How SMPS evolution has sparked a PSU design revolution

January 31, 2014 // By Patrick Le Fèvre, Ericsson Power Modules
With banner statistics that proclaim DC/DC power-conversion efficiencies around 95% becoming another tick-box in the collective mindset, 30-plus years of taming “switchers” marks the end of the road for analogue evolution and the dawn of the digital revolution in PSU design.

Back in the tumultuous days of 1976 when “power to the people” advocated revolution over evolution in human affairs, Swedish engineer Magnus Lindmark kindled a PSU design revolution with a resonant circuit that increased the speed of switch-mode power supplies (SMPS) by an order of magnitude. Power bipolar-junction-transistors (BJTs) hampered switching speeds at a time that predates commercial use of power MOSFETs by several years, choking SMPS transient-response performance in particular. Yet six years after the US authorities published Lindmark's patent in June of 1978, linear regulators continued to dominate the power-supply landscape with switchers normally serving only applications that demanded more than 250W. As an era of cheap energy supply burned itself out, heat dissipation remained linear’s major issue with supplies routinely contributing 60% of their input energy toward global warming. As for low-power SMPS, cost and complexity restricted uptake to military and aerospace projects needing minimal bulk and power consumption. Exceptions include design classics such as 1972's HP-35 pocket scientific calculator and the Apple II personal computer of 1977, both using switchers to efficiently generate multiple rails within minimal area.

Pre-Lindmark, a representative switcher ran at around 20 kHz leaving it with, perhaps, a mere 2 kHz of open-loop bandwidth to maintain regulation. But line-frequency power transformers and heat management measures are heavy consumers of aluminium, copper, and iron whose costs were continually rising. A shock price increase as demand outstripped supply contributed toward silicon vendors developing SMPS control ICs to rationalise design. In turn, the growth in SMPS uptake encouraged investments in silicon and design methods that saw linear power supplies relegated to local low-current provisioning, serviced by a people's army of three-terminal regulators; or to niche low-noise applications in data-acquisition and audio/RF power amplification. Power MOSFETs arrived to solve multiple BJT issues – such as device matching – while furnishing greater efficiency and reliability, setting the course for a stream of evolutionary improvements to SMPSs that