However, because it is difficult to achieve Zero Voltage Switching (ZVS) with the half-bridge for every condition, blocking diodes are usually connected in series with the MOSFETs and fast recovery diodes (FRD) are connected in parallel.

MOSFETs have built-in diodes, but if the half bridge circuit does not operate at the ZVS condition, then the reverse recovery current of the built-in diode flows into the other MOSFET when that MOSFET is turned on. This generates a tremendous amount of heat (through R_DS(ON)), and the resulting higher temperature will make the reverse recovery current in the second MOSFET even greater when it turns off. Then that high reverse recovery current will increase the power dissipation and reverse recovery current in the first MOSFET in the same way.

This positive feedback increases the MOSFET’s temperature until the amount of emitted heat equals the amount of generated heat.

Generally, the built-in diode of a typical MOSFET has a large reverse recovery current, so LCD TV makers use the half bridge solution with blocking diodes in series to prevent the built-in diode from conducting.

A solution to resolve the power consumption design issues in LCD TVs can be the Ultra FRFET™, a MOSFET with a lifetime control process of reverse recovery current.

Some advantages of using the Ultra FRFET are:

• Soft reverse recovery characteristics
• Small reverse recovery current which maintains good EMI performance in LCD TV applications
• Reduced switching loss (both turn on and off) due to low Q_g
• Supports high diode dv/dt immunity (20V/ns) compared to normal MOSFET (4.5V/ns)
• Good reliability at high temperatures and high frequency operation

The Ultra FRFET works well without blocking diodes and FRDs in LCD TV high voltage backlight inverter applications and is suitable for dimmable ballast applications.

Tags: digital broadcasting, Fairchild Semiconductor, high-voltage backlight inverters, LCD TV performance, MOSFETs, reverse recovery current, Ultra FRFET

New Power Semiconductors Cut Data Center Energy

February 18, 2010

Power devices - they’ve been around for what seems like forever and the number keeps increasing exponentially. It may surprise you that there are still many valuable developments in power devices, especially when it comes to reducing the amount of power they consume.“ 

We all know that computers demand an enormous amount of power, but did you know that data centers are one of the biggest users? More than 1% of all the power consumed in the U.S. is used to feed servers. (This is equal to the production of five 1000 MW power plants!) Surprisingly, less than half of the power consumption goes into the computing function in a typical data center. The culprit is power conversion and conditioning, distribution and environmental control.

Fortunately, new developments in high-performance semiconductor components like IGBTs and Super-Junction power MOSFETs for rectification, battery charging and DC/AC inverting can make today’s computers much more energy efficient - and designing in these devices isn’t complicated.

You can learn more in a recent article published in Electronic Products, “New power semiconductors cut data center energy.”

By Steven Sapp, Ritu Sodhi and Sampat Shekhawat

Tags: data center power consumption, environmental control, high-performance IGBTs, power conditioning, power conversion, Power devices, power distribution, Super-Junction power MOSFETs

Getting More Efficiency with Synchronous Rectification MOSFETs

January 26, 2010

Dear Dr. Efficiency, I used a synchronous rectifier MOSFET with large current and low RDSON in a forward converter structure for synchronous rectification (SR) and expected there would be a significant improvement in efficiency.  However nothing happened.  Why was that?  I thought that the SR MOSFET should make a significant contribution to efficiency improvement.  - XM Wang
Hello XM,
You are on the right track, but may need to make some corrections. To deal with the power loss, we have to consider two aspects.

The power loss of semiconductor switches mainly comes from two sources: the conduction loss resulting from the loss on the RDSON that is generated when ID goes through the body diode during dead time, and the switching loss which can be roughly classified as three components: the loss caused from current-voltage cross when the MOSFET switches between turn-on and cut-off, the loss on the parasitic capacitance during switching, and that caused by the trr time of the body diode in the MOSFET.

The above equations show the close relation between the switching loss and the switching frequency. In particular, the loss during dead time mainly depends on the switching frequency, since the dead time is usually fixed. In general, for a given output power, the higher the frequency the more switching loss portion dominance; the lower the frequency the more conduction loss portion dominance.

In a SR structure, the body diode is already turned on by freewheel current before the MOSFET turns on. Since the voltage drop on the body diode is usually less than 2V, the conduction loss is not significant in equation (1).

Equation (2) shows the effect on the switching caused by the parasitic capacitance (Coss) of the MOSFET. This Coss is the equivalent capacitance between drain and source, with a voltage equal to Vds applied. Thus, this portion of loss is proportional to the switching frequency and Vds. You can check if the loss in your design is mainly caused by Coss by paralleling a smaller capacitance between the drain and the source to make the two MOSFETs have almost the same Coss value, then analyze the results by using the equations given above. You’ll get an idea about why the efficiency improvement is not significant.

