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Wireless Charger RX (Qi/PMA) Rectification & Power-Path Control

Wireless Charger RX (Qi/PMA) Rectification & Power-Path Control

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Power Supplies & Adapters · Wireless Charging

This page focuses on the receiver module + power-path: how the RX coil output is rectified and regulated, how power is routed to battery/system/wired ports, and how thermal/FOD risk is contained without unstable charging.

What this page solves – RX-side view

In phones, TWS cases, watches, and vehicle charging pads, the receiver side (RX) converts the magnetic field into usable DC power, then decides where the power goes and when to reduce or stop power. Typical pain points include hot surfaces, intermittent charging, dropouts with small misalignment, and unsafe heating from metal objects.

Scope boundary: RX coil → rectifier → regulation → power-path switching and protections.
Not covered in depth: TX driver design and negotiation strategy, USB-C policy engines, or detailed Li-ion charging algorithms.

System context for wireless charging RX power path Block diagram showing TX pad on the left, coupling to RX coil, then rectifier and regulator inside the RX scope, with power routed to battery, system rail, and wired port. RX scope is highlighted. TX pad TX driver (see TX page) RX module + power path (this page) RX coil Rectifier AC → DC Regulation + power-path DC regulation · routing · protections Loads / system VBAT VSYS VUSB Covered here Sibling / adjacent pages
Figure F1 — System context: this page covers the RX module and the power-path routing that feeds VBAT / VSYS / VUSB.

RX system boundaries, rails & power classes

Typical Qi/PMA receiver products target 5 W / 10 W / 15 W classes. Higher power often shifts toward proprietary extensions; the same RX fundamentals still apply, but thermal and FOD margins become the main limiter.

  • Rectified rail (VRECT): typically ~5–20 V dynamic, depending on coupling and negotiated power.
  • Battery rail (VBAT): 3.0–4.4 V (single-cell Li-ion), managed by a separate charger stage.
  • System rails (VSYS): 3.3 V / 5 V logic or intermediate rails feeding PMICs and loads.
  • Wired port (VUSB): may coexist as an input or output, requiring clean OR-ing and role control.

Anti-overlap rule: detailed Li-ion CC/CV taper states and charge safety policies belong to a general charger page. This page only explains how the RX front-end supplies and cooperates with the charger and system rails.

RX boundaries, rails and power class map Diagram showing RX boundary with input magnetic link and output rails VRECT, VBAT, VSYS and VUSB, plus 5/10/15W power classes. 5 W 10 W 15 W Higher power → tighter thermal/FOD margins RX boundary (focus of this page) Magnetic input Qi / PMA signaling Rectify · Demod · Regulate stable DC + control signals VRECT ~5–20 V dynamic Power-path control OR-ing · role selection · protections System / ports VBAT 3.0–4.4 V VSYS 3.3 V / 5 V VUSB wired coexist RX outputs: stable power + status signals to MCU; charger policy remains a separate block.
Figure F2 — Rail map: VRECT is dynamic; VBAT/VSYS/VUSB coexist and require clean routing, protections, and thermal/FOD constraints.

Rectification: from coil AC to primary DC bus

On the RX side, the coil delivers an AC-like waveform whose amplitude shifts with alignment, coupling and requested power level. The rectifier stage must turn that variable input into a stable primary DC bus for downstream regulation, while keeping loss and heat under control.

Key design choices
  • Diode bridge: simple and robust, but forward drop becomes a predictable heat source at higher power.
  • Synchronous rectification: lower loss and better thermal margin, but requires MOSFET control and timing robustness.
  • OVP & surge handling: clamp events (coil transients, load steps) using TVS/clamp paths so the primary DC bus stays inside safe limits.
IC role mapping (no vendor names)
  • Rectifier control: integrated sync-bridge controller or RX controller with rectifier-drive outputs.
  • Current / voltage sensing: supports power estimation and FOD hooks (input power vs delivered power sanity checks).
  • Protection: OVP clamp gating, thermal foldback inputs and fault signaling to the system controller.
Rectifier and loss map for a wireless RX front end Diagram showing coil input feeding a diode or synchronous bridge, bulk capacitor, DC bus clamp and sensing nodes highlighting main loss and heat points. F2 · Rectifier & loss map (RX-side) Coil input Variable AC alignment / coupling shifts Rectifier bridge Diodes Vf loss Sync FETs Rds(on) loss Primary DC bus Bulk capacitor OVP clamp / TVS I/V sensing (FOD hook) Input power sanity checks → thermal safety Loss & heat concentrate at diode drops, FET conduction and clamp events → link to thermal design
Figure F2. Coil AC rectification choices (diode vs sync), primary DC bus clamping, and sensing hooks that support FOD and thermal protection.

