Planar Magnetics an alternative to conventional core shapes
To address an ever-increasing demand for more power in less space, designers are turning to Planar Magnetics as an attractive alternative to conventional core shapes where low-profile magnetic devices are required.
These devices provide functions critical to the effective operation of dc-dc converters and have a greater consistency of performance than traditionally wound devices.
What are Planar Magnetics?
Basically this is a construction method of inductive components whose windings are created from multi-layer printed circuit boards. These windings are then placed in low profile, E/E or E/I ferrite core combinations.
Planar devices can be constructed as stand-alone components or integrated into multi-layer printed circuit boards, with slots cut to accept the ferrite E core. (See Fig.1)
Back to School Basics
Some understanding of how the design works is always beneficial, so we will briefly discuss some dc-dc power basics.
DC-DC Power Basics
Switch Mode DC-DC Converters
Typically, switch-mode converters take a dc voltage, chop it, filter it, and then regulate the lower (or higher) dc output voltage. Other than voltage conversion, dc-dc converters can help isolate different portions of the host system; reduce cross talk and ground loop effects.
As the power switch devices transition through their operating range, they generate heat, which then must be dissipated. Losses occur in the magnetics and other passive and active components. Switch-mode operation will also produce output spikes and noise.
The overall design goal for a dc-dc converter is to reduce the impact of heat and noise while achieving low output impedance, excellent dynamic response, a low cost, and minimal size.
Isolation is the galvanic electrical separation of the input supply from the output load. Some circuits require isolation to function properly, reject noise, and enhance human and equipment safety. Leakage may occur across the common-mode isolation barrier, which is created with transformers and/or opto-couplers. In order to satisfy certification requirements (UL/TUV), isolation distances must be carefully adhered to. Isolation adds cost, size, and complexity to the converter design.
Board-mounted dc-dc converters are available in a variety of through-hole and surface-mount packaging formats. Isolated dc-dc converters, referred to as ‘bricks’, provide standard pin-outs and footprints in progressively smaller sizes (full brick, half brick, quarter brick, eighth brick). Despite the trend toward open-frame designs, some bricks offer heat-sinkable insulated metal base plates and/or encapsulation. Converters are also available in non-brick formats.
The dc-dc converter used in this presentation utilizes switch-mode power conversion circuitry to deliver a highly efficient power source in a very low profile presentation. This design integrates stand-alone and integrated planar magnetic devices which are comprised of 4 multi-layer PCBs in 2, low profile ferrite E/I cores, in an open framed quarter-brick package. (See Fig.2)
Stand Alone Magnetic
The Input Inductor consists of a single multi-layer PCB containing CU inductor windings embedded in a ferrite E/I core. After gluing the ferrite E/I core together the unit is assembled onto the Main Board through a pair of swaged I/O pins. (See Fig.2 & Fig.3)
This planar inductor board is comprised of 8 (3oz) layers and 2 external no-flow dielectric layers. The PCB utilizes thin core dielectrics to maintain its strict maximum thickness requirement, set by the E core’s minimum opening. The input inductor’s 16 turn winding is achieved by passing a single trace through several blind & buried, and through-hole vias. (See Fig.4)
A no-flow dielectric layer provides basic insulation and also primary/secondary isolation for the magnetics. Alternative materials include Kapton tape, screened dielectrics, and 1080 prepreg.
Development of the windings, including their shape, rotation, and exact placement within in the PCB, is best accomplished by using a 2D or 3D mechanical design tool. The resulting DXF can then be imported into the PCB design tool for use as a template. Adding a thin line, centered within the winding track, will greatly aid accurate trace placement in the PCB design tool.
The direction of the winding rotation is not critical, however maintaining a consistent clockwise or counter-clockwise direction will prevent reversed windings, which may result in reduced product efficiency and/or product failure.
Documentation should clearly indicate where the E core comes into contact with the PCB. This becomes a no-burr keep-out to prevent PCB interference with the ferrite E-core as it passes through the board.
Heavy CU/thin core boards may be more prone to delamination than thin CU designs, so watch for it. Keep your vendor apprised of any quality concerns.
Solder mask in not a UL approved insulator. Using multiple coats of solder-mask as a replacement for no-flow dielectrics should not be considered where a short could endanger human or equipment safety.
Integrated Planar Magnetic Transformer
This integrated planar transformer consists of a main transformer board sandwiched between two small winding boards, in a ferrite E/I core.
Be sure to calculate the total PCB cumulative maximum thickness to insure no air gaps develop in the ferrite core joint. (See Fig.6)
Integrated Planar Transformer Construction
After parts placement, the 2 small winding boards are connected to the main transformer board via their edge-plated I/O pads. These I/Os must be correctly aligned with their respective surface mount lands on the main board. Remember to maintain primary/secondary isolation where the winding boards come into contact with the main transformer board and/or the ferrite E/I core. (See Fig.7&8)
Main Planar Transformer Board
The main transformer board is comprised of 6 (4oz) layers and 2 (2oz) layers in an 8 layer PCB. The 2oz CU plating on the outer layers of the main transformer board allows for placement of the surface mount components that make up the remainder of the dc- dc converter circuitry.
To create the odd shaped start and finish of the main windings intelligent copper was used which was then connected to a glued set of gang vias.
The 2 heavy current windings are achieved by large multiple parallel tracks passing through ganged through-hole vias to feed the output switching devices. (See Fig.9)
Top Winding Board Layout
Individual tracks on the Top Winding Board start and finish on the primary side, via edge-plated pads. The 2 pads on the secondary side are for board adhesion. (See Fig.10)
Tracks on the Bottom Winding Board start and finish on both primary and secondary sides. Pay careful attention to the primary/secondary isolation requirements where the boards meet! (See Fig.11)
The schematic netlist does little to maintain design integrity, as inductors made from a single coiled trace use the same netname at both ends of the symbol.
Visual inspection is required. (Schematic not shown)
Because of the ever-expanding capabilities of the manufacturing industry and the many complex manufacturing processes involved in planar magnetics, designers are encouraged to develop and maintain good communication with their vendors.
Review of Key Design Elements
- There are 2 types of planar magnetic devices in this design
- Stand alone and integrated magnetics
- It’s best to understand how your design works
- Different planar devices require different construction strategies
- Develop templates in a 2D/3D CAD tool, then DXF import
- Watch the winding rotation
- Clearly document E core/PCB keep-outs
- Solder mask is not an approved insulator
- Use no-flow dielectrics or prepreg to maintain isolation
- Be aware of heavy Cu/thin core delamination concerns
- Maximum PCB thickness is a critical requirement for E cores
- Use multiple parallel layers for increased current capacity
- Planar boards use blind, buried, gang and through-hole vias
- Always maintaining primary/secondary isolation distances
- Maintain accurate alignment of I/Os on the main transformer board
- The schematic netlist is no guarantee of design integrity
- Work within the manufacturing industry capabilities
- And last… Communicate with your PCB vendor