Power-supply performance — efficiency and internal loss: Is it correct to say that the efficiency rating of a power supply is determined by its internal power loss (i.e., loss mechanisms set how much of the input becomes useful output)?

Introduction / Context:
Efficiency is a headline metric for any power supply. It directly affects thermal design, size, acoustic noise (fan speed), reliability, and operating cost. This question asks whether the efficiency rating is “determined by internal power loss,” a phrase that connects input power, output power, and the losses inside the converter or regulator.

Given Data / Assumptions:

• Efficiency definition: eta = P_out / P_in.
• Internal loss: P_loss = P_in − P_out.
• Steady-state operation; transient storage effects neglected for simplicity.

Concept / Approach:
From the definitions, eta = P_out / P_in = 1 − (P_loss / P_in). For a given input condition, larger internal losses directly reduce the fraction of power delivered to the load, lowering efficiency. Thus, internal power loss mechanisms—conduction loss, switching loss, magnetic core loss, gate-drive loss, and control/housekeeping power—collectively determine efficiency for specified operating points. Rating tables often show efficiency versus load because losses change with current and duty cycle, but the governing relationship remains the same: minimize losses to maximize efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Thermal measurements correlate: hotter supplies (more watts lost as heat) show lower efficiency under the same conditions. Efficiency improvements come from reducing loss sources (lower Rds_on, synchronous rectification, better magnetics), confirming the statement.

Why Other Options Are Wrong:

• Incorrect: Contradicts the algebraic identity linking efficiency and loss.
• Only true for linear supplies: Both linear and switching supplies follow the same energy balance.
• Only true above 50% load: Efficiency depends on loss at any load; the formula holds universally.

Common Pitfalls:
Confusing nameplate efficiency at one load with efficiency across all loads; ignoring fixed overhead losses that dominate at light load and conduction/switching losses that dominate at heavy load.

Final Answer:
Correct.