Regulation paths: battery charger and system rails

After rectification, the primary DC bus must be shaped into rails that match the product’s power tree. Most designs feed a battery path (VBAT charging) and a system path (VSYS and logic rails), while coordinating with Qi/PMA power negotiation and fast-changing coupling conditions.

What matters on the RX side
  • VBAT path: buck or buck-boost shaping a swinging input into a controlled battery-charge input.
  • VSYS path: a separate or shared regulator for 5 V / 3.3 V logic rails and “always-on” domains.
  • Negotiation vs loops: protocol requests set the power budget, while local regulators enforce voltage/current stability under alignment changes.
  • Light-load efficiency: maintain low loss during trickle/standby states and “set-down sleep” behavior.
Anti-overlap boundary
Battery algorithms (full CC/CV taper details) belong to battery/charger pages. This RX page focuses on handling a fast-varying input bus, power-path handoffs, and protection hooks that are unique to wireless coupling.
Regulation and power-path branches from the RX DC bus Block diagram showing rectified DC bus branching to battery charger, system rails and optional wired port path, with negotiation and thermal/FOD hooks. F3 · Regulation branches & power-path control Rectified DC bus ~5–20 V (dynamic) RX controller Qi/PMA link Comm demod Battery path Buck / Charger VBAT 3–4.4 V System rails Buck / LDO VSYS / 5 V / 3.3 V Optional wired path Power Mux / Ideal Diode VUSB / accessory Protection hooks Thermal derate FOD power check negotiation + local loops coordination Protocol sets the budget; regulators enforce rails; thermal/FOD gates keep the system safe
Figure F3. The rectified DC bus branches into VBAT charging, VSYS/logic rails, and optional wired paths; negotiation, light-load behavior, and thermal/FOD hooks coordinate safe delivery.

Rectification: from coil AC to primary DC bus

On the receiver side, the coil produces an AC-like waveform whose amplitude moves fast with alignment, load, and negotiated power. Rectification turns that waveform into a primary DC bus that downstream regulation and power-path blocks can control safely. The engineering goal is not “rectify at all costs” — it is maximize delivered power per °C while keeping the bus stable under rapid coupling changes.

Two practical rectifier choices (RX-side)
Option Strengths Typical pain points When it fits best
Diode bridge Simple, robust start-up, fewer control loops. Forward drop → heat; efficiency collapses at higher current; more thermal headroom required. Low to mid power, cost-driven designs, benign thermal envelope.
Synchronous rectification (MOSFET bridge) Low conduction loss; higher delivered power at the same temperature; better for 10–15 W class. Gate timing, dead-time, reverse conduction risk, and fault handling become non-trivial. Performance/thermal-limited products (phones, in-car pads, tight skin-temp limits).

Regardless of bridge type, the receiver must survive fast bus excursions. Practical protection is layered: OVP clamp for abrupt coupling changes, surge absorption for cable/ESD events around the system, and controlled inrush into the DC bulk capacitor so the rectifier does not become the hottest point on the PCB.

IC role checklist (rectification stage)
  • Bridge control: integrated rectifier controller vs discrete MOSFET + driver; verify reverse-conduction prevention.
  • Bus protection: OVP clamp strategy (active clamp or TVS) and recovery behavior (auto-retry vs latch).
  • Sensing hooks: current/voltage measurement path for power estimation and FOD inputs (used later by protection logic).
  • Thermal intent: identify the top 2 heat sources (bridge devices, bulk cap ESR, sense resistor) and budget skin temperature.
Rectifier and loss map for wireless receiver DC bus Block diagram showing coil input, rectifier bridge, DC bulk capacitor and highlighted loss hotspots such as diode drop, MOSFET conduction loss and capacitor ripple. RX Coil AC waveform Rectifier Bridge Diode or Sync MOSFET D FET FET Hotspot: Vf·I or Rds(on)·I² Primary DC Bus Cbulk + OVP clamp Hotspot: ripple ESR loss Sense Hooks Vbus / Ibus → power estimate (FOD) Design intent: efficiency under fast coupling changes Survive OVP events · limit inrush · expose sensing for protection loops
Figure F2 — Coil AC → bridge → DC bus, with the main loss hotspots highlighted for later thermal/FOD decisions.

