# 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

<table id="bkmrk-property-effect-vic-" style="width: 100%; border-collapse: collapse; margin: 20px 0;"><thead><tr style="background: #007bff; color: white;"><th style="padding: 10px; border: 1px solid #ddd;">Property</th><th style="padding: 10px; border: 1px solid #ddd;">Effect</th><th style="padding: 10px; border: 1px solid #ddd;">VIC Relevance</th></tr></thead><tbody><tr><td style="padding: 10px; border: 1px solid #ddd;">High inter-winding capacitance</td><td style="padding: 10px; border: 1px solid #ddd;">Built-in C between A and B</td><td style="padding: 10px; border: 1px solid #ddd;">May replace discrete capacitor</td></tr><tr><td style="padding: 10px; border: 1px solid #ddd;">Near-unity coupling</td><td style="padding: 10px; border: 1px solid #ddd;">k ≈ 1 between windings</td><td style="padding: 10px; border: 1px solid #ddd;">Efficient energy transfer</td></tr><tr><td style="padding: 10px; border: 1px solid #ddd;">Cancellation modes</td><td style="padding: 10px; border: 1px solid #ddd;">Some flux cancellation possible</td><td style="padding: 10px; border: 1px solid #ddd;">Affects net inductance</td></tr><tr><td style="padding: 10px; border: 1px solid #ddd;">Lower SRF</td><td style="padding: 10px; border: 1px solid #ddd;">High C<sub>parasitic</sub> reduces SRF</td><td style="padding: 10px; border: 1px solid #ddd;">Consider in frequency selection</td></tr></tbody></table>

## Connection Configurations

#### 1. Series Aiding (Same Direction):

End of A connects to start of B → Fluxes add

L<sub>total</sub> = L<sub>A</sub> + L<sub>B</sub> + 2M ≈ 4L (for k=1)

#### 2. Series Opposing (Opposite Direction):

End of A connects to end of B → Fluxes subtract

L<sub>total</sub> = L<sub>A</sub> + L<sub>B</sub> - 2M ≈ 0 (for k=1)

#### 3. Parallel Connection:

Starts connected, ends connected → Current splits

L<sub>total</sub> = L/2 (for identical windings)

#### 4. Transformer Mode:

A is primary, B is secondary → Voltage transformation

V<sub>B</sub>/V<sub>A</sub> = N<sub>B</sub>/N<sub>A</sub> = 1 (for bifilar)

## Calculating Bifilar Capacitance

#### Approximate Inter-Winding Capacitance:

C<sub>winding</sub> ≈ ε₀ε<sub>r</sub> × (l<sub>wire</sub> × d<sub>wire</sub>) / s

Where:

<div class="formula-box" id="bkmrk-lwire-%3D-length-of-ea" style="background: #e7f3ff; padding: 20px; border-left: 4px solid #007bff; margin: 20px 0;"><div class="formula-box" style="background: #e7f3ff; padding: 20px; border-left: 4px solid #007bff; margin: 20px 0;">- l<sub>wire</sub> = length of each wire
- d<sub>wire</sub> = wire diameter
- s = spacing between wires (≈ insulation thickness × 2)
- ε<sub>r</sub> = dielectric constant of insulation

</div></div>#### 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:

<div id="bkmrk-keep-wires-parallel%3A" style="background: #fff3cd; padding: 20px; border-radius: 5px; margin: 20px 0;">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

</div>## Measuring Bifilar Parameters

<table id="bkmrk-measurement-configur" style="width: 100%; border-collapse: collapse; margin: 20px 0;"><thead><tr style="background: #28a745; color: white;"><th style="padding: 10px; border: 1px solid #ddd;">Measurement</th><th style="padding: 10px; border: 1px solid #ddd;">Configuration</th><th style="padding: 10px; border: 1px solid #ddd;">What It Tells You</th></tr></thead><tbody><tr><td style="padding: 10px; border: 1px solid #ddd;">L<sub>A</sub> alone</td><td style="padding: 10px; border: 1px solid #ddd;">Measure A, B open</td><td style="padding: 10px; border: 1px solid #ddd;">Inductance of winding A</td></tr><tr><td style="padding: 10px; border: 1px solid #ddd;">L<sub>series-aid</sub></td><td style="padding: 10px; border: 1px solid #ddd;">A end to B start, measure</td><td style="padding: 10px; border: 1px solid #ddd;">L<sub>A</sub> + L<sub>B</sub> + 2M</td></tr><tr><td style="padding: 10px; border: 1px solid #ddd;">L<sub>series-opp</sub></td><td style="padding: 10px; border: 1px solid #ddd;">A end to B end, measure</td><td style="padding: 10px; border: 1px solid #ddd;">L<sub>A</sub> + L<sub>B</sub> - 2M</td></tr><tr><td style="padding: 10px; border: 1px solid #ddd;">C<sub>winding</sub></td><td style="padding: 10px; border: 1px solid #ddd;">Measure C between A and B</td><td style="padding: 10px; border: 1px solid #ddd;">Inter-winding capacitance</td></tr></tbody></table>

### Calculating Coupling Coefficient:

M = (L<sub>series-aid</sub> - L<sub>series-opp</sub>) / 4

k = M / √(L<sub>A</sub> × L<sub>B</sub>)

For true bifilar winding: k ≈ 0.95-0.99

## Advantages and Disadvantages

#### Advantages:

<div id="bkmrk-built-in-capacitance" style="background: #f8f9fa; padding: 20px; border-radius: 5px; margin: 20px 0;"><div style="background: #f8f9fa; padding: 20px; border-radius: 5px; margin: 20px 0;">- Built-in capacitance may simplify circuit
- Excellent magnetic coupling
- Matched characteristics between windings
- Compact construction

</div></div>#### Disadvantages:

<div id="bkmrk-lower-srf-due-to-hig" style="background: #f8f9fa; padding: 20px; border-radius: 5px; margin: 20px 0;">- Lower SRF due to high parasitic capacitance
- Difficult to adjust windings independently
- Insulation must handle full voltage difference
- More complex to wind correctly

</div>**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 &amp; SRF →*