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Bifilar Windings

Bifilar Winding Technique

Bifilar winding is a special technique where two wires are wound together in parallel on a core. This configuration creates unique electromagnetic properties that are particularly relevant to VIC designs, including inherent capacitance between windings and special transformer-like coupling.

What is Bifilar Winding?

In a bifilar winding, two conductors are wound side-by-side along the entire length of the coil:

    Standard Winding:         Bifilar Winding:

       ─────────────            ═══════════════
       │ │ │ │ │ │              ║A║B║A║B║A║B║
       └─┘ └─┘ └─┘              ╚═╝ ╚═╝ ╚═╝

    Single wire wound           Two wires (A & B)
    around core                 wound together

    Cross-section view:

    Standard:    Bifilar:
      ○ ○ ○       ○ ● ○ ●
      ○ ○ ○       ● ○ ● ○

    ○ = Wire A    ● = Wire B

Bifilar Winding Properties

Property Effect VIC Relevance
High inter-winding capacitance Built-in C between A and B May replace discrete capacitor
Near-unity coupling k ≈ 1 between windings Efficient energy transfer
Cancellation modes Some flux cancellation possible Affects net inductance
Lower SRF High Cparasitic reduces SRF Consider in frequency selection

Connection Configurations

1. Series Aiding (Same Direction):

End of A connects to start of B → Fluxes add

Ltotal = LA + LB + 2M ≈ 4L (for k=1)

2. Series Opposing (Opposite Direction):

End of A connects to end of B → Fluxes subtract

Ltotal = LA + LB - 2M ≈ 0 (for k=1)

3. Parallel Connection:

Starts connected, ends connected → Current splits

Ltotal = L/2 (for identical windings)

4. Transformer Mode:

A is primary, B is secondary → Voltage transformation

VB/VA = NB/NA = 1 (for bifilar)

Calculating Bifilar Capacitance

Approximate Inter-Winding Capacitance:

Cwinding ≈ ε₀εr × (lwire × dwire) / s

Where:

  • lwire = length of each wire
  • dwire = wire diameter
  • s = spacing between wires (≈ insulation thickness × 2)
  • εr = dielectric constant of insulation

Typical Values:

For magnet wire on ferrite: 10-100 pF per meter of winding

Bifilar in VIC Context

Meyer's designs reportedly used bifilar chokes in several ways:

As Primary/Secondary Pair

L1 and L2 wound as bifilar on same core:

  • Tight coupling between primary and secondary
  • Built-in capacitance may serve as C1
  • Simpler construction (single winding operation)

As Choke Sets

Matched pairs for symmetrical circuits:

  • Identical L values guaranteed
  • Common-mode rejection possible
  • Push-pull drive configurations

Winding Techniques

Tips for Bifilar Winding:

  1. Keep wires parallel: Twist them together before winding or use a jig
  2. Maintain tension: Even tension prevents gaps and loose spots
  3. Mark the wires: Use different colors or tag ends carefully
  4. Wind in layers: Complete one layer before starting next
  5. Insulate between layers: Add tape for voltage isolation

Measuring Bifilar Parameters

Measurement Configuration What It Tells You
LA alone Measure A, B open Inductance of winding A
Lseries-aid A end to B start, measure LA + LB + 2M
Lseries-opp A end to B end, measure LA + LB - 2M
Cwinding Measure C between A and B Inter-winding capacitance

Calculating Coupling Coefficient:

M = (Lseries-aid - Lseries-opp) / 4

k = M / √(LA × LB)

For true bifilar winding: k ≈ 0.95-0.99

Advantages and Disadvantages

Advantages:

  • Built-in capacitance may simplify circuit
  • Excellent magnetic coupling
  • Matched characteristics between windings
  • Compact construction

Disadvantages:

  • Lower SRF due to high parasitic capacitance
  • Difficult to adjust windings independently
  • Insulation must handle full voltage difference
  • More complex to wind correctly

VIC Matrix Calculator: The Choke Design section includes options for bifilar windings. It can calculate the expected inter-winding capacitance and adjust the SRF estimate accordingly. When designing bifilar chokes, the calculator helps ensure compatibility with your target resonant frequency.

Next: Parasitic Capacitance & SRF →

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