The Advancements in Energy Storage: Bifilar and Trifilar Coil Winding Techniques.

Electromagnetic coils are produced by winding a conducting wire in the shape of a coil, spiral, or helix. The shape and dimensions of a coil are designed to fulfill a particular purpose. Parameters such as inductance, resistance, and strength of the desired magnetic field greatly influence the design of coil windings.

The Challenges

Modern equipment is becoming more miniaturized which leads to a demand for micro coils. However, several challenges face the designer of modern equipment which uses micro coils:

  • Economic mass production
  • Safe production process. No risk of damaging sensitive components
  • Highly reliable connections—no oxidization
  • Very tight tolerances
  • High quality to ensure the long life of each component
  • Repeatable physical and electrical characteristics
  • Robust, automated manufacturing process—eliminates human error caused by various skill levels and worker experience

These size constraints pose multiple challenges in both manufacturing and connecting these components to each other and their support systems. In addition to these challenges, there are times when bifilar or trifilar-wound coils are needed. Having two or three conductors in a coil increases the difficulty of producing these coils at a reasonable cost and to high enough standards.

Benatav’s custom winding and connecting technologies make us one of the few companies in the world that can meet your micro bifilar or trifilar-wound coil requirements.

Why Choose Bifilar and Trifilar-wound Coils?

Here are some reasons why you might choose a high quality bifilar or trifilar-wound coil from Benatav for your project:

  • Your device needs windings that create a current that flows in parallel directions or opposite directions.
  • A bifilar-wound coil can be a better option than a wire-wound resistor.
  • Your application requires a means of removing energy stored in the stray magnetic flux.

What is Bifilar and Trifilar-wound?

A bifilar-wound coil contains two closely spaced, parallel windings. This can be achieved by winding the two coils using either two insulated conductors or a single wire with two insulated strands. This single wire is often produced from different colored enameled wires bonded together.

Some bifilar-wound coils have adjacent coils in which the turns are arranged so that the potential difference is magnified (i.e., the current flows in the same parallel direction). Others are wound so that the current flow is in opposite directions. In the latter case, the magnetic field created by one winding is equal and opposite to that created by the other, resulting in a net magnetic field of zero, neutralizing any negative effects in the coil. In electrical terms, this means that the self-inductance of the coil is zero.

This arrangement is used in the transmission and reception magnetics of Ethernet cables and can commonly be seen in the form of a ferrite bead clamped to the outside of USB, laptop power supply, and HDMI cables. Bifilar-wound coils are often used to construct wire-wound resistors with negligible parasitic self-inductance.

A trifilar-wound coil is created using three strands of wire spooled together.

Bifilar-wound Relays and Transformers

In some relay windings and switched-mode power supply transformers, a different type of bifilar-wound coil is used to suppress back-EMF that may damage the device driving the relay. In this case, the primary coil is energized to operate the relay, and the secondary coil is short-circuited inside the case. When the current through the primary is interrupted, as happens when the relay is switched off, most of the magnetic energy is intercepted by the secondary coil, which converts it to heat in its internal resistance. This is only one of several methods of absorbing the energy from the primary coil before it can cause damage. The main disadvantage of this method is that it significantly increases the switching time of the relay.

In a switching transformer, one winding of the bifilar-wound coil is used to remove energy stored in the stray magnetic flux that fails to link the primary coil to the secondary coil. Due to their proximity, the wires of the bifilar-wound coil “see” the same stray magnetic flux. One wire is usually connected to ground through a diode, so when the other wire of the bifilar-wound coil no longer has a voltage applied across it by the switching transistor, the stray magnetic flux generates a current in the clamping coil. This current causes the primary side voltage to appear across the clamping coil, resulting in an equal voltage across the primary winding. Without the clamping coil, the stray magnetic flux would attempt to force a current to flow through the primary coil. Since the primary coil is switched off and the switching transistor is in a high resistance state, the high voltage that would appear on the switching transistor would exceed its electrical breakdown and potentially damage it.

Winding and Handling Bifilar and Trifilar-wound Micro Coils

Equipment used to wind and handle regular-sized coils cannot be used to mass-produce micro coils, which require the development of specialized equipment. To manufacture a micro coil with as many as 2000 turns, the thickness of the wire needs to be greatly reduced. The thinnest insulated copper wire currently available is 8.8 microns (0.00034 inches). Benatav is one of the few companies in the world working with such fine wires, and we are even working on producing insulated wires smaller than 8.8 microns.

Connecting Ultra-fine Wires

Traditional solder-based connections are not suitable for connecting such fine wires, as they can lead to undesired side effects such as oxidation, inferior conductivity, and low durability. To overcome these problems, Benatav has developed a proven, cutting-edge thermo-pressure technology that enables precise soldering with tight tolerances at very high temperatures. Our thermo-pressure bonding technology allows us to connect ultrafine wires without experiencing the drawbacks mentioned earlier.

Using our in-house engineering expertise, Benatav has developed state-of-the-art methods and robots that enable the winding of micro coils and the connection of their wires to a degree that was not possible before. These robotic technologies eliminate the inconsistencies of manual production, allowing us to manufacture and connect micro coils on a scale and to standards that were previously unattainable.

Contact us regarding your micro coil requirements

Whatever your micro coil requirements, due to our extensive specialist experience, chances are that we have already solved any issues that may prevent us from producing your coils at highly competitive rates.