Power supply design Tips from EE-Times

With the emphasis today on the need for more efficient and cost-effective power solutions, EE-Times created this column to provide helpful tips on a variety of power management topics. This column is geared towards design engineers at all levels. Whether you’ve been in the power business a long time or just coming on the power scene, you’ll find some nuggets of information that just might help you with your next design challenge.

  • #1, July 2008: “Picking the right operating frequency for your power supply”
    Selecting the optimum operating frequency for your power supply is a complex tradeoff
    involving size, efficiency, and cost. In general, low-frequency designs tend to be the
    most efficient, but are the largest and most costly. Moving higher in switching frequency improves
    size and cost at the expense of circuit losses. This article use a simple buck power supply to
    illustrate these tradeoffs.
  • #2, August 2008: “Taming a noisy power supply”A noise-free power supply is not an accident. A good power supply layout in particular is essential to minimize lab time when bringing up a new design. A few hours or even minutes spent looking over the layout can save days of troubleshooting.
  • #3, September 2008: “Damping the input filter–Part 1”While switching regulators are often preferred over linear regulators because they are more efficient, the switching topology leans heavily on an input filter. This circuit element, combined with the supply’s typically negative dynamic impedance, can contribute to oscillation issues. Here’s how to avoid the problem.
  • #4, October 2008: “Damping the input filter–Part 2”
    A general criterion has been established that the source impedance of an input filter should be at least 6dB less than the input impedance of a switching regulator as a safety margin to minimize the chance for oscillation. This article is about designing such a filter.
  • #5, November 2008: “Buck-boost design uses a buck controller”
    Electronic circuits typically operate from regulated positive output voltages, often provided by buck regulators. If a negative output voltage is also required, the same buck controller often can be configured in a buck-boost topology. A negative output voltage buck-boost, sometimes called a negative flyback, operating at 50% duty-cycle provides an output voltage equal to the input voltage, only opposite in polarity. It has the ability to “buck” or “boost” the output voltage to maintain regulation by adjusting the duty cycle as the input voltage fluctuates.
  • #6, December 2008: “Accurately Measuring Power Supply Ripple”
    Measuring power supply ripple properly is an art…..
  • #7, January 2009: “Efficiently driving LEDs offline”
    Transition Mode SEPIC Functions as a Simple LED Driver.
  • #8, January 2009: “Reduce EMI by varying power supply frequency”
    Have you ever tested for EMI and found that no matter what you do in the way of filtering, you are still a few dB out of specification? Here is a technique that may help you pass the EMI requirements or possibly simplify your filter design.
  • #9, March 2009: “Estimating Surface Mounted Semiconductor Temperature Rise”
    Semiconductors mounted in thermally enhanced packages require the circuit board to function as the heat sink and provide all necessary cooling.
  • #10, April 2009: “Simply Estimate Load Transient Response”
    This Power Tip from Texas Instruments Robert Kollman presents a simple method to estimate the transient response of a power supply by knowing the control bandwidth and output filter capacitor characteristic.
  • #11, May 2009: “Resolve Power Supply Circuit Losses”
    This Power Tip presents a simple method to help resolve differences between calculations and actual measurement. It is based on the Taylor series expansion that states (after some liberties are taken) that any function can be resolved into a polynomial.
  • #12, July 2009: “Maximize Power Supply Efficiency”
    This article shows how you might use the Taylor series to maximize your power supply efficiency at particular load current.
  • #13, July 2009: “Don’t get burned by inductor core losses”
    Have you ever powered up a buck regulator, tested it at full power, then have a permanent reminder left when you perform the inductor finger-tip temperature test? Maybe, excessive core losses and ac winding losses are the culprits. With a 100-kHz switching frequency, this generally isn’t an issue because core loss constitutes around 5% to 10% of the total inductor loss. Hence, the corresponding temperature rise.
