Power Topologies
The standard ways to convert one voltage to another, plus two application standards you'll meet often: USB-PD and PoE.
Converter topologies​
| Topology | Direction | Isolated? | Use when |
|---|---|---|---|
| LDO (linear) | Step-down | No | Small drop, low noise, low current |
| Buck | Step-down | No | Efficient step-down (12 V → 3.3 V) |
| Boost | Step-up | No | Output above input (battery → 5 V) |
| Buck-boost | Up or down | No | Input crosses output (Li-ion 3.0–4.2 V → 3.3 V) |
| Flyback | Up/down | Yes | Isolated supply, mains→DC, multiple outputs |
Flyback (isolated)​
- A transformer-based switcher that galvanically isolates input from output — the go-to for AC-mains → low-voltage DC and anywhere isolation/safety is required.
- Needs care: transformer design, primary snubber, output rectifier, and an isolated feedback path (opto-coupler or auxiliary winding). Respect creepage/clearance across the isolation barrier (see Power Electronics).
USB Power Delivery (USB-PD)​
- Negotiates higher voltages/currents over USB-C (5 V up to 20 V/48 V EPR) via a PD controller that talks on the CC lines.
- Design notes: a PD controller/PHY (sink or source), correct CC pull-ups/downs (Rp/Rd) and VBUS handling, plus a buck to step the negotiated VBUS down to your rails.
Power over Ethernet (PoE)​
- Delivers power and data over one Ethernet cable. A PD (powered device) front-end: rectifies the power pairs, presents the 25.5 k detection signature, handles classification, then feeds an isolated flyback/forward converter (PoE is isolated by spec).
- Use a dedicated PoE-PD controller + the right magnetics; mind isolation and the higher ~48 V input.
Isolation = safety
Flyback, PoE and mains supplies cross an isolation barrier. Honour creepage/clearance, use rated isolation parts, and don't route traces across the barrier gap.