Regulation paths: battery charger and system rails

After rectification, the RX-side DC bus is not “a stable supply.” It is a negotiated and movable input. Regulation therefore splits into at least two paths: energy storage (battery path) and instant system load (VSYS path). The core receiver challenge is to keep user experience stable (no drop-outs, no oscillation) while the coupling and requested power vary.

Typical rails (RX-side boundary view)
  • Rectified bus: dynamic (often in the ~5–20 V region, depending on power class and alignment).
  • VBAT: single-cell Li-ion domain (commonly 3.0–4.4 V). This page focuses on how wireless RX feeds a charger path, not the full CC/CV textbook.
  • VSYS: system domain (phones/wearables often 3.3–5 V ecosystems, sometimes with intermediate rails).
  • Logic rails: small bucks/LDOs for MCU/ASIC, demod, sensing and gate drivers.

In Qi/PMA systems, power negotiation and local control loops must cooperate: the receiver requests a power level (or incremental power), while the local buck/charger path shapes actual current draw. Good RX designs avoid “chattering”: repeated request/limit cycles that create hot spots and audible artifacts. Practical stability comes from input-current shaping, well-defined UV/OV windows, and explicit low-load behavior (sleep, trickle, wake).

IC role checklist (regulation paths)
  • Front-end converter: buck or buck-boost tolerance to fast bus movement; verify start-up and brownout behavior.
  • Battery path interface: how the RX path hands power to a charger block (current limit, handoff, fault reporting).
  • VSYS continuity: load transient response and priority rules (system-first vs battery-first) under weak coupling.
  • Low-load & standby: explicit “place-and-sleep” current and wake trigger (alignment detect, comm wake, load step).
Scope guardrails (avoid content overlap)
  • Do not expand into full Li-ion charge-state algorithms; only cover RX-side power delivery constraints and hooks.
  • USB-C PD negotiation, cable roles and external power muxing belong to the sibling page: USB-C Power Path / Load Switch.
Regulation paths from RX DC bus to VBAT and VSYS rails Block diagram showing RX DC bus feeding a charger path to VBAT, a system rail regulator to VSYS, logic rails, and a control loop between comm demod and power request. RX DC Bus ~5–20 V dynamic RX Control limits · faults · sleep Comm demod Battery Path buck/buck-boost → charger VBAT 3.0–4.4 V System Path VSYS continuity first VSYS 3.3/5 V Logic Rails MCU · sensors · drivers 3.3 V / 1.8 V request/limit power
Figure F3 — The RX DC bus feeds at least two regulation paths (battery + system), coordinated by comm demod and RX control policies.

Power-path control: wireless + wired coexistence (RX-side)

Most products must support both wireless input and wired input without user-visible glitches. The RX-side power-path goal is to guarantee one rule: the system rail stays valid, while preventing reverse-current, uncontrolled backfeed, or thermal runaway when two sources are present.

Practical priority policies (choose one explicitly)
  • Wired wins: if VBUS is present and valid, wireless input is reduced or disabled to minimize heat.
  • Wireless assist: wireless provides steady baseline while wired handles peaks (requires careful current sharing).
  • Battery-first continuity: when both inputs are weak/unstable, VBAT supports VSYS via a controlled boost path.

The RX-side mux is typically implemented with back-to-back FETs (or ideal-diode controllers) to block reverse flow. Pay attention to edge cases: hot-plug VBUS while on a pad, alignment loss during a load step, and fault recovery (auto-retry vs latched off) that can create “on/off flicker” if not rate-limited.

RX-side protections that actually matter in the field
  • Reverse blocking: prevent VSYS/BAT from backfeeding into the rectified bus or into VBUS.
  • Input current limit: cap receiver draw to avoid thermal spikes and to keep negotiation stable.
  • Short-circuit response: define fast trip vs hiccup; ensure safe restart intervals to avoid repeated heating.
  • Thermal tie-in: OT event must force power request reduction, not only local shutdown (avoid oscillation).
Scope guardrail

This section stays on RX-internal power-path decisions (wireless bus ↔ VSYS/VBAT handoff). Full USB-C PD role negotiation and external load-switching belongs to: USB-C Power Path / Load Switch.

