With the advent of the harmonized CE safety and electromagnetic compliance standards in the European Union (EU) during the last several years and their general adoption worldwide, there now are spin-off benefits to the end users of CE-compliant programmable DC power supplies. Manufacturers of these EU-compatible products now offer supplies that are more reliable, safer, and easier to use and have lower output noise.
While the older UL and CSA safety certifications from the United States and Canada ensured that products designed for use in North America were safe and had been tested to the national standards of these two countries, they were generally less stringent than European requirements. Additionally, the FCC Part 15 J Class A or B limits on both radiated and conducted RF noise meant that high-frequency switch-mode power supplies contained some noise-filtering circuitry. Unfortunately, FCC-compliant units often did not comply with the more stringent CISPR 11 limits in effect in Europe.
For more than 10 years, nearly all new programmable DC power supplies rated at over 100 W have been based on high-frequency switch-mode designs. Typically, U.S. and Canadian manufacturers designed and tested their products to the dominant American standards and tended to leave accommodation of the stricter European regulations to the end user.
Often, this required extra filtering and more testing once the product was installed in a system. This resulted in project delays and the allocation of space in equipment racks to hold unplanned-for filters.
Improving Efficiency and Life Spans
The EMC Directive ensured that the electromagnetic energy radiated and conducted on the AC power lines was below the limits defined in EN 50081-1 and EN 50081-2. Now, products also must withstand external electromagnetic effects such as external high-voltage discharges and operation in RF fields listed in EN 50082-1 and EN 50082-2.
Since these standards are more stringent than the U.S. FCC Part 15 requirements, many North American power supply companies suddenly found themselves with older switch-mode products not in compliance. Consequently, they were unable to sell these products in the European market.
Overcoming this hurdle required a considerable engineering redesign effort to lower the EMI generated by the units. In some cases, this change was as simple as adding some new filtering stages to the AC input and DC output where space allowed. Often, though, the best plan was to introduce product designs that eliminated the majority of the noise at its source.
Traditionally, high-frequency switch-mode supplies, which rely on generating an AC waveform in the range of 100 kHz to 200 kHz to drive the main power transformer, have used power transistors to hard-switch the unregulated input voltage. This means that a transistor turning on will have the whole raw input voltage across it, typically in the range of 350 V, as it changes state.
During the actual switching interval, there is a finite period as the transistor begins to conduct when the voltage begins to fall at the same time as current begins to flow. This simultaneous presence of voltage across the transistor and current through it means that power is being dissipated within the device during this period. A similar event occurs as the transistor turns off with the full current flowing through it (Figure 1).
Designers using a hard-switching topology are in a no-win situation when it comes to reducing this wasted power while still meeting the EMC Directive. As the switching period is reduced through the use of improved driving circuitry, the faster rise and fall times generate more high-frequency energy that is radiated and conducted out of the unit as unacceptable RFI. In this way, older, hard-switching topologies are a compromise between electrical efficiency reduction and EMC trade-offs.
More recently, power-conversion topologies have been developed that simultaneously reduce the power dissipated by the main power transistors during the switching interval and nearly eliminate much of the RF energy generated. The most common technique uses a constant-frequency resonant switching scheme which reduces the actual energy being switched by the active device to nearly zero.
This method, commonly called zero-voltage switching or soft switching, combines the parasitic output capacitance of the power transistors (typically MOSFETs) and the parasitic leakage inductance of the power transformer as a resonant circuit. With this resonant circuit, the output inductance, the parasitic drain-source body diodes of the MOSFETS, and an appropriate switching sequence allow the voltage across each transistor to swing to zero before the device turns on and current flows.
Likewise at turn-off, the voltage differential across the transistor swings to zero before it is driven to the nonconduction state. With this scheme, current is flowing through the transistors only when they are fully on and doing useful work transferring energy to the output of the supply.
The power dissipation within the transistor that would normally occur during the switching interval has effectively been eliminated (Figure 2). Unwanted high-frequency voltage and current transients during the switching period—the culprits that supply much of the RF noise radiated and conducted out of the power supply—also are dramatically reduced due to the smooth resonant transition. With the noise effectively reduced at its source with this sort of scheme, enhanced filtering at the input and output of the unit ensures that the unit is well within the noise limits set by international standards (Figure 3).
