Posted on 23 July 2019

Planar Transformers are Essential for Truly Efficient Electric/Hybrid Vehicles

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Weight advantage makes them more efficient than conventional ‘wire-wound’ equivalents

When the first mass-produced motor car, the ‘Model T’, rolled off the production line in 1908, Gerald Ford famously told the world that “you can have it in any colour, as long as it’s black”. Today, a century later, the motor car has become the backbone of our developing world, and it now appears that we can still only have one colour, although ‘GREEN’ is the new black. “Necessity is the mother of invention” said Plato, and after years of waiting it has taken a global economic downturn for the definition of ‘necessity’ to become: ‘replace the petrol engine with something more efficient and less polluting’. Finally some political and financial backing, from the likes of Obama and Brown, has turned the automotive world’s attention away from ‘gas-guzzling’ to focus more on ‘energy-saving’

By Dean Curran, Managing Director at Himag Solutions Ltd., United Kingdom


Efficiency goes hand-in-hand with the electric/hybrid vehicle revolution and designers now need to squeeze every last ounce of efficiency from each part of the vehicle. It has often been the case that the power-supply design engineer was the last person to be brought onto a project: a topology already dictated; available space limited. However, with efficiency now at the top of the agenda, automotive drivetrain design engineers are embracing energy saving technology, and planar transformers are at the forefront of the revolution.

Planar transformers often offer an electrical efficiency of up to 99.5% and provide higher power density than their conventional counterparts. In addition to this, not only are planar transformers considerably smaller than their conventional counterparts, they also offer a significant weight advantage: the lighter the moving object, the less energy required to move it. Hence, when you consider the added efficiency that the reduced weight of a planar transformer provides to an electric/hybrid vehicle, together with the improved electrical efficiency, it is simple to understand why the demand for planar transformers is growing rapidly.

Planar transformers are ideal for all switch-mode power supply applications and the choice of magnetic components for your design is as important as selecting the best topologies and switching frequencies. Electric/hybrid vehicle battery charging designs often require a power rating in excess of 2kW, at relatively high current levels. These applications ideally lend themselves to a copper ‘lead-frame’ planar transformer construction, comprising thin, flat, pre-formed copper layers. Himag Solutions is a world leader in the high-frequency 2kW+ power range, and has now successfully manufactured single planar transformer components providing up to 30kW of power throughput.

E64 1kW-9kW Planar Transformer

Having established the weight advantage provided by a planar transformer, what is it that makes them so much more electrically efficient than their conventional ‘wire-wound’ equivalents? There are a few elements, one being the ‘construction’ method of a planar transformer: the pre-formed copper lead-frames (or PCB layers) are easily interleaved between one another, primary, secondary, primary, etc., up to five or six times is not uncommon, and are accurately positioned in-line with one another in the vertical dimension. This interleaving reduces the losses: with particularly improved coupling between windings, a significant reduction in leakage inductance and reduced skin effect. Furthermore, by managing the interleaving within the design, the leakage can be very accurately controlled to meet the customer’s design specification.

Invariably planar transformers are custom designed (or ‘bespoke’ as we say in the UK) for the specific end-user application. This means that instead of the power supply design engineer having to design his system around limited standard ‘off-the-shelf’ transformers, he can design in the knowledge that the planar transformer will do exactly what he needs it to do. Needless to say, the very nature of the custom designed planar transformer means that yet more efficiency is achieved: it is obvious that if you have to ‘make do’ with a standard part, your system will not run as efficiently as a custom designed part specifically matching your needs.

E102 7kW-30kW Planar Tramsformer

The advantages offered by the planar winding construction are also opening the market for ‘retro-fit’ planar transformers. Himag Solutions is now frequently asked to design a planar transformer to replace a conventional transformer that runs too hot: the planar equivalent not only runs cooler but also improves the inverter drive waveforms, thus reducing switching losses in the power converter as well. We have successfully applied planar transformer design techniques to conventional ferrite cores to meet the ‘retro-fit’ space constraints, whilst achieving an overall more efficient, cooler, component.

E102 7kW-30kW Planar Transformer Dimension Drawing

Beyond the electrical efficiency of planar transformer design, advantages are also found in heat dissipation. Planar transformers use ‘flat’ ferrite cores that are very low profile but offer a higher surface:volume ratio than their conventional counterparts, providing significantly improved cooling. Furthermore the particular open construction method used by Himag Solutions is very flexible and provides an excellent thermal path from the planar structure to the main heatsink via the ferrite cores. In addition to this, it is also very easy to incorporate ‘spacers’ between the windings to allow airflow through the centre of the transformer, improving thermal performance yet further, especially when forced air cooling is available.

The final thing to mention is the dreaded ‘C’ word: COST! For some time now planar transformers have been disregarded as being too expensive. Contrary to popular opinion, the costs have significantly reduced in recent years through cheaper material costs, as the market grows, and improved manufacturing techniques. It is common for a 5kW planar transformer to cost in the region of $23 nowadays, and in volumes of 100k+ prices below $10 per piece are easily achievable. Moreover, in the 2kW-30kW power range, the use of pre-formed copper ‘lead-frames’ in single and multi-turn forms can achieve virtually any desired turns ratio at low cost, allowing rapid design iteration and ultimately improved time to market.

Advantages of Planar Transformers

• High Efficiency
• Low Weight
• Low Profile
• Low Leakage Inductance
• Highly Efficient Cooling
• Multiple Topologies
• Multiple Winding Options
• Customised Terminations
• Dimensional Accuracy
• Tightly Controlled Parasitics

It is extremely exciting to be leading a planar transformer company through the efficiency revolution that we are embarking upon. With hybrid/electric vehicles at the forefront, we are also experiencing huge demand from the defence/military, solar/wind power, medical and induction heating/charging sectors. The planar transformer market is set for significant growth over the next few years and one thing is certain, if you’re not using or considering using a PLANAR TRANSFORMER, then your competitors surely are.



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