  • #14, July 2009: “SEPIC converter makes an efficient bias supply”
    Have you considered using a single-ended primary inductor converter (SEPIC) topology for a bias supply? If you don’t need isolation, it just might make sense. The SEPIC has several features that make it more attractive than a non-isolated flyback. MOSFET and output rectifier ringing are controlled to reduce electromagnetic interference (EMI) and voltage stress. In many cases, this lets you use lower voltage parts, which may cost less and be more efficient. Also, a multiple-output SEPIC improves cross regulation between outputs, which may eliminate the need for linear regulators.
  • #15, September 2009: “Design a low-cost, high-performance LED driver”
    As LED production costs fall, they’re being used more frequently in applications ranging from handheld devices, to automotive, to architectural lighting. Their high reliability (operational lifetimes of greater than 50,000 hours), good efficiency (175 Lumens/W), and nearly instantaneous response make them a very attractive light source. However, driving LEDs is not without its challenges.
  • #16, September 2009: “Snubbing the forward converter”
    Do you struggle with picking snubber components? Figuring out how much capacitance and resistance to add can seem challenging. Here’s a quick way to get through the problem.
  • #17, November 2009: “Snubbing the flyback Converter”
    Now, we look at snubbing the FET turn-off voltage in the flyback converter.
  • #18, December 2009: “Your regulator’s output-voltage accuracy may not be as bad as you think”
    Output voltages are falling and voltage regulation requirements are getting tighter. However, your job may not be as difficult as it might seem on the surface. Even though you are forced to design with resistors with tolerance of one percent or worse, you may still be able to provide very precise output voltages.
  • #19, January 2010: “Easily create multiple negative output voltages”
    The C’UK converter excels in this application.
  • #20, February 2010: “Watch those unintended resonant responses”
    Have you ever snapped on the input voltage to your power supply and found that your power supply has failed? A rapid input-voltage rise time and high Q resonant circuit that can produce twice the voltage of the input supply may be the problem. Similar problems can occur if you rapidly interrupt current flow in inductive elements.
  • #21, March 2010: “Watch That Capacitor RMS Ripple Current Rating!”
    One of the often overlooked stresses in power supplies is the input capacitor RMS (root mean square) current. If not properly understood, excessive current can cause the capacitor to overheat and fail prematurely.
  • #22, April 2010: “Avoid These Common Error Amp Pitfalls”
    Here is a short collection of power-supply error-amplifier pitfalls that you can easily avoid. They include improperly calculating the gain of the error amplifier, asking the amplifier to do something it can’t, and improperly laying the circuit out.
  • #23, May 2010: “Improve a power-supply’s load-transient response–Part 1”
    This power tip will focus on closing the feedback loop in an isolated power supply with a TL431 shunt regulator. It will discuss a method to widen the power supply control loop bandwidth to improve transient load and line response.
  • #24, June 2010: “Convert parallel impedances to series impedances”
    This Power Tip shows you how to do a quick conversion of parallel-to-series complex impedances (and vice versa).
  • #25: July 2010: “Improve a power supply’s load transient response–Part 2”
    This Power Tip, a follow-on to Power Tip 23, focuses on closing the feedback loop in an isolated power supply with a TL431 shunt regulator. It discusses a method to widen the power supply control loop bandwidth to improve transient load and line response.
  • #26: August 2010: “Current distribution in high-frequency conductors”
    In this Power Tip, we will look at the effective resistance of conductors in free space and wound structures.
  • #27: September 2010: “Paralleling power supplies using the droop method”
    In this Power Tip, we will look at a simple method to parallel supplies.
  • #28: October 2010: “Power Tip 28: Estimating transient temperature rise in a hot-swap MOSFET-Part 1”
    In this Power Tip #28 and the next (#29), we will look at a simple method to estimate the temperature rise of a hot-swap MOSFET. A hot-swap circuit is used to limit inrush current when plugging a capacitive input device into a voltage bus that is energized.
  • #29: November 2010: “Power Tip 29: Estimating transient temperature rise in a hot-swap MOSFET-Part 2 “
    In this Power Tip, we conclude looking at a simple method to estimate the temperature rise of a hot-swap MOSFET.