RX-side power-path control for wireless and wired coexistence Block diagram showing wireless DC bus and wired VBUS entering a power mux with back-to-back FETs, feeding VSYS and VBAT paths with reverse blocking and protection blocks. Wireless DC Bus from rectifier Wired VBUS cable present Power Mux / OR-ing back-to-back FETs reverse blocking VSYS system rail VBAT charger handoff Protection & Policy OCP · OVP · OTP · retry wired/wireless priority
Figure F4 — Two inputs (wireless bus + VBUS) converge into an RX-side power mux with reverse blocking, then feed VSYS/VBAT with clear priority and protection policies.

Thermal + FOD protections: keep skin temperature and safety bounded

For Qi/PMA receivers, thermal behavior is not an “add-on feature.” It is the product. The RX must prevent two outcomes: uncomfortable hot surfaces and unsafe heating from foreign objects (FOD). Effective protection is a closed loop: measure → estimate loss → request less power → enforce shutdown when necessary.

Where the heat really comes from (receiver view)
  • Rectifier bridge: diode Vf loss or MOSFET Rds(on) conduction loss (see rectification stage).
  • Downstream conversion: switch + inductor + controller loss, especially under weak coupling.
  • Coil and shielding stack: eddy-current heating, ferrite loss, and metal proximity effects.
  • Foreign object: additional unexpected loss that does not translate into useful delivered power.

A practical FOD approach is based on power balance: compare the input power (estimated from RX-side bus V/I and comm state) to the delivered power to VSYS/VBAT. When the loss exceeds a threshold for a sustained window, the receiver must reduce requested power or shut down. This is why earlier sections insist on clean sensing hooks — without trustworthy V/I telemetry, protection becomes guesswork.

Protection behaviors that prevent oscillation
  • Derating first: request lower power before hard shutdown; avoids repeated hot-restart cycles.
  • Time-over-threshold: use debounce windows so transient misalignment does not trigger false FOD.
  • Latch vs retry: latch on confirmed FOD; rate-limit retries to prevent “pulsing heat.”
  • Sensor placement strategy: at least one NTC near the coil hot spot and one near the power stage hot spot.
IC role checklist (thermal + FOD)
  • Temperature inputs: NTC/IC sensors support, sampling rate, and fault thresholds (warn/trip).
  • Power estimate: availability of RX-side V/I telemetry; integration window and thresholds for loss detection.
  • Actions: ability to request power reduction (via comm), reduce switching, and enforce safe shutdown.
  • Logging: fault codes for field diagnosis (FOD suspected vs thermal limit vs short-circuit).
Thermal and foreign object detection loop for wireless receiver Diagram showing sensing of temperature and power, loss estimation for FOD, derating requests to transmitter, and shutdown/latch actions when limits are exceeded. Sensors NTC / temp IC V/I telemetry Heat Sources bridge · buck · coil foreign object Loss Estimator Pin − Pout = Ploss FOD threshold + ToT Thermal derating Action 1 Request less power via comm Action 2 Shutdown / latch rate-limited retry Closed loop: measure → estimate loss → derate → protect Avoid oscillation with time-over-threshold and controlled restart policy
Figure F5 — Thermal and FOD are a control loop. Good receivers derate via comm first, then enforce shutdown/latch with rate-limited retries.

Foreign Object Detection (FOD) & power control

FOD is not a single “sensor feature” — it is a loss-accounting loop. A Qi/PMA receiver estimates how much power enters the RX front-end versus how much reaches the regulated outputs. If the gap grows abnormally, the system assumes extra loss is being converted to heat (often in a foreign object or a stressed component) and must reduce power or stop.

What the RX actually measures for FOD

  • Input-side power proxy: VRECT (rectified bus voltage) × IRECT (bridge/bus current estimate). IRECT can come from a shunt, sense-FET, or calibrated RDS(on) method.
  • Output-side delivery: summed power to VBAT/VSYS rails (buck/charger telemetry, inductor current, or rail V×I estimate).
  • Thermal corroboration: coil/NTC temperature and sometimes PMIC die temperature are used to confirm abnormal loss patterns.

