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You might ask, what do medical power supplies, SCRs and high-voltage relays have in common? And the answer is they were all products covered in the August 1975 issue of Solid-State Power Conversion, the predecessor of Power Electronics Technology. As in the first installment of “Then & Now” that appeared last month, the three products highlighted in this installment provide a glimpse of the state of the art as it existed 30 years ago. We'll then look at currently available products for perspective on where each technology is at today.
Supplies Generate Minimum Magnetic Field
Although designed specifically for use in diagnostic, patient monitoring and therapeutic equipment, these new supplies provide features that make them a viable alternative to in-house design of power systems for ultra-sensitive loads. Featuring leakage current of less than 4 µA and special shielding to limit the magnetic field strength, five models comprise the new ME Series: A single output rated at 5 V @ 4 A; Two dual-output models rated at ±12 V @ 1.2 A and ±15 V @ 1 A; Two triple-output models rated at 5 V/±12 V @1.2 A; and 5 V @4 A/±15 V @ 1 A. Regulation is specified at 0.1%. (Single output: $150/10-24 pieces; 60 days)
Solid-State Power Conversion, August 1975, p. 49
Medical Power Supplies
“Thirty years ago, there were no specs for medical power supplies,” says Richard Mentelos, president of Ram Technologies (Guilford, Conn.). There were only a few standards extant at that time, but they were not really accepted by anybody, he says.
Around 1974-1975, UL 544 was introduced in the United States, and Europe began working on IEC 601, but the latter did not come into being until the 1980s. The current versions are IEC 60601-1 and IEC 60601-2, and in the United States there are corresponding UL specifications.
Ram Technologies specializes in a niche market within the medical power supply area, in PC-based medical products. They manufacture medical power supplies that are like an ATX power supply but are medical grade. In its designs, Ram employs the latest in active power-factor correction, as well as synchronous-rectification and distributed output, with all the outputs highly regulated. Their power supplies exhibit very low noise and are designed for extremely high reliability.
One would certainly expect medical power supplies to have far more stringent test requirements than power supplies slated for nonmedical applications — information technology (IT) applications — such as computers, scanners and data-communications equipment. And indeed, that is the case. In the IT sector, the maximum leakage allowed is 3.5 mA, but for medical power supplies it is 300 µA, more than 10 times lower. (That is in the United States; in Europe it is slightly higher at 500 µA.)
The story is similar with high potting. Normally IT power supplies are high-potted at 3000 Vac, but power supplies intended for medical equipment must survive 4000 Vac. That is between the primary and secondary of the power transformer, making sure that the output is well isolated from the primary power source. And there is also a high-potting requirement from the primary to ground.
“This is tough for us, given that these days power supplies are smaller than they have been in the past,” notes Jason Bandivas, a project engineer at Globtek (Northvale, N.J.). “Smaller supplies mean smaller components sitting closer to each other, side by side, so that the likelihood of arcing during the high-pot test is higher,” he adds.
And smaller medical power supplies delivering more power is clearly the trend these days, says Bandivas. Before, 50-W to 65-W supplies with a 3-in. × 5-in. footprint were the industry standard, but today supplies with the same footprint deliver 150 W to 180 W. The height is typically less than 1 U (1.35 in.).
Globtek recently introduced a series of medical/IT-grade 65-W power supplies that delivers outputs from 3.3 V to 48 V. Although designed for medical purposes, it can be used for IT and thereby deliver the benefits of low leakage and stiffer high-pot ratings — regardless of application.
Series E relays are heavy-duty, air-insulated types capable of long-term use at currents and voltages from 1 mA to 100 A, 12 kV to 300 kV peak. Designed for use in OEM high-voltage power supplies, these units will sustain long, reliable operation under stress conditions such as high-current pulse circuits, momentarily accidental overcurrents or short circuits. (From $59/1 piece; stock-16 weeks)
Solid-State Power Conversion, August 1975, p. 53
From the size of a shoebox then to the size of an apple today: That's how much some high-voltage relays have diminished in size over the last 30 years, according to Bernard Bush, engineering manager of the Kilovac product team of Tyco Electronics (Harrisburg, Penn.).
Back in the 1970s, these were open-air relays. While still in production, newer designs using epoxy to seal the relays in pressurized gas are much smaller and provide higher performance at competitive prices. Originally, glass-to-metal sealing techniques devised in the early part of the 20th century for vacuum tubes were used to miniaturize high-voltage relays. Sealing the contacts and actuator mechanism in a vacuum significantly increases resistance to arcing while preserving the unit from environmental degradation. For example, a 0.001-in. gap may stand off 20 V to 50 V in air before arcing occurs. In hard vacuum, the standoff increases to over 1000 V. In addition, the oxygen-free environment maintains low and stable contact resistance without requiring the expensive and sometimes toxic silver alloys required for air designs.
