Planar Transformers - Driving down drive dimensions.

Over the last few years there have been enormous changes in variable speed drive design.  AC drives have come to dominate the market as they have become more powerful, with all the dynamic performance of dc drives.  They have also become extremely compact, where now the limiting criteria are the physical cable access limitations and connection clearances.  There are a number of reasons for this including the obvious ones of powerful microprocessors and digital signal processors, located on multi-layer pcbs, replacing discrete components on multiple boards.  Less well recognised is the role of planar transformers in reducing the dimensions of the main switched-mode power supply, a critical area of drive design.
 
The switched mode power supply (SMPS) supplies all the drive electronic power needs - control voltages, fans and also the floating gate-drive circuit power supply unit.  This dictates the need for a very complex transformer with multiple outputs, running from the main dc link..   The SMPS transformer for this is therefor of necessity, highly complex with a number of different windings and demanding insulation requirements due to the 600 volt dc bus main supply voltage feeding it and also the high dv/dt between the drive PSU windings.  In a wire-wound transformer these factors would necessitate a bulky and complex construction.  However, the space limitations and design considerations of modern drives require a device very much more compact, efficient and flexible.  In recent development phases, designers have been adopting planar transformers which utilise solid insulation throughout.
 
Planar transformers have coils encapsulated within multi-layer pcbs, sandwiched between state-of-the-art low-profile ferrite cores to make small, flat components ideally suited to pcb mounting. Multiple outputs are facilitated by the lightweight, multi-layer pcb construction resulting in major benefits in size and weight reduction of up to 60% particularly for multiple output transformers.
 
Watt for watt planar transformers offer greater density than traditional wire-wound transformers, coupled with a high degree of layout flexibility.  Planars can be developed specifically to fit the available space and pin layout, with clearances incorporated only where needed and with a great deal more flexibility than the fixed layout of conventional wound-bobbins transformers.

Planars also make a significant contribution to reliability and quality consistency. In conventional transformers, turns count can be inconsistent as can insulation taping .  Wire-wound transformers depend upon clearances to meet the safety standards (IEC664) and are not encapsulated.  In a planar transformer, turns ratio is guaranteed as the circuits are etched within pcb layers and thus are completely repeatable, with guaranteed winding polarity. The solid insulation also excludes air from the construction and greatly minimises corona and partial discharge, enhancing reliability and longer life. Over a production period spanning five years and over 500,000 units, Planar Magnetics Ltd recorded failure rates of under 50ppm for a device designed specifically for an advanced generation ac drive.
 
From a magnetic standpoint, magnetic coupling between windings is superior which simplifies power supply design, especially in the workhorse of motor drive auxiliary power supplies, the flyback topology.  This is the main applications area for planars in drives.  Electro-magnetic compatibility is superior so EMC compliance is more easily achieved and temperature operating range is typically from – 40oC to +155oC.   Planars are also thermally more efficient than wire-wound transformers because the surface area to volume ratio is high and with the decreasing volume of modern drives, with stringent demands on thermal management, efficiency is critical.
 
Although planar transformers are relatively new, one UK company,  Planar Magnetics Ltd is already taking a world lead in the technology. Founded in 1996 as part of the Gloucester based Cardinal Technology Group, initially to supply transformers for digital variable speed motor drive applications, including the fieldbus signal transformers , Planar has broadened its market to include transformers and inductors for mainframe computer power supplies, telecommunications rectifiers, battery chargers and a wide range of industrial applications.