Control actions when “loss” rises

  • Derate first: request a lower power level (or reduce requested operating point) before forcing a hard stop.
  • Gate the power-path: if VBAT/VSYS are safe to maintain from a battery, open the wireless path FETs to stop heating immediately.
  • Fault semantics: use debounce/time-over-threshold to avoid false trips during alignment transients; use latching for repeated excursions to prevent “heat oscillation.”

IC role checklist (RX-side only)

  • Bridge/rectifier telemetry hooks: VRECT sense, bridge current estimate, and fault flags (OVP/OCP).
  • Regulator/charger telemetry: rail current, inductor current, charging state pin/IRQ for “output power” proxy.
  • Protection outputs: a deterministic “STOP/DERATE” output that gates the power path quickly, plus an IRQ to the host MCU.
FOD loss accounting loop on a Qi/PMA wireless RX Block diagram showing VRECT and IRECT sensing versus output power estimate, a loss estimator and derate/stop actions through power request and power-path gating. TX pad field source RX coil AC pickup Tcoil / NTC Rectifier VRECT bus IRECT estimate Cbus Regulators VBAT / VSYS Pout estimate Loss estimator (FOD) Pin(VRECT×IRECT) − Pout Pin proxy Pout proxy Tcoil Actions Derate request / Stop Gate power-path FETs FO extra heating

Thermal management & protection behaviors

RX-side thermal protection is a system behavior, not a single OTP threshold. Heating can come from coil loss (misalignment/eddy currents), rectifier conduction, DC/DC switching loss, or battery-side absorption. A robust receiver design assigns each heat source a sensor, then defines staged responses: derate → re-negotiate → isolate → report.

Where heat usually concentrates (practical RX map)

  • Rectifier bridge: diode drop or MOSFET RDS(on) conduction; also switching loss if using synchronous control.
  • DC/DC stage: inductor + high-side FET, especially during high VRECT swings and partial load.
  • Coil + shielding: alignment error and metal proximity create eddy-current heating; this often dominates “hot surface” cases.
  • Battery absorption: VBAT is “cold” only when it accepts power; near end-of-charge, input power can turn into heat elsewhere.

Staged thermal policy (recommended behavior)

  1. Soft derate: reduce requested power level and lower switching stress while keeping user experience stable.
  2. Re-negotiate point: if temperature keeps rising, step down to the next supported power class (e.g., 15W → 10W → 5W).
  3. Power-path isolate: open wireless input FETs (or disable buck) when rapid temperature rise is detected.
  4. Latch & report: repeated events should latch until cooldown or user removal; log a reason code (OTP / FOD / OVP / OCP).

Protection signals to implement (RX-side, non-overlapping)

  • OVP on VRECT: clamp + shut down switching; avoid stress on buck/charger and power-path FETs during bursts or coil lift-off.
  • OCP / SCP: fast current limit on the regulated rails plus a separate “hard short” trip with timer/blanking for transients.
  • Reverse current blocking: prevent VBAT/VSYS backfeeding into the wireless front-end when the field collapses.
  • Thermal OTP with hysteresis: distinct thresholds for coil surface (user safety) and silicon die (reliability).
  • Fault outputs: a clean FAULT pin/IRQ to the host and a deterministic gate-off path independent of firmware.
Thermal and protection behavior on a wireless RX power path Diagram showing heat sources, sensors, protection comparators, and staged actions including derate, renegotiate, isolate, and fault reporting. Coil / shield alignment loss Rectifier bridge loss DC/DC + rails switching loss Tcoil NTC Tdie V/I sense Protection logic OTP / OVP / OCP / reverse block / debounce Stage 1 Derate lower request Stage 2 Re-negotiate step down class Stage 3 Isolate open power path FAULT/IRQ to MCU log

BOM-ready IC shortlist (with real part numbers)

This section turns the RX-side architecture into a purchase-friendly shortlist. It focuses on receiver IC choice and the minimum support BOM (rectification/regulation, power-path, and thermal/FOD protection) without drifting into TX-pad design or full Li-ion algorithm theory.