With these soft-switching techniques, the reduction in wasted power often will improve the efficiency of a unit by more than 2%. While this does not sound significant, it can account for a saving of more than 20 W in a 1,000-W power supply.
This 20 W is all the power that would have been dissipated by the main power transistors, the most critical and most heavily stressed semiconductors in any switch-mode power supply. Reducing the power here lowers their junction temperature and increases thermal operating margins and the life of the whole power supply.
So not only does a soft-switching, CE-compliant power supply generate significantly less electrical noise, it also achieves greater efficiency, longer mean time between failures, and higher immunity to the effects of other equipment operating nearby.
Improved Safety, Reduced Personal Risk
The Low-Voltage Directive portion of the CE requirements came into full force on Jan. 1, 1997. For power supplies, this means compliance with the EN 601010-1 safety standard. While products designed to meet older North American standards normally were safe, the requirements of this new standard place the power supply user at even less risk than before.
Many of the regulations impact the internal design of a power supply. And while they do increase safety through the use of thicker insulators and greater clearances in high-voltage circuits, the changes are not obvious to the user.
The most useful new safety feature is the style and implementation of the AC input and output power connectors. In the past, these often were exposed-screw terminal strips or exposed bus bars which required adding insulating covers or installing the whole unit in an extra enclosure. New, fully compliant designs use improved connector systems that prevent accidental contact through the use of plastic-shrouded connectors and connector covers featuring cable-strain relief clamps. The CE-compliant power supply poses much less risk to the operator than ever before while reducing the overall installation and setup time.
Power Supplies That Work Anywhere
One of the last CE requirements coming into effect (likely in the first years of the 21st century) is IEC 1000-3-2. It will specify limits on the harmonic currents that may be drawn by line-connected industrial equipment. These limits will help ensure that
cleaner power is available to all equipment connected on the line and reduce the overall power losses in the AC distribution system.
The most common means by which power supplies comply with this standard is using active Power Factor Correction (PFC) circuitry in the AC input section. This circuitry, essentially another power converter in series with the main power supply, forces the unit to draw current at a level that closely tracks the sinusoidal shape of the line voltage (Figure 4). Doing this gives an input power factor of very close to 1, so that nearly all of the current is drawn at the fundamental line frequency and perfectly in phase with the line voltage.
A typical noncompliant, single-phase input switch-mode supply may have an input power factor in the range of 0.65 to 0.7. It draws current in very high, narrow peaks; causes heavy distortion of the line voltage; and requires an AC line rated to supply in excess of 30% more current than for the PFC-equipped unit. Obviously, there are some benefits to having a near-unity power-factor-corrected input.
While this standard is being implemented largely to aid the power utilities, again there are some benefits for the user. With a near-unity power factor input, the input-current requirements are greatly reduced—nearly all the current flowing is doing work within the power supply.
For example, a non-power-factor-corrected, 1,000 W-rated supply running off a 120-VAC line will draw up to 16 A, clearly in excess of a standard 15-A distribution line rating. The same unit with PFC will draw only about 11 A, well within the capability of the same 15-A line.
Another feature often available as a side effect of a typical PFC circuitry implementation is the capability of the unit to operate off a wide range of AC input line voltage, typically 85 to 264 V. This means a single unit can be used virtually anywhere in the world with no need for voltage-range selection.
The Winners
The international standards that programmable power supplies must meet before they can be used in Europe have resulted in impressive performance gains. These improvements, intended for Europe, have delivered valuable side benefits for power supply users in North America and throughout the Asia-Pacific region.
CE-compliant power supplies are likely to be more reliable, efficient, and easier to use and set up. They should work as expected—right out of the box, with few surprises, anywhere in the world.
About the Author
Mark Edmunds is vice president of engineering at Xantrex Technology. He joined the company in 1983 after receiving a degree in electrical engineering from the University of British Columbia and has held his current position since 1993. His professional experience includes several R&D collaborations with the electrical engineering departments at UBC, the University of Victoria, and the University of Illinois. Xantrex Technology, 8587 Baxter Place, Burnaby, BC V5A 4V7, (604) 415-4600, e-mail: [email protected].
Copyright 1999 Nelson Publishing Inc.
May 1999