  • #30: December 2010: “Power Tip 30: Low-voltage buck IC makes simple, inexpensive bias supply”
    In this Power Tip, we are looking at a simple circuit to convert a high AC input voltage to a much lower DC voltage usable in applications such as e-metering. In this particular application, there is no need to isolate the output voltage from the input voltage. Here, the rectified AC input voltage can be as high as 375 VDC, and output in the range of 5 Volts at currents of several hundred milliamps. These high-volume applications are often cost-driven, so a low parts count/low cost circuit is required.
  • #31: January 2011: “Pick the right resistance ratio of synchronous buck MOSFETs”
    In this Power Tip, we will look at a trade study of conduction power dissipation in a synchronous-buck power stage as a function of duty factor and the ratio of FET resistances. The results of this trade study provide a useful starting point for the selection of the FETs.
  • #32: February 2011: “Beware of circulating currents in a SEPIC coupled-inductor–Part 1”
    In this Power Tip, we establish the leakage inductance requirements for the coupled-inductor in a SEPIC topology. The SEPIC is a useful topology when electrical isolation between the primary and secondary circuit is not required and when the input voltage is higher or lower than the output voltage. It can also be used in place of a boost converter when short circuit protection is required.
  • #33: March 2011: “Beware of circulating currents in a SEPIC coupled-inductor–Part 2”
    In this Power Tip, we continue our discussion from Power Tip 32 – Part 1 of establishing the leakage inductance requirements for a coupled inductor in a SEPIC topology. Previously, we discussed the fact that the coupling capacitor’s AC voltage is impressed across the leakage inductance of the coupled inductor. The voltage across the leakage inductance can induce large circulating currents in the power supply. In Part 2, we show measured results of a power supply built with a loosely coupled and tightly coupled inductor.
  • #34: April 2011: “Design a simple, isolated bias supply”
    Have you ever come across the need to generate an isolated power supply for gate drive, isolated sensing or communication circuits? In this Power Tip, we will take a look at a circuit that can do this with minimal parts count, complexity, and cost. This circuit finds use when you have a low input voltage available and the powered circuits allow some (five percent) supply-voltage variation.
  • #35: May 2011: “Minimize transformer interwinding capacitance effects”
    Have you ever designed a low-power flyback converter with a high turns ratio? If so, you probably encountered problems with interwinding capacitance. In this Power Tip, we take a look at techniques to reduce the capacitance effects that allow higher frequency operation.
  • #36: June 2011: “Higher-voltage LEDs improve light bulb efficiency”
    There is much interest in replacing incandescent screw-in light bulbs with bulbs that use LEDs as the light source. Typically, a small number of LEDs—between five and nine—are connected in series and a power supply has to convert the line voltage to a low voltage, typically tens of volts, at currents around 350 to 700 mA. There are a number of trade-offs in determining how to best isolate the consumer from the line voltage. Isolation can be accomplished either in the power supply or in the mounting of the LEDs. In these lower-power designs, physical isolation of the LEDs is a common choice as it allows the use of a cheaper, non-isolated power supply.
  • #37: July 2011: “Trade line range for input-capacitor ripple current”
    An interesting tradeoff occurs when you select the input filter capacitor in a low-power, offline power supply. You trade the ripple-current rating of the capacitor for the voltage range over which the supply needs to operate. By increasing the input capacitor, you apply more ripple current in it and narrow the operating input voltage range of the power supply by decreasing the droop in the input capacitor.
    This impacts the transformer turns ratio and various voltage and current stresses within the power supply.
    A larger capacitor ripple current rating means less stress and a more efficient power supply.
  • #38: August 2011: “Simple latch circuit protects power supplies”
    Have you ever needed a simple, inexpensive latch circuit? This power tip shows one that can provide fault protection in power supplies with only pennies of components. It is basically a silicon controlled rectifier (SCR) implemented with discrete components.
  • #39, September 2011: “You get more than just better efficiency by going synchronous”
    Have you ever been asked to design a power supply with good load-transient response at light load?
    If so, and you allowed the power supply to go discontinuous, you probably discovered that the gain
    in the control loop decreases greatly at light loads. This can result in poor transient response and
    the need for a massive output filter capacitor. A simpler approach is to make the power supply continuous at all loads.
