Part 1 established the $k \cdot Q$ rule; Part 2 covered distance and frequency. This part is about circuit architecture: how the compensation network shapes what the link can deliver, and why system efficiency is a chain that no single stage can rescue.
5. Compensation topologies: the four basic configurations
| Topology | TX side | RX side | Output character | Best for |
|---|---|---|---|---|
| SS | Series | Series | Current-source type | Constant-current drive, LEDs |
| SP | Series | Parallel | Voltage-source type | General-purpose power |
| PS | Parallel | Series | High input impedance | High voltage, low power |
| PP | Parallel | Parallel | High gain, self-bootstrapping at light load | Weak coupling, long range |
In long-range, weakly coupled scenarios ($k < 0.1$), a parallel-compensated receiver uses resonance to amplify the induced voltage by $Q$ — making PP the default choice for cross-material and cross-distance applications. Paired with a regulator stage that has enough voltage headroom, it tames both extreme operating points at once: no-load overvoltage and full-load droop.
6. The end-to-end efficiency chain
System efficiency is a chained product — no single stage can compensate for the rest:
$$\eta_{total} = \eta_{drv} \times \eta_{tx} \times \eta_{space} \times \eta_{rx} \times \eta_{rect} \times \eta_{reg}$$
| Stage | Typical | Dominant loss mechanism |
|---|---|---|
| Driver $\eta_{drv}$ | 90–96% | Switching loss, gate drive |
| TX resonant cavity $\eta_{tx}$ | 92–97% | Coil ESR, capacitor ESR |
| Space link $\eta_{space}$ | 30–85% | Set by $k \cdot Q$ (Part 1) |
| RX resonant cavity $\eta_{rx}$ | 92–97% | Same as TX |
| Rectifier $\eta_{rect}$ | 90–95% | Schottky forward drop |
| Regulator $\eta_{reg}$ | 85–95% | LDO dropout / DC-DC |
The space link is almost always the weakest stage — which is why, across this series, coil geometry and $Q$ get more engineering attention than any circuit trick.
The Wireless Power Transfer Engineering Series — Part 1 · Part 2 · Part 3 · Part 4