Power Up for Control
Power supplies are found everywhere in manufacturing. Yet, when did any engineer stop to consider what advances in power technology contribute to control strategy implementation? Control architecture objectives include reducing enclosure size, enhancing safety for technicians troubleshooting problems within the enclosure, distributing control throughout a production line or process plant,...
Power supplies are found everywhere in manufacturing. Yet, when did any engineer stop to consider what advances in power technology contribute to control strategy implementation? Control architecture objectives include reducing enclosure size, enhancing safety for technicians troubleshooting problems within the enclosure, distributing control throughout a production line or process plant, and improving the plant's electrical power quality. Modern power supplies contribute to each of these objectives.
In addition, power supply manufacturers are designing to meet standards like UL 508 for control systems and IEC 61000 for power quality. As other features become available for most products, mean time before failure (MTBF) becomes a competitive advantage. Parallel operation, allowed by new technologies, is an easy way to increase current supplied inexpensively.
Manufacturers are incorporating today's advances in microelectronics to pack more features into ever-smaller packages.
'Customers want reduced power supply size with no reduction in power output and increased efficiency,' says Jeff Kuzniar, Entrelec's (Irving, Tex.) engineering manager. 'Today, switching power supply technology is mainly based on primary switching with flyback regulation for low and medium power and forward regulation for increased power. The trend here is to integrate more components into the basic circuitry to reduce the number of parts.'
Switch mode reduces size
Switching power supplies are increasingly replacing linear ones. These were originally found in computer equipment, but are taking on industrial hardness and are found in more control cabinets.
Charlie Cook, Wago's (Germantown, Wis.) engineering manager, says, 'The reason to change to switching power supplies comes down to packaging and price. At sizes over 100 W, they cost less. Since they generate less heat, package size is reduced. Efficiencies of linear supplies were generally around 60%. For 100 W, this means 40 W went to heat. That's a lot of heat for a small enclosure. Switching power supply efficiencies are generally over 90%. Result? Less heat and smaller packages enable smaller control enclosures.'
Mounting power supplies is often a problem. Frequently, they are bolted on top of an already large enclosure. This can be a problem for many process plant applications. Engineers in discrete manufacturing try to eliminate large enclosures in favor of small ones for several reasons. Cost of components is a major reason, but cost of real estate that a production line consumes is another. Being able to look across an entire machining line rather than getting lost in a maze of steel is a benefit.
One solution is reduces the size of every component possible and makes them easily mountable. Buddy Mitchell, product manager at Wieland Electric (Burgaw, N.C.), says, 'The major trend we see driving power supply design is the request for DIN-rail-mounted, compact, and easy-to-wire packaging. DIN-rail mounting eliminates additional requirements for traditional open-frame power supplies. Not only do they install more quickly, but incorporation of screw clamp technology from terminal blocks enhances fast and efficient wiring.'
According to Arnold Offner, interface group manager at Phoenix Contact (Harrisburg, Pa.), 'Customers now insist on touch-proof designs that require no additional soldering, but rely on screw connections. Power supplies now come with conformal coating, ac and dc input capability, and compatibility with worldwide variations in line voltage. Also, UL 508 Listing has become an important criterion for designers who wish to run the system at maximum rating.'
Mr. Offner's point about UL 508 regards the code that unlisted power supplies must be derated 50% in a system. By achieving UL listing, manufacturers allow users to employ smaller units in the application. This is another cost and space saving measure.
The point made about touchproof designs leads to another sought-after feature-safety. Touchproof designs predominate in terminal blocks and motor starters by preventing accidental touches of line voltages during troubleshooting. Of course, just moving to 24-V dc as a standard control voltage also promotes safety.
Matt Polk, Cutler-Hammer (Westerville, O.) product manager, notes another feature-the ability to source large outrush current requirements. He continues, 'These would be associated with pulling in motor starter coils. It also works well in starting up DeviceNet network systems.' (See CE , July '99, p. 9, for a discussion of soft start motor control with 24-V dc control.)
Rick Frosch, Lambda (Melville, N.Y.) chief engineer, reports, 'The biggest thing is surge standards. A power supply must be robust enough to handle line surges. IEC 61000-4 specifies the standard test. In addition there is VDE 0160, a German specification much tougher than IEC. It is not simple to meet that spec, but we are designing for it. The power supply must handle inductive spikes generated by very large motors or generators.
'Another power-related specification is power factor correction (PFC). This refers to the requirement to make the current and voltage sinewave synchronous. IEC 61000-3-2 defines a specification for 75 W and above. I expect 50 W units to fall under its jurisdiction in the future.'
Power factor correction essential
Omron Electronics' (Schaumburg, Ill.) product marketing specialist, Roy Ortiz, adds, 'PFC limits the harmonic current on the input side of the power supply, making the input 'cleaner' and more sinusoidal. PFC satisfies EN 61000-3-2 and, in turn, reduces peak current demand. Most switching power supplies create line harmonics whenever the line current is not sinusoidal. Harmonic currents do not contribute to load power, but cause unwanted heating.'
