Switching to Efficient Power

by Laurent Butaud, Future Electronics France

The relative efficiency and simplicity of Switch-Mode Power Supplies (SMPS) underpin their increasing popularity. Poor design, however, can compromise that efficiency, and the switching operation is inherently noisy. Consequently, it is important for digital engineers to understand the sources of inefficiency and noise, so that they can minimize their effects on digital circuits.

READ THIS TO LEARN ABOUT The types of noise that SMPS can induce Design techniques and guidelines for minimizing noise High performance components that can be used in SMPS designs

The electronic appliance industry owes a great debt to the venerable switch-mode power supply. Nearly all TVs and computers use them and most industrial equipment is equipped with an SMPS.

SMPS have also found their way into portable products where they are able to maintain a constant supply voltage independent of battery charge. High power SMPS can be found in telecommunication exchanges, power stations, trams and even trains.

In theory, an SMPS works with 100% efficiency, although less-than-perfect components limit efficiency in practice to between 70% and 95%. This high efficiency improves reliability and keeps the operating temperature down. In addition, the higher the switching frequency, the smaller the size and weight of the SMPS components, resulting in reduced cost and board size.

Because of the importance of power supplies in digital equipment, digital design engineers must understand the power supply’s operation, and know how to improve efficiency while suppressing noise. Otherwise, engineers can find their carefully crafted circuitry misbehaving due to spikes from a poorly designed power supply.

Fortunately, the principles of operation are relatively simple and technical guidance is available from Future Electronics. Future also supplies components from top vendors such as Murata, National Semiconductor, International Rectifier, Sipex and Vishay that integrate many SMPS functions, easing the design process.

 

 

Types of SMPS

SMPS can be produced in several topologies including buck, boost, SEPIC and flyback. All comprise switching controllers, transistors, inductors and capacitors. Examples of various topologies are shown in Figures 1 to 5. These examples are based on Sipex, National Semiconductor and ON Semiconductor devices. The choice of SMPS topology depends on the application: buck (or step down) is used when output voltage is lower than input voltage; boost (or step up) is used when output voltage is higher than input voltage; SEPIC is useful when an input battery voltage varies around the output voltage (as the battery discharges); and flyback power supplies are used for higher power outputs.

Because of its emphasis on efficiency, SMPS design minimizes the use of inefficient or ‘lossy’ components and employs efficient resistors, switches, capacitors, inductors and transformers. The primary design problem is how to combine these components and control the switching so that the desired spikeless output is obtained.

In practice, there is no such thing as a lossless component, so some compromise is to be expected. A more practical way of defining the design challenge is to describe it as overcoming the performance characteristics of less than ideal components. (Note that protection techniques and derating are used because in practice parts tend to fail when overstressed.)

Modern electronic devices need clean, spike-free power in a range of stable voltages. Unfortunately, SMPS are based on a controller, driving either one or two transistors (either synchronous or nonsynchronous). This arrangement results in noise at the switching frequency and its harmonics. The noise from the power supply can harm the performance of the digital electronics.

Relatively small effects can have significant impact on the end user experience. For example, a poorly designed SMPS can cause a hi-fi to crackle or generate image distortion on an LCD TV.

 

 

Improving Noise Suppression and Efficiency

A well designed supply will use various techniques to minimize noise. The main types of noise in a system are input, output, radiated and microphonic.

Input noise generally comprises reflected ripple, in which the input current noise of an SMPS interacts with the source impedance of the raw supply voltage, corrupting AC-line or battery voltages. Output noise is voltage noise that may upset noise sensitive loads such as audio or video electronics.

Radiated noise can be electromagnetic or electrostatic and usually originates in magnetic components, such as transformers, inductors, switches and rectifiers. And finally, microphonic noise is audible sound, the usual cause of which are low frequency switching waveforms that excite coil windings and cause them to mechanically vibrate against each other.

A simple input filter can dramatically reduce input noise. Designers have to be aware, however, that input current noise not suppressed by the input filter capacitor returns to the battery and AC adapter. This same noise can then corrupt other loads connected to the battery.

If noise causes the battery wire or AC adapter cable to act as an antenna, the resulting Electromagnetic Interference (EMI) may violate regulations. This makes the choice of the capacitor critical. A good choice for this application is Murata’s ceramic capacitor family, which can drive up to 3A in an 0603 package. To improve filtering, the Murata BNX002 EMI filter works well.

