voltsecond

Volt-second is a unit used in electromagnetism to describe the amount of magnetic flux change produced by a voltage applied over a period of time. In SI units, one volt-second equals one weber of magnetic flux. In practice, designers often talk about volt-seconds per turn, which relates to the flux swing per turn in a coil.

According to Faraday’s law, the emf e in a coil with N turns is e = -N dΦ/dt, where Φ is the magnetic flux per turn. Integrating over time gives ∫ e dt = -N ΔΦ. Therefore, the volt-second (V·s) represents the change in flux per turn scaled by time, and the time integral of the applied voltage relates to the total flux change of the core. For a given coil, the flux change per turn is Φ_max = (V t)/N, so the volt-second per turn equals the usable flux swing per turn.

In transformer and inductor design, the volt-second rating is a practical constraint: the product of applied voltage and pulse duration, divided by the number of turns, should not exceed the core’s allowable flux change per turn. Equivalently, V t / N ≤ Φ_max, where Φ_max = B_max A_core (B_max is the maximum flux density and A_core the core cross-sectional area). This reflects the core’s saturation limit and the need to reset flux during operation.

In switched-mode power supplies and pulsed transformers, volt-second balance is essential to prevent core saturation. Designers choose operating waveforms and duty cycles so that the per-turn volt-seconds over a cycle cancel out, ensuring a stable flux in the core.