  • #40, October 2011: “Common-mode currents and EMI in non-isolated power supplies”
    Have you dismissed common-mode currents in a non-isolated power supply as a potential electromagnetic interference (EMI) source?
  • #41, November 2011: Powering DDR memory
    Power dissipation within CMOS logic systems is primarily related to clock frequency, input capacitance of the various gates within the system, and the supply voltage. As device feature sizes and, hence, supply voltages have been reduced, significant gains have been made in lowering dissipation at the gate level.
  • #42 (Part 1): Discrete devices—a good alternative to integrated MOSFET drivers
    Many times in power-supply design, an engineer is faced with the problem of limited drive current available from his control IC, or too much power being dissipated in it due to gate-drive losses. To mitigate these issues, external drivers are often used. Semiconductor manufacturers (including TI) have ready-made MOSFET-driver solutions in the form of integrated circuits
  • #43: Discrete devices—a good alternative to integrated MOSFET drivers (Part 2)
    In Power Tip #42, we discussed an emitter follower used in MOSFET gate-drive circuits and saw that drive currents in the 2-A range are achievable with small SOT-23 transistors. In this Power Tip, we look at self-driven synchronous rectifiers and discuss when discrete drivers are needed to protect the synchronous rectifier gates from excessive voltages.
  • #44: Handling high dI/dt load transients, Part 1
    With many central processing units (CPUs), specifications require that the power supply must be capable of providing large, rapidly changing output currents, typically as the processor changes operating modes.
  • #45: “Handling high dI/dt load transients, Part 2”
    In Power Tip #44, we discussed capacitive bypassing requirements for loads with rapidly changing currents. We found it imperative to have low equivalent series inductance (ESL) capacitors physically close to the load, as less than 0.5 nH can create unacceptable voltage excursions.
  • Power Tip 46: Time your synchronous-buck FETs properly
    In this Power Tip, we investigate the importance of timing between the high-side and low-side FET gate drives in a synchronous buck regulator. Timing optimization is becoming increasingly important as engineers strive to eke out the best possible efficiency in their power supply.
  • Power Tip 47: Tame conducted common-mode emissions in isolated switchers (Part 1)
    In this Power Tip, we continue our discussion of common mode currents which began in Power Tip 40. There we discussed how common-mode currents are created by large voltage swings found in switching stages, which drive currents into the capacitances to chassis ground.
  • Power Tip 48: Tame conducted common-mode emissions in isolated switchers (Part 2)
    We continue our discussion of common-mode currents which started in Power Tip 47 part 1.
    We discussed that we can return common-mode currents to their source by using a chassis capacitor, which also reduces the source impedance of the noise. However, there is a safety limit as to how much capacitance we can use, which determines the remainder of the common-mode filter.
  • Power Tip 49: Avoid these common multi-layer ceramic capacitor pitfalls
    Multi-layer ceramic (MLC) capacitors have become extremely popular in power electronics due to their small size, low equivalent series resistance (ESR), low cost, high reliability, and high ripple current capacity. Commonly, they are used in lieu of electrolytic capacitor to enhance system performance.
  • Power Tip 50: Avoid these common aluminum electrolytic capacitor pitfalls
    Aluminum electrolytic capacitors remain a popular choice in power supplies due to their low cost. However, they have limited life and are sensitive to both hot and cold temperature extremes. Aluminum electrolytic capacitors are constructed with foils placed on opposite sides of paper saturated with an electrolyte. This electrolyte evaporates over the capacitor’s lifetime, altering its electrical properties. If the capacitor fails, it can be spectacular as pressure builds up in the capacitor, forcing it to vent a combustible and corrosive gas.
  • Power Tip 51: Be aware of capacitor parasitics
    Power supply ripple and transient specifications establish the requirements for the amount of capacitance you will need.
  • Power Tip 52: Making over the wall wart
    With worldwide sales of cell phones closing in on two billion per year, the cell phone charger’s size, cost and efficiency is under scrutiny.
