A SHORT HISTORY OF THE EVOLVED POWER SUPPLY
Address by Frank Toich, Sales Manager, at Kepco's 50th Anniversary Celebration
1926 Ad for Motorola Battery Eliminator
The power supply industry dates back to the early 1920s, when crude devices were first developed to serve as "B" battery eliminators to power radios in both the commercial and consumer markets.
The market for separate power supplies evaporated around 1929, when most radios manufactured included a built-in power supply. The need for stand-alone power supplies remained relatively small in the 1930s and into the 1940s. The dominant technology during this period consisted of vacuum tube linear regulators.
Power supplies used vacuum tubes for both the power and control elements. Typically, a voltage regulator (VR) tube, the predecessor to today's zener diodes, was used to produce a stable reference. Control was pretty much limited to the manual twisting of knobs. In those days we did not care too much about dissipation. Under normal circumstances, vacuum tubes ran pretty hot -- and unless the plate of the tubes glowed red, or glass started to melt, no one worried much about it.
Model 700 Vacuum tube power
supply, 0-350V, 0-750mA
In the mid 1940s, three companies set up shop in a relatively obscure community in Queens, New York. These companies, who eventually became leaders in the industry, were Lambda, Sorenson and Kepco. While all three companies exist today, only Kepco maintains its independence and original ownership and continues to operate out of Queens, New York.
Early Kepco Laboratories logo
A milestone in the industry occurred in the 1950s when semiconductors were first introduced into the power supply design. As semiconductor designs proliferated in the market (transistors replaced tubes), concerns about dissipation and heat dominated the thinking of power supply designers. germanium transistors did not have the ability to glow in the dark, as did tubes, they simply melted and quit. Designers of these products suddenly had to take their thermodynamics seriously.
Kepco Type SC, the first "transistorized" power supply
Products using transistors were limited to low voltage models at modest power levels or hybrid designs which used semiconductors in the control circuit and vacuum tubes in the power stage to make possible higher voltage products. In the 1950s, and early 1960s, power supply products adopting Mag-Amp technology satisfied those applications requiring substantially higher power.
Kepco Type KM, A Mag-Amp Design
This same time period also brought us the concept of the first remotely programmable power supplies. A pioneer in this field was Dr. Kenneth Kupferberg, one of the founders of Kepco, who, in his career, was credited with 14 patents.
In the 1960s, the world was still analog. Computers were still in their early phase of development. The big debate focused on analog computing [op amp control for simulation and modeling] and that strange concept, called digital computing. In this time frame, linear series pass power supplies were seen more as power amplifiers than a power source. This amplifier concept exploited the high gain and linearity of the transistors and created what were, in effect, high power operational amplifiers. As op-amps, they were made to scale, sum, integrate, or manipulate signals. To accomplish this, power supplies were being produced which allowed access to all of the control nodes. Both input and feedback control elements could be removed and substituted by the user to permit manipulation of the output to satisfy many diverse applications.
The 1960s also saw the introduction of true bipolar (four quadrant) source/sink units, and the concept of ferroresonance for correction of source voltage variation in a highly reliable, low parts count package.
Kepco Model BOP, Featuring Bipolar 4-Quadrant Power
(See BOP specifications)
In the 1970s, an energy crisis, which affected the entire industrial world, provided the switching power supply with an opportunity to re-surface and establish a significant position in the electronic marketplace.
The design and manufacture of switching power supplies can be traced back at least to the 1950s. At that time, these products were produced in huge quantities, mostly to replace vibrators. In those days, vibrators converted an automobile's 12V into high voltage d-c by mechanically switching (the first switch-mode power supply)! Later, germanium transistors were used to switch electrically.
The fundamental problem, which inhibited the advancement and greater use of this topology, was its relatively low frequency range (within the mid-audio spectrum) which caused these products to whistle annoyingly.
The big breakthrough in the 1970s was the development of low loss ferrite (transformer core material), coupled with the readily available, higher speed silicon transistors that made possible the practical reality of high frequency products which could operate above 20KHz where they were inaudible.
During this same decade, the high-gain series pass linear power supply was enhanced with a new level of intelligence, the ability to follow commands from a host computer on a standard communications bus.
Digital control was being grafted onto the front end of linear power supply products. The very first interfaces consisted of resistor chains that were parallel with reed relays, to create BCD Digital control. Then came digital to analog conversion [DAC], for voltage control, and finally, in mid-decade, the power supply industry adopted the instrumentation bus standard introduced by the Hewlett Packard Company as HPIB. This was adopted as IEEE-488 by the Institute of Electrical and Electronic Engineers, and later renamed GPIB by Instrumentation Manufacturers. Prior to this industry standard, the industry was limited to the RS232 serial bus which was very slow and restricted to relatively limited distances between controller and instrument.
In Europe, this is known as the IEC bus.
The 1980s saw many new start-up companies enter the market producing switch-mode products. Many of these new companies were based in the Pacific Rim, first in Japan, and eventually shifting to Taiwan and Hong Kong.
During this decade, the quality and performance characteristics for switchers were substantially improved. Operating frequencies also increased from the 25-50KHz range, on up to 100KHz and even 1 Megahertz as FET's replaced bipolar transistors.
Here we are now, more than half way into the 1990s, and we have already experienced numerous developments. For example, this industry, driven by market demands, has produced switching products which operate at increasingly higher frequencies and are constructed utilizing surface mount technology (SMT), substantially reducing their physical size. We have seen these same products offering such features as wide range input, to accommodate source voltages worldwide, active power factor correction, to minimize harmonic distortion on power lines, and forced current sharing, to provide these products with the capability of fault-tolerant operation.
Kepco Model HSP, N+1 Redundancy with Hot Swap
(see HSP Specifications)
Modern fault-tolerant power systems typically employ a technique known as parallel N + 1 redundancy. The advantage of this method over the traditional paralleling scheme, is the ability to distribute power (current sharing) and minimize the stress on individual units. The popularity of the N + 1 redundant system approach with current sharing has increased so rapidly it has become a de facto standard in the industry.
Kepco Model VXI-27, a VXI
interface drives up to 27
remote power supplies
(See VXI-27 specifications)
Another trend which has enjoyed increased interest, is that which is sometimes referred to as point-of-use stabilization; distributing the power at some intermediate voltage (48V, 150V, 400V). This technique is also known as "distributed power." It relies on the use of a bulk supply to perform the conversion of a-c, from the mains, into d-c, which then, in turn, powers any one of a number of lower power d-c to d-c converters placed directly at the point of load. This technique of power distribution has lowered the system wire count resulting in more manageable harness sizes making the products easier to build and reducing their overall size.
Instrumentation power supplies now interface with the IEEE 488.2 bus, support VXI and embrace various soft-panel architectures.
What's on the horizon for the next phase of the power supply evolution -- Stay Tuned!
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Vol.7, No.1