Digital Influences on Power Supply Design
Design complexity is driven by consumer demand. If you offer bells and whistles, then more bells and whistles are demanded.
Evolution of Digital Power Supply Design
Back in the 1950s and 60s, consumer electronics were practically non-existent; telephones were mechanical, washing machines were a motor and a clockwork timer and a basic car would probably have no “electronic” circuits at all – unless you stretched to a radio perhaps. (Arguably, the ignition coil could be considered an early boost converter?).
The advent of television was a major influence on electronics – to generate, control, transmit and decode even a black-and-white signal was enormously complex and was all designed with analogue electronics. Incredible advances were made in analogue design but TV also pushed the early switched-mode designs in power supplies – notable the flyback supply, so named as it caused the electron beam on the screen to flyback across the screen to start a new line of the picture.
We take increasing density and ever-smaller components for granted now, but it was not always so; My father started a company in the early 1970s to produce a new oscilloscope using not valves, but the new-fangled transistors and integrated circuits which were just appearing on the market. Instead of a suitcase-sized beast weighing around 30kg his offering was about the size of a desktop PC and weighed about 7kg. Despite this and very decent performance, people were suspicious of the small size – “what am I getting for my money?” was the question being asked and the device had only a limited market for several years.
By the mid ‘70s these integrated circuits had evolved into early microprocessors – initially just 4-bits – which gave designers a wealth of opportunities. One such was in a 3-phase 400Hz AC power source produced in Dorking, Surrey which used an early Motorola 6802 micro to produce the gate pulses for the three AC switched-mode converters and which also looked after the high-level operation of the converter (enable, overtemp etc.) These micros were far too slow to even consider using them for processing the control loop but they introduced the concept of embedding digital circuits to the power supply industry.
Advantages were numerous; repeatability, simple set-up, control options and so on became common-place and with the advent of control interfaces such as IEEE-488, remote programming became a practical reality. Phase imbalance, phase angle shifting and so on all became simple options to add to the equipment and became valuable marketing tools.
One side-effect of the “digital” tag however was a perception of perfection; this 3-phase source had analogue meters for output voltage and current. In the early 1980s, one customer, who makes aircraft wings in the West Country, wanted one of these supplies to power-test their wings. This “special” was designed and manufactured; it had wheels, bright yellow livery and a flashing light on the top. The customer also wanted a digital frequency meter, so one was fitted. It showed 398.6Hz – within the ±1% tolerance required and it always showed 398.6Hz as it was derived from a crystal clock.
This was not acceptable! They were buying a digitally controlled 400Hz converter so the display had to show 400Hz. The frequency meter was duly removed and a simple display fitted – hardwired to show “400.0”. All was good….
Modern-Day Power Design
By the 1990s digital devices were becoming ever more complex and execution speeds were increasing into 10s of MHz.
An opportunity arose to redesign a 5kV electron beam supply for a company in Manchester; the decision was made to replace all the TTL logic circuitry which controlled and monitored the supply with a simple PLD – only a few hundred gates but reconfigurable very easily. This turned out to be a valuable decision.
One thing that had not been appreciated was that during the e-beam spectrometer’s test process, the customer would turn up the supply by 10% to “spot-knock” the inside of the vacuum chamber. This resulted in a number of arcs occurring during the process which normally ran overnight. With the original logic circuit a current trip would be detected and the system would shut down. The PLD circuit mimicked this behaviour but, once the problem was identified, a simple binary up/down counter was added which allowed up to 16 arcs in a period of some minutes before a trip would occur. A new PROM was posted to the customer and the problem went away.
In the last 10 to 15 years, further integration has developed microcontrollers with on-board PWM, comparators and amplifiers allowing the adoption of fully-digital control loops. Alongside high-performance PCB design software and manufacturing processes, this has allowed the design of a high precision 800V programmable source with a 1.8V processor core embedded at its very heart. The processor not only handles every aspect of the power supply’s operation but also produces a comprehensive data stream which is transmitted across the internet to give a global view of system performance and utilisation. Bells and whistles which would have been unimaginable 50 years ago!
The Future of Digital Power Supply Design
With the ever-growing demand for data, prognostics and performance power supplies in another 10 to 15 years will certainly be very different beasts from those of today.
Here at Celab we design for the future, whilst also assisting with any obsolescence issues faced in the meantime. Our power supply designers are constantly innovating, ensuring our customers not only get products to the correct specifications, but products that will exceed expectations for years to come.