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Design group aims to develop supplies for the low power sector

When it comes to power supplies rated at 250W and beyond targeted at medical and industrial applications, TDK Lambda claims to hold the largest market share. But tackling the much larger market for less powerful supplies has presented a problem for the company, as its product offering in this range has been determined largely by what has come out of the parent company in Japan.

"From a technology point of view, Japan is a very conservative market in general; not just in power supplies, but also electronics in general," said Martin Southam, director of marketing at TDK-Lambda EMEA. In Japan reliability is everything as companies can pay unlimited damages if anything goes wrong. "It drives a very conservative approach," he continued, "so, generally, supplies are vastly over specified and heavily derated and that has an impact in terms of size and cost. They are great products and last forever, but in Europe we are more cost and size focused."

So the company set up a low power design team in Devon, with dedicated engineers and a laboratory, to develop power supplies to meet European requirements for low power medical and industrial equipment. This market sector continues to grow as equipment is migrated away from hospitals and into doctor's surgeries and the home.

Southam commented: "Our approach to this sector is to focus on the design life of the end product. When you start to use low power products, it sometimes happens that people compromise on the power supply in order to get them small enough. This can have an impact."

Problems appear because power supplies have a standard footprint – 2 x 4in in the case of this new 100W supply – and there is a limit to how much you can squeeze into this space. So TDK Lambda set out to develop a power supply that not only adhered to this form factor, but also provided the reliable life and all round performance needed for these markets.

Andrew Skinner, the company's chief technology officer, outlined the challenge. "It is simple – we needed less parts, otherwise we wouldn't have enough space to dissipate heat from the power components."

One of the ways the team cut down on parts was to take out the full power factor correction unit. Skinner said: "Other products will last for their specified lifetime if they work at room temperature, but not if they work at their maximum specified temperature. What we have done, by adding a couple of parts, is allow it to work across the full temperature range without having a full power factor correction circuit, which would pretty much have doubled the component count. We have done this just by putting a bit of thought into the rectification."

A power supply is at the mercy of its electrolytic capacitors, usually the first components to fail as they have a finite life – the electrolyte evaporates over time.

Capacitor life in the ZM series is 47,000 hours (5.4 years), according to the spec sheet, and this is more than adequate for the typical applications. Southam commented: "Most companies use good quality caps, so that is not a differentiator itself, it is the approach to design." That approach, Southam reiterated, is to establish what the life of the end product is and to ensure the power supply will outlast it.

Capacitors basically have three functions in a power supply. The first controls the start up voltage to the control chip. For cost reasons, it is common to put in a small, low power electrolytic capacitor, not high voltage, usually buried in the power supply. "The problem," claimed Southam, "is because they are small can sizes and because they are embedded deep in the power supply, they can get very hot and wear out relatively quickly. You only notice it when you turn the power supply on and off again and it won't start."

TDK is using ceramic capacitors in the ZM series instead of the more typical electrolytics. Southam said: "Ceramic capacitors don't suffer from the same wear out as an electrolytic. They are more robust devices but do cost a bit more."

The long term performance of electrolytic capacitors depends on heat, which Southam says is taken into account when designing around the bulk capacitor – the main hold-up capacitor in the power supply. "Careful attention to the design, in terms of the components around it, can reduce heat and some people put in the minimum rating possible, which doesn't help. We have seen electrolytic caps have a lifetime of a year or less."

The third capacitor type – the output capacitor – can can get hot, depending on how they are clustered and the ripple current passing through them, which can heat them internally. Skinner commented: "What we have done is introduce a ceramic capacitor prefilter, so we are filtering some of the ripple current out before it reaches these parts – it means extra parts and extra cost but it does bring extra lifetime."

For all that the ZM series – which will have 80W and 180W versions next year – represents a new market for TDK Lambda, there are more radical design changes in the offing. These will be the fruits of a technology centre the company has opened in Bristol to do early stage and blue skies research before it switches to more product specific design in Devon.

There are currently two main focus areas. Skinner commented: "There is digital control – we are doing quite a bit of algorithm work and doing a lot of work with suppliers on upcoming controllers.

"The other is new switching devices, seeing as silicon MOSFETs have pretty much come to the end of their evolution. These new devices operate at higher frequencies and, to get the best out of them, you need to have a circuit around it that optimises its benefits. That means we need to look at doing things differently."

Smallest power solution
The ECE60 is a new series of single output encapsulated 60W AC/DC power supplies from XP Power. Suiting applications that require high power density, slim profile and low no-load power consumption, the ECE60 is available in either PCB mount, chassis mounted with screw terminals or in a DIN rail mount configuration. Claimed to have the smallest footprint and the highest power density of any 60W supply, the PCB mounted model measures 91 x 38 x 28mm and has a power density of more than 10W/in3.

There are eight models, with nominal outputs from 3.3 to 48V DC. A peak load capability allows up to 130% of rated output power to be available for up to 30s to accommodate a higher start up current, when powering a motor for example. Using this approach allows an engineer to avoid having to specify a more expensive higher rated power supply just to provide for an occasional peak load requirement.

Tim Fryer

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