Power Supplies

Figure 1: Line Frequency Power Supply

Becoming the best at power supply design is much more than using the right components or applying exhaustive testing schemes. Designing the "best" of anything is a commitment to a strict no-compromise philosophy. In the case of power supply design, that philosophy must be founded on reliability over cost or features. Reliability implies efficiency; efficiency leads to meeting green objectives. In this article I would like to share with you a few recent developments in power supply design, coupled with an inside look into how we design power supply systems to be reliable and sustainable.

Linear Power Supplies

To understand the significance of the term "switchmode", we should first investigate the prior power converters that did not operate in this way. Typically these are called “linear,” but again there is some discrepancy between the term as used colloquially and the types of converters to which it applies. First let’s investigate what we meant by the conversion of power into something more useful to the electronic circuits in the products we make. A typical AC mains voltage is 120 Volt AC 60Hz sinewave, whereas the electronics may require +5 Volts DC and -5 Volts DC for video circuits, or +80 Volts DC, and -80 Volts DC for an audio amplifier. Obviously we need to scale the voltage and provide multiple outputs, but we also need to provide electrical isolation from the AC mains for safety reasons. A transformer is a very useful device that provides electrical isolation, scaling, and multiple outputs all at once, but it only works with an AC signal. Transformers just can’t pass DC for reasons we won’t get into here. Since we already have an AC voltage on the AC mains, it’s a simple matter to use a transformer as an AC to AC converter, then follow it with some simple diodes to provide an AC to DC conversion as shown in Figure 1. Now we have converted 120 Volts AC to +5 Volts DC, -5 Volts DC, +80 Volts DC and -80 Volts DC with just a handful of simple components! This is called a "line frequency power supply".

But there’s a catch, of course, actually several of them. This simple converter lacks line regulation, meaning that if the AC mains voltage sags by 15%, then so do all of our output voltages. Maybe our circuits will work anywhere in that range, and maybe they won’t. And if the line voltage doubles from 120VAC to 240VAC, then theoretically the output voltage would double, but practically speaking we would have to use a different transformer. Universal input is much more difficult to achieve, and typically the voltage range must be selected or configured at the factory. We can add a regulator to make sure that the output voltage does not change if the AC line voltage fluctuates within limits. This can be accomplished with minimal effort by adding a “linear” regulator (as shown in Figure 2), which basically functions as a controlled resistance between the input and output voltages, turning a considerable fraction of the load power into waste heat.

The efficiency of a linear regulator running from a line frequency transformer can be typically 50-75%, creating large amounts of waste heat simply by their nature of operation. This heat is power that is first drawn from the AC mains, then processed by the transformer creating losses, then dissipated using a thermal management system that may include a large and heavy heatsink and possibly a fan to move the heat out of the product and into the local ambient where it may need further attention, being removed by the equipment room cooling system. At least the linear regulator is cheap to buy; it certainly isn’t cheap to operate.

This type of power supply was used in the past, and in some places it still is, but the industry has generally adopted “switchmode” power conversion to reduce size, weight, and waste heat. This type of supply takes advantage of the fact that an isolation transformer’s size and weight are proportional to it’s operating frequency.

So instead of using the convenient and “free” AC frequency provided by the AC line, 60Hz in our example, we can use 60kHz for a reduction in size and weight. In Figure 3, this is done by rectifying the line frequency AC to DC, then using active power electronics to create a high frequency AC to pass through the transformer, after which it is rectified to DC again. This results in a great reduction of size and weight.

In this type of circuit the output voltage is a function of the input voltage and the switch duty cycle. So it is capable of universal input operation if a feedback loop is closed, varying switch duty cycle to maintain constant output voltage. Since the circuit runs directly from rectified 120VAC or 240VAC, there is significant difference in dominant losses for the two cases, usually favoring the low voltage case. There are ways to mitigate these losses and increase efficiency at the expense of increased complexity and design effort. Here we will review some of them briefly.

Figure 4: Transistor Capacitance with Hard and Soft Switching Waveforms

Transistor Loss

Transistor switching losses are very significant for traditional “hard switched” converters. This means there are losses each time a transistor changes state. These losses are from several factors but the most significant is the discharge of capacitance from the transistor itself. If the transistor's capacitance is allowed to transition “softly”, switching can occur nearly without losses as shown in Figure 4. In order to keep the switching losses from getting too large, as well as for cost cutting, a traditional design uses as small a transistor as possible since a smaller geometry has smaller capacitance. Unfortunately, this increases the resistance, so conduction losses are increased. In the resonant transitions converter, the transistor can be very large since it’s capacitance no longer leads to higher switching losses. Then resistance can be made arbitrarily small with a large geometry device, reducing conduction loss as an indirect result of resonant switching.

Figure 5: (a) Dissipative Clamp Topology; (b) Resonant Clamp Topology

Clamp Loss

A traditional converter has some unavoidable energy stored, which can be dissipated with a traditional clamp circuit comprised of a diode and capacitor to clamp stored energy into, and a resistor to dissipate the energy as shown in Figure 5(a). This standard method actually dissipates more than just the stored energy, since it sits across the reflected output voltage all the time. A “resonant clamp” topology is a relatively recent development that recycles the clamp energy in a nearly lossless way. With this approach, an extra transistor is used at a significant increase in cost and complexity, but efficiency is improved as shown in Figure 5(b). Transformers are designed to maximize stored energy instead of minimizing it.

Diode Loss

The conversion of transformer AC to output DC has traditionally been handled by a simple diode, but at the expense of an additional transistor, we can reduce the conduction losses dramatically as shown in Figure 6. The timing of this added transistor must be carefully synchronized to the main transistor, but losses can be cut an order of magnitude.

