High frequency, switching transformers are effectively manufactured as very small, cost-efficient planar transformers. The PCB that replaces the typically wound coils, is flat, extremely durable, high temperature resilient and moisture resistant. They can feature very high innerlayer voltage dielectric breakdown ratings. This article will illustrate how the PCB planar transformer is gaining industry approval.

Planar’s Start

20-layer HVPF high voltage (4000 volt) planar.

The first planar transformers were manufactured from individual double-sided FR4 PCBs with FR4 insulators. Although they featured the technology of the day, the construction methodology could not easily handle the higher turn count in a sufficiently small package required for today’s advanced power supplies. Wire and solder type assembly was required to join the individual layers, forming the multilayer turns. The required solder tab area added to both the cost and size. The required FR4 spacers prevented effective high voltage dielectric figures. The construction method dictated that the transformer had to be potted after assembly to provide the necessary environmental protection, at an additional cost.

During this preliminary planar growth period, the advantage that the individual layer approach offered was for higher current simple applications. The heavy current, secondary coils could be inexpensively punched from a copper sheet and interlaid with the primary, double-sided PCBs. As manufacturers developed methods of effectively producing heavy copper circuits, the single-layer construction technique quickly lost favor due to its high assembly cost and bulky size.

Complicated Multi-Layer Designs Emerge

Planar designs progressed with more turns and more layers. Today, it is not uncommon to manufacture planars with eight separate coil circuits in 24 layers. They can feature blind vias between eight layers and buried vias between fourteen layers. The innerlayer spacing required to fit this wonder into a standard thin sintered core is 0.004". As the spaces and layers get thinner, the voltage rating quickly drops below a usable value. UL certification requires three separators between circuits. The thinnest prepreg available presses out at 0.002". Therefore, the effective minimum spacing is 0.006". This limits the minimum overall thickness and layer count that can be manufactured, effectively reducing the number of core choices. As the thickness of the planar circuit grew to accommodate the rising layer counts, a new problem surfaced. A long, small via is difficult to plate properly at ratios exceeding 6:1. With typical via hole diameters of 0.018" and board thickness growing greater then 0.160", it resulted in plating ratios approaching 9:1, presenting a challenge to even the best PCB manufacturer.

As planar designs evolved, engineers found ways to produce more power from the little circuits. Correspondingly, they needed more copper to handle the larger currents. As the copper thickness grew, so did associated manufacturing problems.

Thick copper traces require very controlled plating and etching systems to manufacture the line edge definition required. Thick copper traces in a multi-layer construction prevent proper filling of the space between layers. The prepreg cannot flow enough to fill in the large gap between the traces.

There are a few reasons to combine planars and controller boards and a couple of good reasons not to combine them. The reduced cost of combining planars and the driver circuitry all on one board is the biggest advantage. The pre-connection of the leads as part of the main controller board is another advantage. One reason not to include the planar on a controller board is that the process to make a normal multi-layer is not the same for most complicated planar designs. Normal multi-layers have too large an inner layer spacing to make good planar transformers. The biggest reason, however, is the heat produced by most planars is too high for FR4 material. The area where the planar is built into the controller board will overheat and turn brown. The other reason is the cost is greater to replace the entire controller board if only the planar fails, compared to the lower cost of replacing a separate planar attached to the top of the controller board.

The sintered core itself can be quite poor in dimensional tolerance. Cores can be out by as much as 20 mils in any stated dimension. Some manufacturers wrap the board with transformer tape as a preventive measure against arcs. One problem that I have encountered is poor sidewall strength of the finished PCB. When a circuit with tracks close to the sidewall is routed, the wall can degrade and fall away, exposing track and possibly compromising the voltage dielectric rating.

Manufacturing of Early Planar Transformers

Micro 4-layer poly sensor coil.

Planar transformers work with high frequency. The high frequency allows a smaller core with different properties. It would be difficult to design a planar that works on 60 Hz. The core would be quite large and cumbersome. Originally, the cores were fabricated by sintering metaized powered into molds. The resulting core featured very loose and varying tolerances. The inside edges were square, posing problems for the PCB manufacturer who cannot rout square inside corners. The first planars used single- or double-sided PCBs. They were quite thick and the exposed tracks necessitated separators of ether FR4 or transformer paper. Their voltage rating was compromised by the open design, and often required potting to protect the edges of the board from arcing over to the core. As the business grew, higher current units were made by punching out windings from thick copper. The primary was still PCBs. Termination was through solder attachment: push in pin or bolt and nut technology. These types of planars are still sold and manufactured by the hundreds of thousands. Their low cost makes them attractive.