How to choose fast (3 RX core families)
  • Regulated 5 V receiver → best when the product already has a battery charger or system PMIC; RX exports a stable DC rail.
  • Receiver + direct Li-ion charger → smallest BOM for wearables/TWS; fewer conversion stages and less board area.
  • EPP / multi-mode / higher power → when thermal headroom is tight, power is higher, or multi-standard compatibility matters.
Receiver BOM options: regulated 5V RX, RX+charger, and EPP/multi-mode RX Block diagram comparing three wireless power receiver BOM approaches and showing where typical receiver IC part numbers fit, plus the supporting power-path and thermal protection blocks. RX BOM options (pick one RX core, then add power-path + thermal/FOD) A) Regulated 5 V RX B) RX + Li-ion charger C) EPP / multi-mode / high power RX coil RX coil RX coil RX IC regulated 5 V BQ51013C/B • BQ51020/21 • BQ51025 Power-path / OR-ing USB vs wireless → VSYS TPS2121 • LTC4412 • TPS229xx Battery charger / buck 5 V → VBAT / rails BQ25895 • TPS62840 • MP2145 Thermal / FOD hooks NTC + current sense TMP117 • INA180 • NCP15XH103 RX IC with Li-ion charger BQ51050B smallest BOM Support BOM focus tuning caps, TVS, layout TVS (SMF series) • NTC (10k) watch rectifier heat Thermal success factors ferrite sheet + copper + vias limit current on misalignment thermal pad under RX IC RX IC EPP / multi-mode STWLC33 P9415-R • P9415 When this option wins higher power, more thermal headroom or multi-standard requirement System integration notes I²C status hooks to MCU separate rails for VSYS / VBAT keep power-path deterministic Tip: lock down the RX core first, then tune power-path behavior (USB/wireless priority) and thermal limits under misalignment.

Purchase-friendly shortlist (ICs + “must-have” support parts)

The list below is organized by function so procurement can match a BOM line to an electrical requirement. For each line, the “why it exists” is included to avoid wrong substitutions that later create heat, dropouts, or unsafe FOD behavior.

BOM Line What it must do (RX-side) Example part numbers (real) Selection notes / do-not-substitute
Wireless RX IC (regulated output) AC→DC + Qi comm, exports a stable rail (often 5 V) for system PMIC or external charger. TI BQ51013C / BQ51013B • TI BQ51020 • TI BQ51021 • TI BQ51025 Prefer versions with the needed WPC/Qi revision and control interface (e.g., I²C on some variants). Do not replace with “power-only rectifier” parts that drop comm/limit behavior—thermal and dropout issues often appear under misalignment.
Wireless RX IC + direct Li-ion charger Collapses stages: AC→DC + charging control in one device (portable/wearables). TI BQ51050B Smallest BOM, but layout/thermal design becomes the product’s “charger heatsink.” Do not swap to a non-charger RX if the product expects direct VBAT behavior.
EPP / multi-mode / higher power RX Used when power is higher, thermal margins are tight, or multi-standard support is required. ST STWLC33 • Renesas P9415-R • Renesas P9415 Match the target ecosystem (e.g., Qi EPP vs multi-mode). Do not ignore package thermal resistance—high-power RX failures are often “layout failures” in disguise.
Power-path priority (wireless vs USB) Deterministic source selection / OR-ing so VSYS never backfeeds and transitions do not brown-out the SoC. TI TPS2121 (power mux) • ADI LTC4412 (ideal-diode ctrl) • TI TPS229xx (load switch family) The wrong substitution shows up as reverse current, port heating, or “charging stops when a cable is plugged.” Validate reverse blocking and switchover behavior.
Battery charger (if RX outputs 5 V) Handles CC/CV, safety timers, thermistor input, and battery protections while RX controls input power requests. TI BQ25895 (example) • TI BQ25606 (example) Keep responsibilities clean: RX negotiates and provides a rail; charger enforces battery limits. Avoid “host-only charging” if thermal shutdown must be hardware-enforced.
System rails (buck/LDO) Generates stable 3.3 V / 1.8 V rails from RX/VSYS with good light-load efficiency. TI TPS62840 (example) • MPS MP2145 (example) Prioritize low IQ and good efficiency below ~100 mA, otherwise standby drain becomes user-visible.
Thermal sensing Protects skin-contact surfaces; controls derating early enough to avoid “hot then drop.” TI TMP117 (digital temp sensor, example) • Murata NCP15XH103F03RC (10 kΩ NTC, example) Do not place the thermistor far from the heat source. Thermal failures are often “sensor placement” failures, not IC failures.
FOD / input current measurement hooks Enables safe operation around coins/keys and metal rings; supports power estimation and thermal protection logic. TI INA180 (current sense amp, example) • shunt 10–20 mΩ (example) The goal is trend accuracy (heat / foreign object), not lab-grade metrology. Keep the shunt + Kelvin routing clean to avoid false FOD triggers.
BOM hooks (procurement-friendly “must-not-miss” lines)
  • Shielding / ferrite sheet: often decides whether EPP power is usable without skin-temperature complaints.
  • TVS / clamp network: protects against spikes during misalignment or fast attach/remove events.
  • Thermal interface: copper + vias under RX IC matters as much as the IC choice; treat it as part of the BOM.
  • Power-path determinism: a “simple diode OR” frequently creates drop/heat and reverse-current surprises; keep a defined mux/ideal-diode strategy.

