Medical Electronics Manufacturing Fall 1998
Electronics manufacturers who comply with global safety requirements are developing new, more powerful types of high-performance power supplies.
By complying with stringent international regulations, U.S. and Canadian electronics manufacturers are producing new types of power supplies that are not only safer, but capable of delivering more powerful performance benefits. Among these exciting developments are soft-switching power conversion technology, new internal design strategies, and power factor correction circuitry.
International standards are helping drive the development of advanced types of high-performance power supplies for technologies such as soft switching and power factor correction circuitry.
In the past, many North American manufacturers of programmable power supplies limited their design efforts to meeting the needs of domestic markets and ignored foreign regulatory requirements. Because the largest markets for programmable supplies were in North America, the continent's manufacturers had little incentive to meet the needs of the international market. As a result, products distributed outside the United States often required extra filtering and testing following their installment in a complete system. Projects were delayed and additional space was allocated in the equipment racks to hold unanticipated filters.
By the early 1990s, however, the world electronics market had grown to such a size that North American electronics manufacturers could no longer ignore international requirements. Canadian and U.S. power supply manufacturers, who had often complied with just one national standard and typically had carried only North American certification such as Underwriters Laboratory or Canadian Standards Association (CSA), were forced to upgrade their products to international standards to compete in a global market.
Of particular importance to manufacturers was the newly formed European Union (EU) and its unified set of product standards designed to simplify and promote intra-European trade. The new CE marking scheme comprised various directives applicable to power supplies and permitted manufacturers complying with the standards to distribute their products freely. North American manufacturers began to implement the more stringent European standards and, for the first time, were able to accommodate higher ac line voltages used in most other parts of the world.
But these manufacturers benefited from more than a broader marketplace. Suddenly, as a result of international regulatory compliance, they found themselves creating safer products that also offered dynamic new capabilities.
Soft-Switching Power Supplies
When the Electromagnetic Compatibility (EMC) Directive became mandatory on January 1, 1996, many North American power supply firms suddenly found themselves with older, noncompliant switch-mode products that could not be sold in Europe.
The directive places legal requirements that are much more stringent than the previous FCC part 15 requirements on companies selling electronic products in the EU and mandates that electromagnetic energy radiated and conducted on ac power lines stay below the limits defined in EN 50081-1 and EN 50081-2. The electronic products must also be able to withstand electromagnetic effects such as high-voltage discharges and operation in radio frequency (RF) fields as defined in EN 50082-1 and EN 50082-2.
To overcome this marketing hurdle, manufacturers needed to lower the EMI generated by the units. In some cases, they made the change by simply adding some new filtering stages to the ac input and dc output where space allowed. Often, however, they needed to introduce new product designs that eliminated the majority of the noise at its source.
Figure 1. High-frequency output noise on supply output. Top trace: Traditional hard-switching design (up to 300 mV pk-pk). Lower trace: CE-compliant soft-switching version (less than 100 mV pk-pk).
To do so, manufacturers commonly employed a form of resonant switching in the main power transistors that reduced the actual energy being switched by the active device to nearly zero. This current- or soft-switching technique greatly decreased the unwanted high-frequency voltage and current transients that supplied much of the RF noise radiated and conducted out of the power supply (see Figure 1).
Soft-switching techniques also eliminate the power loss that typically occurs as the main power transistors change from a conductive to a nonconductive state and vice versa. By reducing the amount of wasted power, manufacturers often improve the efficiency of a unit by approximately 2%, a savings of more than 20 W in a 1000-W power supply. Without soft switching, the energy saved would be dissipated by the main power switches—often metal oxide silicon field-effect transistors or MOSFETs.
These transistors are the most critical components in any switch-mode unit because a power supply can no longer operate if one of its MOSFETs fails. The components perform the high-power, high-frequency switching within the power supply and allow the supply to control and regulate the dc output. They typically are the most stressed devices in a switch-mode power supply and the ones most likely to fail because of problems elsewhere in the supply.