As the switching frequency increased to gain efficiency, the open style transformer could not cope due to the large spacing between the layers or the increased coil counts. New multi-layer boards were used to lower the distance between windings and to increase the number of layers so that more complicated multi-winding planars can be designed. The spacing became an issue with the poor side-to-side voltage dielectric values of FR4 material. It is not uncommon for a planar transformer to operate at 3000 volts with a very sharp peaky waveform, inductive to creating arcs. FR4 is not up to that task. Different materials were tried, and three prominent ones emerged: BT epoxy, mem 1755 and HVPF.

Manufacturing

Single-sided boards are very simple and quite standard. They can feature heavy copper tracks which are simply etched using thick copper base material copper. The line width and space design must reflect the manufacturing method, for etched 6 oz. heavy copper loses no less than 5 mils per side. When very heavy copper of 10 oz. is etched, the undercut can be as large as 10 mils each side. For plated copper, a reasonable line width ratio is only 3 mils per oz. The process of plating up to get the conductor thickness fills in the gap left by an imageable dry film. Unlike etching, this copper plating filling in process is very accurate and non-destructive.

Double-sided boards are used most often, with the need for plated through holes to connect windings. When double-sided standard boards are specified, a normal type 1 oz. product will work. If heavy copper is required, then there are a few options. First, you can etch each side but it will not be plated and the resolution will be low. The board can be plated up with copper including the holes or the board can start with a medium thickness copper and both etched and plated to get the desired thickness of copper. When the board is designed for nut-and-bolt type interconnects between windings, the thicker copper in the holes can withstand the torque of the connectors.

Importance of Full Copper Area Use

For all types of heavy copper planar designs, the copper fullness is of utmost importance. In the manufacturing process, any copper features away from the main copper circuit area will quickly over-plate and mushroom out of proportion, creating high spots on the circuits.

If your design leaves open areas or remote copper traces, change the CAD program to create a design that fills the board evenly with copper. Widen and extend each trace or winding to fill the board. The extra copper will help with thermal conductance, therefore lowering the operating temperature. The extra copper will lower the resistance of the windings, therefore lowering the heat generated by any DC component in the waveform. Extra copper does not cost any more for the board; it only helps.

Design of Multilayers

12 oz. heavy copper high current coils.

It is possible to make planar multi-layers with heavy copper on all or some layers. The major consideration is proper filling of the spaces between the layers. The thickness of the separation between layers must be at least 0.005" (0.127 mm) thicker than the thickness of the copper layers in order to prevent inner layers from shorting. Layers can have different copper thickness. However, they should be the same on each side of the core. Inner layers can have up to 8 oz. of copper.

When pressing multi-layer boards, the trick is to get the amount, thickness and flow rate of the prepreg correct. Too high a percentage of glue in the prepreg will cause it to squish out; too much prepreg overall will cause the build to be too thick. Too little prepreg or too low a glue percentage will cause dry weave from resin starvation and possible inner layer shorts. It’s a fine line between all the choices in the buildup of the prepreg and glue sheets.

Combination Planars

During the design phase, the basic layout parameters will be established. The planar can be a single multi-layer board or a combination of different board types. One such example is a flex/multi-layer hybrid, with a flex folded secondary and a multi-layer primary. When the secondary windings need to carry a large enough current to not allow two windings side by side on a PCB, a flex circuit can be designed to be bent or folded into shape and placed in the core with a multi-layer primary carrying the finer windings. One should also consider the extra material needed to fold the circuit into the desired shape in the overall thickness of the core. The copper winding is fully insulated with high voltage Kapton covercoat, allowing ease of assembly. Terminals and exit leads can be incorporated into the design. The overall thinness of the folded windings allows a normal multi-layer board to be included with the folded flex in the core. Design the flex windings with 0.020" covercoat/base material past the copper edge. This will provide side insulation within the package.

The entire planar windings can be designed in flex fold technology. Each heavy copper (10 mils thick plus two 1-mil cover coats) winding is only 12 mils thick. Therefore, a three winding high current secondary is a total thickness of 36 mils.

Folded Flex

Enlarge this picture Folded flex 10 oz. high power thin transformer.

