Getting More Efficiency with Synchronous Rectification MOSFETs

January 26th, 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.

 sr-mosfet-blog

 

 

 

 

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.

Author Information: DR. Efficiency

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Power Supplies Get Greener

January 18th, 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.

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Peak current mode PWM and slope compensation

January 8th, 2010

Dr. Efficiency,

Why does my power supply’s output suffer duty cycle fluctuation? I’m experiencing one big duty cycle followed by a small duty cycle. The pulses with large duty cycles almost occupy the maximum T(on) time and only those with small duty cycles can be modulated. My power supply uses a UC3842 PWM controller. Both the feed and the input voltage are smooth. Can poor PCB layout introduce interference? What are the disadvantages of this issue? How do I solve the problem?

 - Mr. Zhang

Mr. Zhang,

According to your circuit architecture, the UC3842 PWM controller used in your power supply works in Peak-Current Mode (PCM) and I think you may be experiencing sub-harmonic oscillation.

The PCM PWM controller has superior load regulation characteristics and anti-input interference capability, which makes it easy to implement current-limiting and over-current protection. It is stable in feedback and easy to compensate, hence widely used.

However, PCM PWM has a unique feature: when in continuous conduction mode (CCM) and with a duty cycle over 0.5, the angle between the rising curve of the inductor current and control voltage are smaller than that between the falling curve and the control voltage.  And in this case, we assume that there is a small disturbance occurring in the initial inductor current in one cycle. Then at the end of this cycle or at the beginning of the next cycle, the disturbance will be amplified and after several cycles of disturbance accumulation, duty cycle fluctuation will become one big duty cycle followed by a small duty cycle, or so-called sub-harmonic oscillation will occur.

This is an inherent feature of any open loop system which uses PCM PWM. It has nothing to do with the feedback or the PCB layout. Here we also understand that even with D<0.5, sub-harmonic oscillation could also be induced, depending on the angle between the rising curve of the inductor current and control voltage and the angle between the falling curve and the control voltage.

Sub-harmonic oscillation can make open-loop systems unstable, more susceptible to interference and in serious cases, it can even reduce the switch frequency by half and decrease the output power. This problem can be solved by making the duty cycle <0.5, or by compensating the current slopes. Slope compensation can be implemented by adding a signal with a fixed slope on the detected current signal or by adding a reverse slope signal on the control voltage to increase the angle between the current slope and the control voltage. With these measures taken, the possibility of sub-harmonic oscillation will decrease and the useable range of duty cycle will be widened.

 However, it should be noted that if the current slope is over-compensated, the advantage of PCM PWM will be off-set. To be specific, the higher the compensation, the more the PWM behaves like a voltage mode PWM. So it is important to have a proper slope compensation design. To facilitate the design procedure, Fairchild has integrated the slope compensation function within its newly introduced FAN6754 and FAN6753 PCM PWM ICs, providing you with more flexibility and a larger duty cycle range during design. In addition, the device also limits the maximum duty cycle, reducing the impact of sub-harmonic oscillation on the system and freeing you from undesirable compensation tasks.

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

Author Information: DR. Efficiency

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

December 21st, 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.

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Green Power Feeding the Grid

November 24th, 2009

Alfred Hesener

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.

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Will Portable Medical Devices Be The Standard?

November 16th, 2009

Medical Electronics to Leverage off Mobile Handset

Telehealth, E-Health,Wi-Health….The mobile handset has changed the way we live and communicate and we will see the same effect on portable medical electronics in the future. A cell phone that doubles as a wireless diagnostic tool may be a few years out due to infrastructure development but small adoptions in medical electronics will provide an immediate impact. Let’s look at a several key design areas that should be examined.

Handsets are everywhere and the consumer is educated on features like resolution, audio fidelity, plug-in and wireless peripherals. The portable medical segment has noticed these features but implementation into legacy medical is not an easy task due to IEC regulations and slow product design cycles. The adoption of mobile handset features into the portable medical segment will take place over time but there are several key areas that designers can focus on now.

Power

Power is at the top of the list. Until an alternative is proven, the battery as we know it is the medium of choice for the coming future. The cell phone’s 3.7V Li-Ion battery has seen growth in copious volumes and has driven the development of new DC/DC products from the IC industry. This includes a range of integrated synchronous buck devices with max recommended inputs of 5.5V and numerous charging schemes. Get rid of that stacked C123 cell battery giving 6-9Vs and you will see some nice power solutions that combine both high performance and cost effectiveness.