Hence, a CE-compliant power supply that uses soft switching operates more efficiently because the thermal and electrical stress on the MOSFETs is reduced. This decrease in stress provides the MOSFETs with a greater operating margin, thereby reducing the likelihood that they and the power supply will fail. The CE-compliant unit with its enhanced filtering and better circuit protection is also less susceptible to external sources of EMI. So, not only does a soft-switching, CE-compliant power supply generate significantly less electrical noise, it achieves greater efficiency, longer mean time between failures (MTBF), and higher immunity to other equipment operating nearby.
Internal Design Strategies
The power supply design changes that have been driven by the Low Voltage Directive (LVD) since its implementation on January 1, 1997, relate to operator safety (EN 601010-1). While these changes are most obvious on external connectors and protective covers that are readily visible, there have been many internal changes as well.
Thicker insulation and greater component spacing help to reduce stray currents that can cause an operator shock. These alterations also help to ensure that other equipment driven by the power supply is protected from stray currents flowing through ground paths. If a fault occurs within the power supply, these safety precautions still protect the operator and the attached equipment from possible damage.
Figure 2. Ac input with safety strain relief cover, a typical feature of CE-compliant power supplies.
The style and implementation of the ac input and output power connectors also benefited from innovative design strategies. Previously, these were often exposed-screw terminal strips or exposed bus bars that required the user to be diligent about adding insulating covers or installing the whole unit in an extra enclosure. The new, fully compliant designs, however, used improved connector systems that prevented accidental contact through the use of plastic shrouded connectors; connector covers with cable-strain relief clamps were also featured (see Figure 2). Thus, the CE-compliant units not only posed much less risk to the operator than ever before, but they also greatly reduced the overall power supply installation and setup time.
Power Factor Correction Circuitry
Power supply manufacturers will soon have to comply with IEC 1000-3-2, one of the final CE requirements that will probably be instituted in the first years of the 21st century. The regulation 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 that overall power losses in the ac distribution system are reduced.
To comply with this standard, manufacturers may employ active power factor correction (PFC) circuitry in the ac input section of the unit. This circuitry, which is 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. This yields an input power factor of very close to 1, like an ideal resistive load (e.g., a light bulb), so that nearly all of the current is drawn at the fundamental line frequency and is 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 0.70 and draw current in very high narrow peaks, causing heavy distortion of the line voltage and requiring an ac line that is rated to supply in excess of 30% more current than for the PFC-equipped unit. Obviously, there are some benefits to using a unit with a near-unity power-factor-corrected input (see Figure 3).
Figure 3. Input current for non-PFC unit (top trace) and for a PFC-equipped unit (lower trace). The non-PFC unit draws 27A pk, while the improved, PFC-equipped version draws only 17A pk at the same output power level (vertical scale is 10A/division). Note the clean sinusoidal waveform for the current drawn by the PFC unit.
Although this standard is being implemented largely to aid the power utilities, there are some benefits for manufacturers as well. A near-unity power-factor input greatly reduces the input current because nearly all the current flowing is working within the power supply. For example, a 1000-W-rated supply without power-factor correction running off a 120-V-ac 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 side benefit of implementing a typical power factor correction circuitry is the unit's ability to operate off a very 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 complications from voltage range selection. It also eliminates the need to order a special product configuration.
A power-factor-corrected unit will comply with the proposed regulations, minimizing impact on other electronic equipment on the line. In addition, it will simplify setup time by allowing the use of standard power outlets and eliminating concerns over setups for a particular line voltage.
International regulations have made power supplies more productive and capable of delivering more impressive performance. Manufacturers based in North America as well as the Asia-Pacific region are already reaping the benefits of these products, which are more reliable and efficient, and easier to set up and use. Moreover, internationally compliant power supplies are more likely to work as expected—right out of the box and with few surprises—anywhere in the world.
Mark Edmunds is vice president of engineering for Xantrex Technology, Inc., a manufacturer of programmable dc power supplies, based in Burnaby, British Columbia.