The output capacitor of a boost converter is subject to abrupt current steps equal to the entire peak inductor current. These high amplitude fast current transitions can generate some output noise when they interact with the equivalent series resistance and inductance (ESR and ESL) of the output capacitor. In addition to the large voltage steps that ESR causes, ESL causes tiny, high frequency spikes at the leading and trailing edges of the switching waveform.

These high frequency spikes (known as ‘hash’ spikes), which can reach hundreds of millivolts in amplitude, can be suppressed with a simple RC filter in the supply line, such as a 0.1Ω resistor in series and a 0.1µF ceramic capacitor connected to ground. Often, the parasitic inductance of wires connecting the power supply and load is enough to suppress hash spikes. The trade-off is then to find capacitors with low ESL and ESR. Murata ceramic capacitors are again a good option here.

For microphonic noise, the designer can usually solve the problem by raising the minimum frequency out of the audible frequency range of 20Hz to 20kHz, or by applying varnish to the windings.

Fixed frequency Pulse Width Modulation (PWM) has the advantage of providing the most stable and predictable noise performance of any control architecture. The designer can choose the switching frequency and its harmonics so that the audio band or a selected RF band remains free of switching noise.

Variable frequency Pulse Frequency Modulation (PFM) SMPS are also popular, because they extend battery life in the sleep and standby modes of operation. On the downside, they are generally noisier both in amplitude and frequency than PWM supplies. For example, at light loads, a PFM system minimizes switching loss by switching at a very low frequency, which improves battery life but at the expense of noise in the audio band. This audible noise needs filtering.

However, with careful component selection it is possible to force the PFM converter to operate above the audio band at the minimum load condition. For example, reducing a PFM regulator’s maximum on-time by adjusting the timing capacitor can raise the minimum switching frequency.

 

 

Keeping Costs Down

Many semiconductor vendors manufacture components that reduce cost and simplify the design of a power supply by integrating multiple elements into a single device. One example is National Semiconductor’s LM5008, which combines the controller and MOSFETs into one chip, while accepting an input voltage up to 100V.

Another way to save cost is to reduce the regulator size while increasing its power. National, for example, manufactures regulators in its LLP package; likewise Vishay supplies compact PowerPAK® MOSFET devices.

However, discrete switching controllers still have their place because they are suited to higher power outputs or specific topologies. They also tend to be less expensive in high volume applications. Sipex offers its SP765x step-down regulator range which, when coupled with Vishay’s IHLP2525 inductor, is a good choice for point-of-load battery powered applications. Alternatively, ON Semiconductor’s NCP1450 stepup controller is good for small battery powered applications in which power efficiency and PCB real estate are critical.

National Semiconductor’s LM5115 step-down controller coupled with an International Rectifier high voltage MOSFET is a good choice for robust high input voltage (up to 75V) industrial applications. There are also specific designs for powering LEDs. ON Semiconductor’s NUD4021 switching LED current source is a good example of this type of circuit. The NUD4021 requires a minimum of extra components, reducing cost without sacrificing the LED’s performance.

Trends in the peripheral components include dual FET references for both high and low side in a single package, ceramic capacitors replacing tantalum, and high performance yet compact inductors. The latter are available from companies such as Murata (for example, the LQR package) and, for higher currents, Vishay’s IHLP family.

 

 

Finding Help

SMPS design can appear daunting for the digital designer at first glance. But it is well worth getting to grips with the concept because of the effect of the power supply on the rest of the design. By applying the principles described above, efficiency can be dramatically improved and switching noise suppressed.

And further help is at hand. Component vendors can provide an array of reference designs, evaluation boards and online design tools that ease the design task.

For example, National Semiconductor’s WEBENCH® is an online design tool for power supplies. The designer specifies input voltage range, desired output voltage and maximum current, and then the system picks the most appropriate topology and product to realize the function. In addition, WEBENCH generates an electrical schematic and bill of materials, as well as simulating the thermal behavior of the printed circuit board and its components.

Sipex has online tools too, one dedicated to its PowerBlox™ family of high current switching regulators for point-of-load designs, and another for driving LEDs.

Moreover, Future Electronics employs applications engineers familiar with all types of switch-mode power who are able to assist customers with their designs. Future also stocks a full range of components for SMPS assembly from leading suppliers such as National Semiconductor, International Rectifier, Sipex, Vishay and Murata.