Engineering Reference
Efficiency, Loss, Voltage Drop and Fuse Terms Reference
Lookup efficiency, power-loss, voltage-drop, inrush and fuse terminology with formulas, waveform conditions, derating and protection boundaries.
Efficiency, Loss and Drop
| Term | Relationship | Unit | Boundary |
|---|---|---|---|
| Efficiency η | Pout / Pin | % | Define input/output boundaries and use average real power consistently |
| Power loss | Pin − Pout | W | Includes all loss mechanisms inside the selected boundary |
| Voltage drop | Vsource − Vload | V | State one-way/round-trip path and operating current |
| Voltage-drop percentage | Vdrop / Vsource × 100 | % | Use the intended source/reference voltage |
| Conduction loss | I²R or VF × I | W | Use waveform-appropriate RMS or average current |
| Temperature rise | P × thermal impedance | °C or K | Thermal path and time scale must match the model |
Inrush and Fuse Terms
| Term | Meaning | Design boundary |
|---|---|---|
| Inrush current | Short-duration startup current | Peak, duration, source impedance and repetition rate all matter |
| Fuse rated current | Continuous-current marking under specified conditions | Not the instantaneous opening current |
| Time-current curve | Opening time versus overcurrent multiple | Use worst-case ambient and tolerance |
| I²t | Current-squared time integral | Compare only under compatible waveform and manufacturer definitions |
| Breaking capacity | Maximum prospective fault current the fuse can interrupt safely | Must exceed available fault current at the installation voltage |
| Derating | Reduction from nominal rating for environment/application | Follow fuse family, holder, ambient and agency guidance |
Worked Check
Pin = 50 W, η = 90% → Pout = 45 W, Ploss = 5 W
The 5 W loss still needs distribution among switching, conduction, magnetic and other mechanisms before thermal design.
Protection Sequence
Determine normal RMS current and startup pulse, apply environmental derating, check time-current tolerance, verify breaking capacity and voltage rating, then coordinate wiring, source and downstream component limits.
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FAQ
How is efficiency calculated?
Efficiency is output power divided by input power using consistent average real-power boundaries.
Is power loss always converted to heat?
Within many electronic components most loss eventually becomes heat, but define the system boundary and account for stored or radiated energy where relevant.
How is voltage-drop percentage calculated?
Divide path voltage drop by the selected source/reference voltage and multiply by 100.
Is fuse rated current the current that opens it?
No. Opening time depends on overcurrent multiple, duration, ambient, tolerance and the time-current curve.
What is fuse I²t?
It is a current-squared time integral used to characterize pre-arcing or clearing energy under defined conditions.
Why does inrush current matter for fuse selection?
A fuse must tolerate normal startup pulses while still clearing actual faults within safe limits.
What is breaking capacity?
It is the maximum prospective fault current a fuse can safely interrupt at a stated voltage and circuit condition.
Can a fuse protect a semiconductor from every fault?
No. Semiconductor damage can occur faster than a general-purpose fuse clears; coordination requires time-current and I²t analysis.