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 C parasitic reduces SRF 

 Consider in frequency selection 

 Connection Configurations 

 1. Series Aiding (Same Direction): 

 End of A connects to start of B → Fluxes add 

 L total = L A + L B + 2M ≈ 4L (for k=1) 

 2. Series Opposing (Opposite Direction): 

 End of A connects to end of B → Fluxes subtract 

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

 3. Parallel Connection: 

 Starts connected, ends connected → Current splits 

 L total = L/2 (for identical windings) 

 4. Transformer Mode: 

 A is primary, B is secondary → Voltage transformation 

 V B /V A = N B /N A = 1 (for bifilar) 

 Calculating Bifilar Capacitance 

 Approximate Inter-Winding Capacitance: 

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

 Where: 

 

 

 l wire = length of each wire 

 d wire = wire diameter 

 s = spacing between wires (≈ insulation thickness × 2) 

 ε r = 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: 

 

 Keep wires parallel: Twist them together before winding or use a jig 

 Maintain tension: Even tension prevents gaps and loose spots 

 Mark the wires: Use different colors or tag ends carefully 

 Wind in layers: Complete one layer before starting next 

 Insulate between layers: Add tape for voltage isolation 

 

 Measuring Bifilar Parameters 

 Measurement 

 Configuration 

 What It Tells You 

 L A alone 

 Measure A, B open 

 Inductance of winding A 

 L series-aid 

 A end to B start, measure 

 L A + L B + 2M 

 L series-opp 

 A end to B end, measure 

 L A + L B - 2M 

 C winding 

 Measure C between A and B 

 Inter-winding capacitance 

 Calculating Coupling Coefficient: 

 M = (L series-aid - L series-opp ) / 4 

 k = M / √(L A × L B ) 

 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 →