Later, ceramic-to-metal seal designs developed, which improved envelope durability and allowed for high-volume production designs using a stacked ceramic/vacuum braze approach. Further improvements were made using pressurized gases to achieve high-voltage dielectric strength. Tyco Electronics manufactures complete lines of high-voltage glass and ceramic relays using both pressurized gas and vacuum. Over the past 10 years, many of its newer high-voltage relays have been produced using epoxy-plastic enclosures. Epoxy sealing has been a boon for 500-Vdc relays. It is less expensive than glass or ceramic, and allows much greater design flexibility with significantly reduced development times. Tyco Electronics says it is the only manufacturer that can employ this plastic-epoxy manufacturing technique because it is a patented process the company owns.
A key application for ceramic high-voltage relays is in high-frequency communications equipment, such as in switching inductor taps in antenna tuning units. In medical fields, high-voltage relays are crucial in heart defibrillators and kidney lithotripters (which are employed to dissolve kidney stones), and in power supplies for magnetic-resonance imaging. As for the plastic-epoxy enclosed relays, they are in demand in the automotive industry for fuel-cell and hybrid vehicles, as well as forklifts, generators, golf carts and welding equipment.
Yet another way of constructing high-voltage relays is to employ what is called “stacked ceramic.” This construction technique is employed by Gigavac (Santa Barbara, Calif.). Here is how Pat McPherson, Gigavac's vice president of marketing and new ventures, explains it: Think of the body of an ink pen made of ceramic — slice off a series of rings and place them between flexible sheets of copper material. While in a vacuum, braise the copper to the adjoining ceramic rings. It is the flexible nature of the copper diaphragm that allows a rod brazed inside the capsule to move and make contact. Add an external insulated rod to the outside of the switch and move it with a relay armature, and the contacts will open and close the contact pair that is sealed in the vacuum. There you have it: a small, inexpensive stacked ceramic high-voltage vacuum relay for RF and dc switching that can be made in large batches.
One of the markets that Gigavac serves are the manufacturers of vacuum-deposition equipment employed in the fabrication of semiconductor wafers. These manufacturers were seeking very high-voltage, very high-current relays that were reasonably priced. Because of Gigavac's low cost of manufacturing, such relays are priced approximately 30% below previous competing relays that had less current-carry capabilities. Gigavac also makes pressurized SF6 gas-filled relays rated up to 70 kV that are used in ESD test equipment, power-supply polarity reversal for DNA analyzers and heart defibrillators.
Then Fast SCR Operates to 10 kHz
Capable of reliable operation at switching frequencies up to 10 kHz, these new units feature guaranteed turnoff times as low as 10 µs, and are rated for continuous operation to 1200 V and 500 A. Maximum peak one-cycle surge current is 7500 A. Especially suited for use in high-frequency power supplies, inverters and choppers, these new units achieve their remarkable speed through use of a divergence gate configuration, in which the gate is distributed over the surface of the silicon strip in an interdigitated arrangement. As a result, when a turn-on voltage is applied, the entire strip is quickly affected by the “fingers” of the gate (by contrast, conventional SCRs use a gate located at a single point on the periphery of the chip; the turn-on signal must then propagate in wave-like fashion until the entire chip is conducting). Forward voltage drop for the new SCRs is an impressive 1.3 V at 500 A. (From $83.60/10-99; stock)
Solid-State Power Conversion, August 1975, p. 52
They block thousands of volts in the off state yet conduct thousands of amperes in the on state. From its humble beginnings as a small stud-mounted device able to handle just a few hundred volts and perhaps 30 A in the 1960s, the voltage and current-carrying capabilities of thyristors have skyrocketed to 12 kV and 400 A, according to Eric Carroll, marketing manager of the semiconductor department at ABB Switzerland Ltd. (Zurich, Switzerland).
Many of us are familiar with one member of the thyristor family: a single PNPN device that is generally known as a silicon controlled rectifier (SCR). Connect two SCRs back to back, and you have the ac version that RCA introduced as the triac over 30 years ago.
Early on, they could be employed in phase-controlled rectifiers, but their turn-off time was too slow for them to be used in inverters. During the 1970s and 1980s, efforts were made to speed up the thyristor. A second, commutation thyristor was added that sends current in the reverse direction to cancel the load current so that it falls to zero, because a thyristor cannot turn itself off, thereby enabling it to recover its blocking capabilities at the end of each conduction cycle.
ST Microelectronics' (Geneva, Switzerland) engineers say one of their big success stories was supplanting electromechanical relays with triacs in cycling compressors in refrigeration units. This raised life expectancy from 200,000 cycles to 600,000 cycles — not surprising, really, when you stop to think that a triac has no moving parts.
The evolutionary trend in thyristors has been toward smaller devices and toward integrating what had been external circuitry. According to Arnaud Edet, product marketing engineer at ST Microelectronics, their customers urged them to integrate more of the ancillary environment into the package that houses the basic device. This meant integrating the external protection devices, power-management section, level-shifting and diagnostic data transfer back to a micro-controller. So indeed, what once had been a simple monolithic thyristor was transformed into a very smart switch!