  • Power Tip 53: Use P-SPICE to design your power supply control loop
    P-SPICE (or any simulator) can be a very effective tool in synthesizing power supply control loops. In this Power Tip, we use P-SPICE to design a control loop for the integrated, synchronous buck.
  • Power Tip 54: Use 2-section filter for low-noise power supply
    Some low-noise applications may require the power supply output ripple voltage to be less than 0.1 percent of the output voltage. These low ripple requirements easily can translate into filter attenuations of significantly greater than 60 dB, which cannot be practically met with a single stage.
  • Power Tip 55: Compensate a low-noise power supply having a two-section filter
    Some low-noise applications may require the power supply output ripple voltage to be less than 0.1 percent of the output voltage. This low ripple requirement easily can translate into filter attenuations significantly greater than 60 dB, which cannot be practically met with a single stage. In Power Tip 54, we discussed the design and time domain simulation of such filters. In this Power Tip, we discuss using P-Spice in closing the feedback loop around such a filter.
  • Power Tip 56: Estimate PWB interconnect inductance
    In this Power Tip, we examine some simple formulas for interconnect inductance in free space and over a ground plane. We will find that the ground plane significantly reduces trace inductance and is critical for optimal performance in power supplies.
  • Power Tip 57: Design a flyback primary switch snubber
    How to best control the voltage stress on the primary switch in a single-ended flyback converter (shown in Figure 1) is a multi-faceted problem. You have to solve a combination of technical issues while still keeping an eye on the overall cost.
  • Power Tip 58: Power supply grounding…which camp are you in?
    If you want to start a heated discussion between a group of power supply engineers, all you need to do is ask them how they layout the grounds in a power supply. You will quickly learn there are two basic strategies. Each side will swear that the other approach has no chance of functioning as they smugly remember how well their last design worked.
  • Power Tip 59: Save check-out time with a good power supply layout
    You can destroy a well-designed power supply with a sloppy layout. There are high power switching circuits as well as sensitive analog circuitry in a power supply, and mixing the two can result in chaotic operation.
  • Power Tip 60: Three simple split-rail power supply topologies
    Have you come across a need for a split-rail, low-power supply from a single input voltage?
    For example, this could be +12/-12 volts from 5 volts or +/-5 volts from 12 volts. This Power Tip presents three power-supply topology options to meet these types of requirements with varying degrees of performance and costs.
  • Power Tip 61: Watch the Conversion Ratio on Boost Converters
    Have you ever needed to provide a boosted non-isolated power supply output from a lower-voltage input? The boost converter is a traditional solution. Still, you need to be mindful of limitations on the control IC.
  • Power Tip 62: Boost or Flyback for Extreme Conversion Ratios?
    In Power Tip 61, we discussed controller duty factor limitations for very high conversion ratios in non-isolated boost converters. We found that the maximum duty factor limitation in the controller limited how high of a boost voltage you could create. So what do you do when you run up against this limit?
  • Power Tip 63: Testing High-di/dt Power Supplies?
    High-performance processors demand subvolt power rails that can rapidly source and sink current. It is a testing challenge to duplicate these large current swings to verify power supply performance. The two main challenges are to minimize the load inductance in the test setup and to make the proper voltage measurement.
  • Power Tip 64: Compensate for Cable Drop Without Remote Sensing
    Sometimes your power supply design specifies better regulation than you can achieve without the complexity of remote sensing. A prime example of this is an offline USB charger where the power supply must compensate for 0.5 volts of cable drop without the cost and bulk of two extra wires. The voltage to the output needs to be in range of 4.75-5.25 volts. Without remote sensing, this cannot be achieved with typical component tolerances and the 0.5-volt drop on the output cable.
  • Power Tip 65: Power Supply Implications of Envelope Tracking
    RF power system efficiency is becoming increasingly important in low-power systems when it comes to wringing out the last minute of operation from a battery, and in high-power base stations where electric bills can be substantial. One obvious way to improve the situation is to emit only the needed RF power. This is relatively easy to do, as the amplifiers operate as AB amplifiers, and the drive signals can be backed off when lower output power is desired.
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