Power Supply Efficiency
Linear vs switching regulator efficiency, load dependence, and losses.
Reference
Topology comparison
| Topology | Typical η | Notes |
|---|---|---|
| Linear (LDO) | (V_out / V_in) × 0.95 | Dissipates difference as heat |
| Zener shunt | < 50% | Toy / reference — wastes energy |
| Charge pump | 70–85% | Discrete ratios (×2, ÷2) |
| Buck | 85–95% | Efficient step-down |
| Boost | 85–93% | Efficient step-up |
| Synchronous buck | 90–96% | Replaces diode with MOSFET |
| Flyback | 75–88% | Isolated; low-mid power |
| LLC resonant | 92–97% | Isolated high-efficiency |
Losses
- Conduction loss
- I² · R in switches / inductor DCR
- Switching loss
- Depends on V × I overlap during transitions
- Diode drop
- V_f · I — use synchronous or Schottky
- Core loss
- Hysteresis + eddy currents in magnetics
- Quiescent
- IC operating current — matters at no-load
- Copper / PCB
- Trace resistance; significant at high current
Load dependence
- Efficiency is usually peaked around 50–80% load.
- At very light load, switching and quiescent losses dominate — pulse-skipping mode helps.
- At heavy load, conduction (I²R) losses dominate.
- Report η at multiple loads (10%, 50%, 100%) — peak alone is misleading.
Rule of thumb
- LDO efficiency
- = V_out / V_in (e.g. 3.3 V from 5 V ≈ 66%)
- Switcher efficiency
- > 90% across useful load range
- Heat dissipation
- P_diss = P_in · (1 − η) = P_out · (1/η − 1)
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