I/O

The USB port is becoming more important for mobile handsets. The portable medical market is aware of this vast adoption, however adding the port and complying with the industry’s IEC60601 standard can be a difficult task. (Optocouplers or magnetic isolation is required at the data pins or after the PHY.) Fortunately, there are simple charge detect ICs available to aid in adding USB power and providing Over Voltage Protection (OVP).

USB devices can minimize the number of external ports on your medical device and can mux other signals such as audio, video and sensors onto a shared connector port. When you need to transfer data off your medical device in a hurry but with shortest impact to design schedules, consider adding an external SDIO memory port. This is a win-win solution that improves time-to-market for the designer. By using an available port on the microcontroller it offers the user a familiar and established memory interface.

Packaging

Portable medical devices are space constrained. The enhanced packaging technology from mobile handsets has already been proliferated widely and can be leveraged in portable medical applications. ICs are available in sub 1×1mm packages in the form of bumped Chip Scale Packaging (CSP) and established MLP form factors. Examples include a simple P channel MOSFET or a new I2C lever translator / repeater. Does your manufacturer like leads? How about the SOT-923F?

Cell phones operate on a platform concept and medical is just starting to adopt this feature to improve time-to-market and leverage extensive testing. Medical electronics symposiums are starting to discuss these topics and more. Look for more product solutions from semiconductor manufacturers in the coming months.

Author Information: ESuckow

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What’s Up with Feedback Loops?

October 23rd, 2009

Greetings to Dr. FAE,
I notice many of your competitors do not use feedback loops in their reference designs. You guys have some parts with no feedback loop (like the tiny 0.6A 6MHz FAN5361 regulator http://www.fairchildsemi.com/pf/FA/FAN5361.html) while some parts, like your new 4A integrated regulator FAN21SV04 (http://www.fairchildsemi.com/pf/FA/FAN21SV04.html) do have an external feedback loop. I wonder: why is this? If I have a choice, I don’t want to mess around with external parts and complicated circuit analysis. It would make my life easier if you’d quit making parts with feedback loops.  - - - Baffled in Buffalo

Dear Mister Baffled,

If we’re talking about making people’s lives easier, you could connect our parts to well-behaved loads with well-behaved sources and we could sell you expensive resistors in fancy packages and make a lot of money.

However, most parts that provide a regulated output have a feedback loop for control and enhanced stability. Sometimes this feedback loop is buried inside the part…in that case we design a control loop we think will work well for a variety of circuits a customer might hook it to. Even a simple device like a Low-Dropout (LDO) regulator has a feedback loop from the output to control the conduction of the pass transistor. This is why, in rare situations, when a load is ill-behaved or a board layout is poor or bypass capacitors are improperly selected-the output will oscillate. In a feedback loop, we’re always trying to balance the aggressiveness of the response (the speed that the regulator will respond to load changes) with stability over temperature, component variation, worst case circuit and device influences. This brings up subjects like Bode plots and phase/gain margin that are uncomfortable for some people.

So, when you look at designing with a part like the FAN21SV04 and see the external feedback loop components, we’re trying to do you a service. You can adjust the feedback compensation to meet the transient requirements of your design.  So I hope you agree - including a feedback loop will make your life easier.

Now if you’ll excuse me, I have to visit my therapist.

Author Information: Dr. F.A.E.

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Dr. FAE: Voltage Feed-Forward Feature for Power Supply Controllers

September 21st, 2009


Dear Dr. Fred A. Engleberry. Could you please explain the advantage of the voltage feed-forward feature of many of your power supply controllers? - -      Puzzled in Peoria

Greetings to you in Peoria.

My IQ is 181 so, certainly, I could explain the advantage of voltage feed-forward. In anticipation of your next question, I will elaborate…

This morning, over a warm, half-caf, double-short, non-fat latté at the local coffee shop, I was enjoying a recreational review of Kreck and Lück’s Novikov Conjecture (Geometry and Algebra) which says the following:

Finally, we indicate the proof of Theorem 16.2 for arbitrary n. The idea is to work inductively. If f is a diffeomorphism on Tn X P with P a 1-connected manifold, one can isotope it so that it preserves Nn-1 X P.