Internal links: Wireless Charger TX (Qi) · USB-C Power Path / Load Switch

Validation, test & debug checklist (RX module + power-path)

Wireless RX validation is less about “does it charge once” and more about whether it stays stable across alignment changes, load steps, temperature rise, and power-source transitions (wireless ↔ wired). A reliable RX module exposes the right debug points (rectified bus, regulation rails, thermal inputs, and status pins) so root-cause can be separated into: coupling/protocol, rectification/regulation, or power-path/protection.

A. Minimum measurement points (bring-out pads recommended)

  • VRECT / VBUS_DC: rectified DC bus after bridge + bulk capacitor (captures coupling swings and OVP events).
  • VBAT: charger output rail (observe CC/CV taper behavior only at the interface level, not full battery theory).
  • VSYS / 5V / 3V3: system rails (droop, load-step response, and brownout during alignment slips).
  • IIN estimate: shunt / sense node used for power estimation and FOD correlation (or controller’s reported telemetry if available).
  • NTC / TJ proxy: NTC divider node or thermal flag pin (verify derating slope and trip thresholds).
  • STAT / INT / PG: state pins (mode changes, OVP/OCP/OTP, negotiation failures, and FOD events).

B. Test matrix that catches the “real” field failures

  • Alignment sweep: center → edge → offset corners; log VRECT ripple, negotiation retries, VSYS droop and recovery time.
  • Load steps: VSYS step (e.g., 0.3 A → 1.5 A) while charging; watch if RX requests more power cleanly or oscillates.
  • Thermal ramp: ambient + board heating; confirm derating begins before “too hot to touch” and does not chatter around thresholds.
  • Foreign object stimulus: controlled metal target (coin/foil) at known positions; verify FOD reduces power fast and latches/report events if required.
  • Wireless ↔ wired transition: plug/unplug USB while on pad; confirm no reverse-current into the pad and no ping-pong between sources.
  • Disturbance: ESD at exposed connectors; EFT/burst on wired rail (if present); verify protection does not soft-brick the RX state machine.

C. Fast fault isolation (symptom → likely root-cause)

  • VRECT collapses first → coupling/alignment or negotiation power limit; check comm demod stability and coil/Q tuning.
  • VRECT OK, VSYS/VBAT droops → regulation loop or current limit; inspect buck/charger compensation, inductor saturation, OCP thresholds.
  • Thermal flag toggles frequently → poor thermal path or aggressive derate slope; check hot spots (bridge FETs, inductor, shielding can).
  • FOD triggers at no-object → power estimation mismatch; verify sense scaling, rectifier loss model, and calibration/offset trimming.
  • Ping-pong between wired and wireless → power-path handoff not monotonic; add ideal-diode control / hysteresis and validate reverse blocking.
Wireless RX validation map: test points, transitions and protections Block-style diagram showing RX coil to rectifier bus, regulation rails and power-path handoff to wired input, with recommended measurement points for validation. RX Validation Map (What to Probe & What to Log) Coupling & Protocol Alignment sweep center → edge → corners Negotiation retries log INT/STAT + power level Key probes • VRECT ripple • COMM demod status • FOD event flag • Retry counters (MCU) Power Chain RX coil Rectifier diode / sync VRECT (probe) Regulation buck / buck-boost / charger Outputs & Protections VBAT (probe) charge rail stability VSYS / 5V (probe) load-step droop Transitions & safety • wired plug/unplug • reverse blocking • OVP/OCP/OTP flags • FOD response time
F10 — Validation map: probe VRECT/VBAT/VSYS, log INT/STAT and thermal/FOD flags, and stress alignment + wired transitions to expose ping-pong and overheat issues.

