Side-by-side
| Aspect | Buck (step-down) | Boost (step-up) |
|---|---|---|
| Direction | V_out < V_in | V_out > V_in |
| Formula (ideal) | V_out = D · V_in | V_out = V_in / (1 − D) |
| Input current | Pulsating | Continuous |
| Output current | Continuous | Pulsating |
| Feedback loop | Well-behaved | Has right-half-plane zero |
| Typical η | 90–95% | 85–93% |
| Typical use | Digital rails, motor drives, LED drivers | Battery-powered boost, LED strings, solar MPPT |
Other topologies
| Topology | V_out | Notes |
|---|---|---|
| Buck-boost | Inverted polarity | Can go up or down; output is negative w.r.t. input |
| SEPIC | Same polarity, up or down | Two inductors, couples for non-inverting operation |
| Ćuk | Inverted polarity | Two inductors; continuous I on both sides |
| Flyback | Isolated, up or down | Transformer-based; common for low-mid power offline |
| Forward / Half-bridge / Full-bridge | Isolated | Higher power; more components |
Notes
- If V_in crosses V_out (e.g., Li-ion pack 4.2 → 3.0 V with 3.3 V output), use a buck-boost or SEPIC.
- For high step-up (> 5×), consider flyback or boost + voltage multiplier; plain boost loses efficiency at high D.
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