power supply

26th March 2013

Sustainability in electronics – Power supply design

Which replacement technology is for me?

In our January newsletter, we looked at the EuP regulations regarding the environmental impact of power supply design. We concluded that conventional transformers can have high standby power as well as being heavy. So what are the alternatives?

The simplest alternative is a resistive dropper. In most cases, mains voltage is far in excess of the voltage required by the logic circuit, so it is possible to drop the spare voltage across a resistor. What is wrong with this? Logic circuits, even those with a modest current demand, can easily result in high power loss in the dropper resistor. Conversion efficiencies will be below 5%, so for anything other than tiny power requirements, this is not a solution.

Whilst a resistive dropper power supply takes spare power and consumes it, the capacitive dropper power supply is more generous; it returns the power to the mains power source! Regulatory standards have been carefully written and make it clear that this “recycling” of power is allowed. The technical term for this is “reactive power flow”. Such power supplies are straightforward, low-cost and lightweight. They require some care with design to ensure reliability, but have many applications.

Capacitive dropper power supplies have a close relation, the capacitive divider. These share some of the mathematics of transformer designs. They are more complicated to design, but capable of delivering much higher powers and do not result in the reactive power flow of the simpler designs. They are lightweight and present very few EMC challenges. Unfortunately, few engineers are aware of these, or able to design them. However, they do have a role and should be considered. A number of patents in the 1960s and 1970s touched on this type of design, but then awareness of the technique waned. Interested readers might want to look at the 1969 patent US3478258, for example:



Up to this point, the power supply designs discussed have been “live”. In other words, the output is as dangerous as the input and should not be touched. Where isolation is required, then a transformer is needed as part of a switched mode power supply. Whilst the use of a transformer may sound familiar, any similarity with a conventional transformer rapidly disappears. The core material will be ferrite, not iron, the number of turns of wire will be fewer and the wire itself will be thicker. The power supplies use a smaller transformer and are reasonably lightweight. The design of these power supplies is far from trivial: breakdown voltages, switching speeds, EMC suppression and protection, isolation distances and control loop stability are just some of the factors that must be taken into consideration.

There is a bewildering array of evaluation boards and design guidance available for development of this type of power supply. In the space of the newsletter we can only give the briefest overview of the options available to the power supply designer. Every application has unique requirements and these will indicate which power supply topology is most appropriate. An understanding of all of the different typologies is critical if a cost effective and reliable solution is to be achieved.

To talk about what might be most relevant to your power supply challenge, please contact william.gardiner@heber.co.uk

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