Transformer Utilization Factor (TUF) for a rectifier A rectifier is fed by a transformer whose secondary RMS voltage and current are Vs and Is. If the DC output power is Vdc * Idc and the apparent AC power at the transformer secondary is Vs * Is, what is the expression for the transformer utilization factor (TUF)?

Introduction / Context:
Transformer utilization factor (TUF) measures how effectively a transformer's VA rating is converted into useful DC output power by a rectifier. It is a convenient figure of merit when comparing rectifier topologies and transformer sizing in power supplies.

Given Data / Assumptions:

• Transformer secondary RMS voltage = Vs.
• Transformer secondary RMS current = Is.
• DC output power = Vdc * Idc.
• Apparent AC power drawn from secondary = Vs * Is.

Concept / Approach:

TUF is defined as the ratio of DC output power to the VA loading of the transformer secondary. Because rectifiers draw non-sinusoidal currents with harmonics, Is can be much greater than the DC current, and the ratio captures how much of the transformer's VA capacity becomes usable DC.

Step-by-Step Solution:

Verification / Alternative check:

Known TUF values (e.g., ~0.286 for half-wave, ~0.693 for full-wave bridge, depending on definitions) are computed using the same ratio, confirming the expression.

Why Other Options Are Wrong:

• Inverting the ratio (option b) gives the reciprocal and is not the standard definition.
• Algebraic sums or squared sums (options c and d) have no physical meaning for utilization in this context.

Common Pitfalls:

Confusing TUF with rectifier efficiency or power factor; TUF focuses on transformer VA usage, not only on losses.

Final Answer:

TUF = (Vdc * Idc) / (Vs * Is)