Finally, in a forward half-bridge structure, the switching loss contributed by trr is also significant since the secondary current is usually kept in continuous current mode. You can check if the loss in your design is caused by trr by paralleling a schottky diode on the SR MOSFET side and then observing whether the efficiency is improved notably. If the efficiency is improved significantly, you can select a MOSFET with shorter trr time for the SR MOSFET.

Now, go back to your question. For MOSFETs within the same series, the lower the RDSON, the larger the value of parasitic capacitance. For example, FDP047N10 from Fairchild Semiconductor has a RDSON of 4.7mohm and a Coss of 1500pF, whereas FDP100N10 has a RDSON of 10mohm, but its Coss is only 710pF. In other words, it is possible that the lower RDSON FDP047N10 may have a larger loss under high switching frequency due to its larger Coss. Other parameters, such as Qg, parasitic body diode in MOSFET, also contribute to the overall power loss, which compromises the efficiency improvement effect of lower Rds. So, apart from the RDSON, which should be as low are also possible, the parasitic characteristic is an important factor in MOSFET selection.

Hope you are satisfied with my explanation. Let’s go and have a cup of coffee.

Tags: conduction loss, efficiency, MOSFET, power loss, semiconductor switches, switching loss, synchronous rectifier

Power Supplies Get Greener

January 18, 2010

Each quarter, Fairchild Semiconductor introduces new products, tips and tools for power and analog applications in the quarterly Benchmarks magazine. The following Benchmarks Volume 1, 2010 “Engineering Connections” article discusses flyback topology as an efficient solution for meeting today’s global demand for lowering power consumption in power supplies.

External power adapters are instrumental for the operation of virtually all small electronic devices. As many as 3.2 billion adapters are currently in use globally, according to industry estimates.

With this worldwide focus on energy savings, regulatory bodies are examining all ways to “go green,” and standards have been developed, specifying higher levels of efficiency for products such as notebook PC power supplies. Flyback topology has proven to be an effective solution, both in terms of cost and technology, for pulse-width modulated (PWM) power conversion in these products. Fairchild has a wide portfolio of PWM controllers  that enhance the performance of flyback converters.

As part of its global focus on energy savings, Fairchild has developed a portfolio of pulse-width modulated (PWM) controllers, which enable notebook power-supply designers to meet the stringent international energy-saving regulations. These include the ENERGY STAR External Power Supply (EPS) version 2.0 requirement that mandates 87 percent average active-mode efficiency to obtain compliance.

Integrated PWM controllers, like the FAN6754, offer designers high-voltage startup to improve energy savings at light load by 25 percent when compared to alternate solutions. It also eliminates external protection circuits by incorporating over-voltage, over-current and over-temperature protection plus brownout and line-compensation functions. Other advantages of Fairchild’s PWM controllers include frequency hopping, which reduces EMI emissions by as much as 5-10 dB, and internal soft start (8ms) to reduce voltage stress on the MOSFET at startup.

Additionally, Fairchild’s PWM controllers incorporate several design features that lower the overall power consumption of notebook adapters, such as a proprietary green-mode function that provides off-time modulation to continuously decrease the switching frequency under light-load conditions. Fairchild’s PWM devices offer a host of robust, accurate protection features built-in to protect the power supply and the load from failure, all without adding external components or circuitry.

Tags: Benchmarks, Energy Efficiency, Fairchild Semiconductor, flyback converter, green, PWM controller

What should we do to achieve high efficiency and power in LED street lamps ?

December 21, 2009

The power requirement of LED street lamps being used in China falls in the range of 100~250W. It is widely agreed that if used properly, these kinds of LED street lamps deliver many advantages. What I want to explain here is the ways to make it possible for these lamps to deliver those advantages. The key factors to be considered are high efficiency, power, reliability and cost-effectiveness.

Some low-power lighting requires PFC, while high-power lamps usually require PFC combined with DC/DC requirements. In China where the AC line voltage is 220V, Boundary-Conduction Mode (BCM) PFC controllers, such as the FAN7530 and the FAN6961, become the ideal choice to maintain a balance between the efficiency and performance-cost ratio. These solutions only need a few components.

Low Rds(on) SupreMOS(TM) MOSFETs, can further decrease switch and conduction loss. When used at the boost output, the HyperFAST 2 high voltage diode family with lower Vf can also lower the conduction loss of the diode itself.

For DC/DC topology, there are many choices such as quasi-resonant (QR), double transistor forward (DTF), active-clamp, LLC and asymmetrical half-bridge (AHB). High-power lighting applications, for example in a 100W lamp, where the output voltage is usually a little high, QR working with a synchronous rectifier can achieve up to 92.5% of total efficiency. Moreover, Fairchild has integrated QR and BCM PFC into one package (the FAN6921), reducing external components and simplifying the control.