One of the distinct advantages of folded flex is the low cost when compared to a typical multilayer/separate heavy copper punched windings technology with insulator sheets and soldered interconnects. The lack of plated vias shortens the length of the overall planar package, reduces the chance of failed interconnects, and lowers the manufacturing time and cost while providing superb insulation. Some designs even allow for a limited interlacing of the windings.

High Voltage Considerations

1 kw high power 12-volt to 72-volt transformer coil for super audio car amplifier.

Planar circuits can operate in a high voltage condition, providing the standard design and construction rules are followed.

When designing a planar circuit for high voltage, be aware of the circuit spacing. High voltage creates shorts by two methods: direct arc over and corona. Arc over suddenly occurs when the voltage potential between two conductors exceeds the ionization value of the insulator between them. Dry air will arc over at approximately 10,000 volts per 0.250". Humid air will arc at as little as 5,000 volts per 0.250" with two sharp points. Aged FR4 board material is rated around 350 volts per 0.001". Kapton has an aged dielectric breakdown rating of 3000 to 4000 volts per 0.001" thickness.

The main culprit is corona: the invisible destructive force that slowly carbonizes the circuit board material. Corona is the ionization of the air and material immediately surrounding a high voltage circuit. It has an especially large effect if the high voltage conductor is sharp and pointed. Its potential damage increases with voltage and can be visually seen in very high voltage circuits as faint blue lightning like fingers coming forth from the surface. To minimize corona, the shape of the traces must be as round as possible. Avoid any sharp 90-degree bends. Make the traces flow with rounded corners. The soldering should be round, avoiding sharp pointed tips that enhance the corona effect. For multilayer or double-sided planar circuits, use as thick a Kapton as you can, therefore, giving you as much insulative material as possible between potentials. Space the high voltage windings away from each other as much as physically possible.

On multilayers, allow the manufacturer as much room between the layers as you can. One flaw with planar high voltage multilayers is that the glue sheets do not press out fully without some air gaps and micro voids, which decrease the dielectric breakdown potential. On planar multilayers, the common practice is to specify multiple layers of glue sheets to lessen the chance of a continuous void between heavy copper layers. Kapton is an ideal material for high voltage work. It is a film with no voids or holes and the rating is very high at over 3000v per mil.

The planar designer has many choices in regards to substrate. The most popular PCB material is FR4: low cost, but features too low a temperature for planar transformer unless a relatively cool running design is used. Maximum temperature is in the 120–130ºC range. Polyamide offers superior temperature application, maximum useable temperature of 180–190ºC. However, polyamide is very expensive and very hydroscopic. Our experience has shown that 1755 phenol epoxy has excellent properties for planar transformers with good high temperature limits of 170–180ºC, excellent resistance to moisture absorption and relatively low cost.

High Voltage Thin Manufacturing System

Enlarge this picture

For today’s thinner high power planar transformer, a newer advanced construction technique is required. A High Voltage Polyimide Film (HVPF™) was developed. Extremely thin planar circuits can now be manufactured featuring very high voltage dielectric breakdown ratings, high temperature capability and heavy copper traces in a multilayer circuit. With over 3000 volts/mill dielectric rating, coil layers can be designed with as little as 0.0015" separation. HVPF utilizing a high temperature glue system has a maximum operational temperature of 180^oF. An outer cover coat of HVPF creates an environmentally sealed high voltage cover, providing protection against moisture, electrical contact and physical damage. The HVPF covercoat negates the need to epoxy pot the assembly for protection. (HVPF is a trademarked product of Sierra Proto Express.)

Manufacturing Concerns

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Designing for heavy current planar transformers requires solving specific manufacturing problems associated with the thicker copper traces. As the copper trace thickness exceeds 4 oz (0.0056"), new design rules apply; line width and space rules relative to copper thickness are typically 0.003" per oz of copper thickness. As the switching frequency rises the phenomena of skin effect needs to be considered and calculated. The formula used is d = 72 / F, where “d” is the effective depth of the skin effect. The calculated depth is from all sides, effectively doubling the usable thickness. Some designs rely on multiple layers in parallel to distribute the higher currents.

In conclusion, the printed circuit planar transformer will most likely grow in acceptance throughout the electronics industry where high reliability PCBs are required.

Robert Tarzwell is the director of technology at Sierra Proto Express (Sunnyvale, CA). He can be reached via email: rtarzwell@megadawn.com