My thinking might be illustrated more clearly with a transfer function from Erickson and Maksimovich, Fundamentals of Power Electronics:

image0013

This formula clearly shows that input voltage is not a variable. It contains a built-in assumption that the input voltage is invariant. Adding input voltage greatly complicates the transfer function.

Dear Doctor FAE, pardon me, but I do not recognize that answer as plain English.

-         PiP

Very well, I shall explain without the crystal clarity of the simple equation. The control loop of a DC-DC converter operates by sampling the output voltage and adjusting the pulse width modulation of the power train. The control loop acts as follows: if the output voltage changes, then we adjust power supply to counteract the change and keep the output stable.

However, if the input voltage changes, the power supply must respond to this change too. We could wait for the effect of the input change to appear at the power supply output, but wouldn’t it be glorious if we could monitor the input voltage and adjust the PWM immediately without waiting for the output voltage to change…if we provided some direct control method that did not complicate the feedback loop?

That’s the advantage of voltage feed-forward.

We do this by allowing the input voltage to directly modulate the slope of the PWM ramp. With an increased input voltage, the slope of the ramp increases and crosses the feedback signal sooner, giving a shorter output control pulse. Get it? Thus, increasing Vin reduces the PWM control signal outside of the output voltage control loop.

Now, if you’ll excuse me… I’ll be seeking a refreshing nap.

Author Information: Dr. F.A.E.

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Why did my FET fail?

August 25th, 2009

We’re introducing a new blogger to the site: Dr. Fred A. Engleberry. Dr. F.A.E. holds a PhD from MIT (Muckton Institute of Talknology) and has several months of valuable experience with applied technology. We are pleased to have Dr. F.A.E. available to answer questions collected from customers around the world.

Dr. F.A.E, “Smoky” from General Specifics Inc. sent a question…”Why did my FET fail?” Without further ado, we’ll turn the session over to Dr. Fred.

First of all, Smoky, you’re probably expecting a lot of annoying questions about your design. Such as, what frequency you’re running at, what the gate drive circuit looks like, what the load is and what supply voltage is present. Some design engineers might try to determine whether there is an avalanche condition beyond what the device might be reasonably expected to tolerate, whether the gate drive is insufficient or oscillating, whether the load is inductive, whether voltage spikes creep too close to breakdown voltages on the gate or drain, or whether the total package dissipation is being exceeded.

However, let’s say in this instance I could see your schematic and Bill of Materials (BOM). Your gate resistor, R42, as noted on the schematic, should be one ohm, but the BOM shows 1,000 ohms. Replace this resistor with the proper value and you will find that your FET turn-on and turn-off rise and fall times will become reasonable and you will avoid gate oscillation – and your FET design will become robust.

Want more information on FETs? Check out our website for MOSFETs at http://fairchildsemi.com/products/mosfets/index.html

Author Information: Dr. F.A.E.

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Perfect Sunrise, Great location, Wonderful people.

August 3rd, 2009

Arriving at Fort Williams in Cape Elizabeth, Maine on Saturday for the 12th Fort Williams in Cape Elizabeth, MaineTD Banknorth Beach to Beacon 10K, my breath was taken away. This didn’t happen from physical exertion but from the majestic beauty of the sunrise over the beautiful rocky coast. I had a feeling that this meant it would be a memorable day.

When the race started, we knew that people from all over the world and over 50 of our own employees were running the picturesque course. Anticipation rose as we awaited the arrival of the first finishers.

At first the runners trickled in but soon the field was flooded with participants of all ages. We congratulated everyone on their successful completion and were excited to spot the green shirts of our co-workers. The Fairchild Semiconductor men ended up finishing #2 in the men’s corporate challenge! Congrats guys!

In our Green Energy Sponsor tent, dscn38351we had fun mingling with interested community members who took home free give-aways like green water bottles. Visitors of all ages rode the bicycle to charge their cell phone.

When I left at the end of the event, I thought how amazing the day was- perfect sunrise, great location, wonderful people!

A big thanks to my fellow employees at Fairchild Semiconductor who ran and helped manBeach to Beacon 2009 Fairchild team our Green Energy Sponsor booth. Our contributions will go to the 2009 beneficiary, Maine Handicapped Skiing. Thanks to everyone at Fairchild who made our Green Energy Sponsorship a success!

Author Information: Katelyn

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