Startup Circuit Loss

Figure 7 shows a startup resistor. This is used to supply start-up power to the 12V control system from the rectified AC line voltage, 170VDC to 340VDC. This is only 4% to 8% efficient, but the worst part is that the resistor stays in circuit, dissipating power for the life of the product. The 240VAC case dissipates 4 times the 120VAC case, due to V2/R. High efficiency designs use an actively controlled high voltage current source to start up the control system that dissipates no power during active mode.

Figure 7: Startup Loss Eliminated by replacing resistor in (a) with transistor (b)

Tranfomer Loss

Power transformer losses can be reduced simply by accommodating the size and cost of a larger part. If the design is not competing on power density or cost minimization, this is a good choice.

Thermal Management Simplification

Another benefit of increased efficiency, or reduced dissipation, is that thermal management can be simplified. Traditional designs often require the mounting of a transistor to a dedicated heatsink. This then requires a thermal insulator and a nut and screw combination for mounting. Then some type of locking nut must be used. Due to repeated thermal expansion and contraction, a compression washer must be used to ensure proper force is applied. To maintain electrical isolation, a shoulder washer is used. All this mass is then affixed to the circuit board somehow (as shown in Figure 8), but it can be susceptible to shock and vibration, causing broken leads.

A more robust approach involves using surface mount power transistors and diodes, and using the PCB copper area for a heatsink. If the dissipation is low enough, this approach removes all hand labor and human error from the process, leaving just the highly mature and reliable process of a solder interface with a low profile result that is much less susceptible to shock and vibration.

Operating Temperature and Life Expectancy

All the efficiency improvements result in lower power draw, but the benefit of increased efficiency goes much farther than that. Heat accelerates component aging; a cooler product lasts longer and is more reliable. The general relationship is a doubling of lifetime for every 10˚C temperature reduction. The benefits of high efficiency are seen as reduced failure rate and increased life expectancy.

Power Factor Correction

Power factor is another area that received a lot of attention from a worldwide compliance perspective in the last 20 years. It has been made mandatory in Europe for all designs over 75 Watts in many product categories. Power Factor is simply Watts / Volt x Amperes. For a resistor, Watts = Volts x Amperes and power factor is 1.00. In the case of a simple line frequency rectifier, current can flow only when the input AC sinewave peaks exceed the DC voltage on the capacitors, leading to high current spikes at the waveform peaks and large deadbands in between. The spectrum of the resulting current contains harmonics of the line frequency, which increases the total current draw, but can deliver no real power. The current is drawn from the line effectively sent back without delivering any total power. Of course this increased current draw creates a very real load on the AC system wiring, which must be sized for the Volt x Amperes, rather than the Watts. In typical designs of linear or switchmode power supplies the power factor can be in the 0.5 to 0.7 range, meaning that the current draw is 50% to 100% higher than required by the power consumption. A technique called power factor correction is used on more sophisticated designs to improve the power factor to close to 1.0 , which reduces the current draw to just that required for the power draw as shown in Figure 10. Although this does not improve efficiency in the power supply, it improves system efficiency through the AC mains back to the generator. Once again, this comes at the expense of increased cost, complexity and design resources, and high reliability can only by achieved with very careful attention.

Energy Star

Energy Star is a voluntary public-private partnership that sets energy efficiency standards for a growing number of product categories. One type of product covered is the External Power Supply, also called a Desktop Power Supply. These are the plastic cased units that power so many consumer and professional products with low unit power but enormous aggregate power. The limits for Energy Star in a Desktop Power Supply are a function of the power rating of the supply; they include idle power and average efficiency. Average efficiency is simply the average of the efficiency measured at 25%, 50%, 75%, and 100% of rated load. The limits are difficult to satisfy and present a significant design challenge. Whereas typical Desktops may have an idle power of 2.0W, an Energy Star Desktop is allowed just 0.3W. Efficiencies may be improved from just 70% on a non-Energy Star unit to 85% in an Energy Star unit, meaning waste heat has been cut in half. Although the numbers may seem small at first, the Watt-hours saved add up quickly over the course of a year, especially considering the high volumes of Desktop Power Supplies sold.

Electromagnetic Interference (EMI)

EMI is a concern in switchmode power converters to a far greater degree than it is in linear supplies. A switchmode circuit operating with 60kHz square waves produces harmonics easily up to the 60MHz region in the form of electrical noise, electric fields, and magnetic fields, which can interfere with operation of the very circuits that the power supply is supposed to be serving. These harmonics can also exit the product via cabling acting as unintentional antennae, or openings in the chassis. Once in the outside world, they can interfere with all types of equipment. The last 20 years have seen an industry-wide focus on compliance as regulating bodies worldwide have been created or strengthened to deal with the problems of electromagnetic interference. Early designs were notoriously noisy from an EMI perspective, but again, companies were able to rise to the challenge and produce switchmode designs effectively as quiet as the old linear ones while retaining all the benefits.

Conclusion

Reliability is a primary concern of professionals, whether they are customers, system designers, or product manufacturers. At Extron, we have adopted the philosophy regarding power supplies for our products that reliability is our primary objective, not cost, not power density, not specsmanship. To achieve this, we start with a highly efficient topology made from the highest quality components available and manufacture it with the same process used for our high-end video products, using the same care and attention to detail in every step of the process. From Purchasing to Manufacturing, from Test Engineering to Quality Assurance, Extron switchmode power supplies ensure more high quality Extron products.