Another popular topology is zero voltage switch (ZVS). Both an LLC and an AHB can have their two bridges working in zero voltage by implementing a simple circuit. When using Fairchild’s highly-integrated solution (for example, a LLC controller and two MOSFETs in FSFR; an AHB controller and two MOSFETs in FSFA2100), the circuit can be further simplified, with few external components. And the body diode of the MOSFET has good fast recovery characteristic, which can reduce the possibility of short-through, yet provide high reliability with high efficiency. When the output voltage is high, an LLC is the better option; when the output voltage is low, an AHB is more suitable for implementing a self-driven synchronous rectifier, and both can achieve over 93~94% efficiency.

The above solutions are highly integrated solutions and require just a few components, thus delivering high efficiency, high power density, optimized thermal performance as well as high reliability.

Click here for more information on LED lighting from Fairchild.

Tags: Energy Efficiency, Fairchild Semiconductor, LED Lighting

Green Power Feeding the Grid

November 24, 2009

PV inverter technology is driven by efficiency and compliance, but reliability is also important

What modern, ecological houses are wearing this season is blue - more precisely, the dark blue of solar cells. And this trend is gaining momentum, despite the financial crisis and reduction in feed-in tariffs from governments surprised by the success they created.

The owners of these systems are less concerned about the looks. What they are concerned about is high and reliable output. (Think of efficiency in the high 90s at more than 7000 power temperature cycles over lifetime!).

Green Power Feeding the Grid discusses how a good, reliable power switch must be the basis of this.

Tags: Energy Efficiency, green, inverter

Lifetime of an LED

July 2, 2009

Richard Chung

LED options are advertised as longer lifetime and more efficient lighting options to the traditional CFL, LFL, and HIDs. Is this really true? What are the factors that determine LED product life?

LED lifetime is measured in lumens depreciation. It is percentage of light output degradation from initial light output. LED Industry standard from IESNA (Illumination Engineering Society of North America) such as LM-80 specifies procedure for determining lumens depreciation. Researching the different LED vendors and their datasheets, the common theme is proper heat sink and thermal management of silicon junction temperature extends the lifetime of the LEDs. Look for lifetime versus junction temperature graphs in LED vendor datasheets that accounts for lumens depreciation. For example, L70 notation means that light output is 30% less than initial output. It is the percentage decrease that a “typical” eye starts to detect a decrease in light output. Different end applications can determine suitable lumens depreciation levels.

Thermal management of LEDs is half the effort to ensure the advertised lifetime. The LED power supply or driver design is the other factor for lifetime. When the LED does not work, the end user does not care if it is caused by LED’s lumens depreciation or the LED power supply that failed.

Because LEDs are non-linear (varying forward voltage versus forward current) devices, a constant current LED driver or power supply is required. Because non-linear LEDs produce power factor less than 1, PFC (Power Factor Correction) LED drivers are needed when it exceed a certain power level and/or harmonic current limits. Requirement to light high brightness LEDs is a power supply. There are components in a power supply that can decrease the life. More details next time!

Work cited:

www.ssl.energy.gov

Tags: efficient lighting, Fairchiild Semiconductor, HID, LED, LFL, lighting

The Race for Highest System Efficiency

June 15, 2009

By Guy Moxey, Low Voltage Power, Product Marketing

With the emergence of important climate saving legislation such as 80 PLUS, Climate Savers and EnergyStar® 5, analog designers for DC-DC power systems are striving to meet the challenge of increasing system level efficiency across all operating power states. From this, power silicon products such as power IC’s and power MOSFET’s are now very much in vogue as these devices create and dissipate the vast majority of any low voltage power conversion in-circuit losses that, in turn, directly relate to the system’s overall efficiency.

Take a typical notebook, typical peak efficiencies for a 46A, 2 phase Notebook VCORE solution with PWM controller and discrete MOSFET implementation are typically @90% peak at current ratings of 10A per phase, reducing down to @86% at full loads of 23A. This 10-13% loss in system efficiency is directly proportional to power and thermal dissipation. The complete notebook system is normally @ 50-60W output and running at 85% efficiency so that translates to a 9W power waste for every note book PC in the form of heat and battery life.

At start up or during a heavy processing sequence, the power system is dominated by conduction losses ( I2R) of the low side MOSFET. Here select a ultra high cell density low RDS(ON) FET housed in a dual sided cooled package so that the losses will be significantly minimized. However, as most PC s spend a majority of their operating life in standby or sleep states, it’s essential that the power system allows for light load efficiency management where gate drive and switching losses are predominant at low output currents below 10A. Here driver impedance and MOSFETs have to be carefully optimized. Gate drive voltages of 5V are preferred with MOSFETs with ultra low gate charge.

By careful MOSFET selection, close optimization with the driver IC, the design can start to move toward a higher level of overall system efficiency. Full load thermal design points can inch upward toward the 90% level, medium to light load levels can be touching 95% and ultra light loads don’t immediately take a dive southward with such velocity. But while we progress and save a few watts of loss over today’s designs, there is still some significant silicon research and development to be done before the utopian power curve can be seen.

Tags: DC-DC power systems, Energy Efficiency, Fairchild Semiconductor, MOSFETs, power IC’s