Another standard pointed out by Mr. Ortiz is National Electrical Code (NEC) Class 2 power source. 'This term used by NEC defines a safe power source, one that has limited energy (for instance, a maximum of 24 V at 4.166 A). The advantage of a Class 2 power supply is that if you have any devices, like sensors, bar code readers, etc., that are not UL approved and are connected to a Class 2 source, the UL inspector will not have to require additional tests on these devices. This feature can also be an advantage on DeviceNet applications, since most DeviceNet applications use small gauge wires.'
'An often misunderstood factor is the expected life of the power supply,' contributes Mike Blanchfield, director of marketing for Automation Systems Interconnect (Carlisle, Pa.). 'While factors such as average load rate, vibration, and ambient temperature affect power supply life, a key issue is heat generated by internal components. Since end conditions cannot be known, manufacturers calculate MTBF. In every case, it is limited by the internal electrolytic capacitor. While the rest of the power supply may have an MTBF of 100,000 hours or more, typical electrolytic capacitors have 30,000. Know the derivation of MTBF calculations. It is best to compare products based on MIL-HDBK-217E.
'This is another important consideration when comparing whether the power supply was rated at full load.'
Power supplies communicate
In these days, when most Control Engineering articles include communications, will there be power supply communications? 'Yes,' says Weidmueller's (Richmond, Va.) product specialist, Al Stewart. 'We have fault relays to signal problems, as well as analog outputs to signal output voltage, load current, and internal temperature. These can be used to predict failure.'
Other important features, according to Mr. Stewart, include parallel operations that increase available current, load sharing, and redundancy. In this case, it is best if the power supplies connected in parallel can adjust their outputs to maintain zero current imbalance. This feature must be designed in. Connecting Weidmueller 'Load Share' terminals enables this feature, he says.
Even this often hidden, yet essential, control component has hidden complexities that, if exploited by the alert control engineer, can provide many ways to enhance overall control system design.
Power Factor Correction in small power supplies
Power Factor Correction (PFC) is rapidly becoming a common feature in smaller control power supplies. European Union initiatives drive this development by demanding power be corrected at the load as an integral part of the system.
Tom Brooks, design and development vp at Taiyo Yuden (U.S.A.) (San Marcos, Calif.), discussed this technology with Control Engineering .
Q. What are some key technology issues in power supply design?
A. PFC is becoming increasingly important. Power has always been enormously price sensitive, and as little as a few cents difference per watt can determine whether your power supply will be spec'd or not. PFC is also becoming cost effective at all levels. Not all suppliers are opting to comply at this time.
Q. Why not?
A. The key factor slowing progress is the confused state of regulatory affairs. EN61000-3-2 has been a moving target ever since its establishment in 1995. Due to constantly changing content and compliance dates, the so-called Harmonic Current Regulation is causing much confusion both here and abroad.
Q. What are some technical issues impacting PFC in low-end power supply design?
A. All PFC approaches are either active or passive. The latter uses inductors, capacitors, and other passive components. It is desirable for low-power, cost-sensitive applications. While it's reliable, cheap, and efficient, the passive approach is heavier and less compact than active, which is also finding favor due to superior performance. Generally speaking, the preferred approach is passive or single-stage active below 75 W, single-stage active with range select from 75-200 W, and dual-stage active above 200 W.
Q. What are the relative merits of single- versus dual-stage active approach?
A. Single-stage looks promising at 150 W and below where it is fairly easy to achieve low current and high voltage. The trick is to design low output voltage/high output current converters that are almost as efficient as the high voltage converters and work over the entire input voltage range of 85-265 V ac. Unfortunately, most existing technologies require either high-cost, high-voltage components or special input voltage selection circuits.
Q. What about dual-stage?
A. Dual-stage techniques provide a method of current shaping or forcing the input current to follow input voltage. This requires one converter (usually Boost) to control input current and another (usually Buck) to regulate output. This is not economical at low power ranges.
Q. Are there any other techniques?
A. Active power filtering is another dual-stage conversion technique. It uses harmonic or current injection to achieve PFC. Unlike boost and single-stage converters, which process all power presented to the converter because they are in series with the ac line, active power filtering is parallel to the line. Through a method of measuring harmonic distortion, currents are injected to cancel reactive loads.
Reduce power consumption of electronic products not in use
The fastest growing segment of power consumption is electronic products not in use. Energy experts estimate approximately 5% of residential electricity consumption is wasted by 'standby' use. Many devices use standby power to maintain signal reception capability, monitor conditions, power internal clocks, charge batteries, and display information.
There was hesitation to reduce the standby requirements of a device because providing very low power levels with a main power supply resulted in greater losses of efficiency. Main power supplies cannot efficiently deliver a typical standby level of 0.25 W power.
Bias' low-power, standby switching power supply packs 0.25 W in a single-component 1 in.3 package. The 50% efficiency is above traditional standby methods and powers CMOS processors for appliances, remote processing, and other electronic circuits.