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FAQs (Wireless Charger RX · Qi/PMA)

These FAQs focus on RX-side rectification, regulation and power-path control. TX driver design, coil mechanical stack, and full Li-ion charging theory are intentionally out of scope.

Why does charging drop or stop when the device shifts slightly on the pad?
Small shifts reduce coupling, so the rectified bus (VRECT) sags and ripple rises. If regulation cannot ride through the sag, the RX requests a lower power level or renegotiates, causing visible dropouts. Validation should log VRECT ripple, negotiation retries, and VSYS droop to separate coupling limits from buck/charger current-limit behavior.
When is synchronous rectification worth it versus a diode bridge?
Diode bridges are simple but waste power as forward drop rises with current, turning into heat near 10–15 W. Synchronous rectification reduces loss but needs gate control, dead-time management, and robust fault handling. It is most valuable when surface temperature and efficiency matter, the enclosure has weak thermal paths, or the design targets EPP-class power without hotspots.
How should OVP and surge protection be applied on the rectified bus?
The rectified bus is dynamic (often ~5–20 V), so protection must clamp true overvoltage without tripping on normal negotiation steps. A practical stack uses a fast OVP comparator/clamp inside the RX controller plus a TVS sized for the worst credible transient. The clamp should protect bridge FETs and bulk capacitors, and avoid injecting noise into demod sensing.
How should VBAT and VSYS rails be separated on the RX side?
VBAT is the controlled charger output, while VSYS must remain stable for the system even during negotiation changes. A common approach is a dedicated charger path to VBAT and a separate buck (or a power-path charger) that maintains VSYS with priority and droop control. This prevents “charging loop events” from resetting the system and reduces user-visible stutter.
What causes ping-pong between wired USB input and wireless charging?
Ping-pong happens when both sources look “almost valid” and the handoff threshold has no hysteresis. VSYS rises slightly on one source, disables the other, then droops and flips back. Use an ideal-diode / power-mux behavior with clear priority rules, reverse blocking, and debounce timers. Validation should plug/unplug repeatedly while logging VSYS, source flags, and reverse current.
How can thermal derating be made smooth instead of “hot / cold chattering”?
Smooth derating requires a continuous power limit curve, not a single hard threshold. Combine an NTC-based slope (reduce requested power gradually) with a separate OTP hard stop for safety. Add hysteresis and minimum-on/minimum-off timing to avoid rapid toggling around the trip point. Thermal validation should test steady-state at several ambient points and include airflow/no-airflow cases.
What is the most common cause of FOD false trips on RX designs?
FOD often relies on comparing “power received” versus “power delivered,” so scaling errors create false alarms. Typical causes include wrong current-sense gain, unmodeled rectifier losses, temperature drift, or calibration not matching the final mechanical stack. Use stable sense components, validate across temperature, and re-check the loss map when switching from diodes to synchronous rectification or changing the bulk capacitor ESR.
Why does the RX sometimes “connect” but never ramps to full power?
A stalled ramp is usually negotiation or local-loop instability. Negotiation stalls when demod cannot reliably decode packets under noise, or when RX reports inconsistent measurements. Local-loop issues include buck/charger current limit set too low, inductor saturation, or unstable compensation under a high-ripple VRECT input. Debug by correlating packet retry counters, VRECT ripple, and the regulator duty/current limit during attempted power increases.
How should standby power be minimized for “place and sleep” behavior?
Standby is dominated by leakage paths and housekeeping rails. Gate off nonessential loads, use high-efficiency light-load regulation (PFM or low-IQ LDO where appropriate), and ensure the power-path does not back-power the system through body diodes. Disable noisy debug pullups and avoid always-on LED indicators. Validation should measure input power at idle on pad and confirm wake-up does not cause negotiation churn.
What layout mistakes most often create RX overheating or unstable comm demod?
Overheating is commonly caused by narrow copper on the rectifier/buck current loops and weak thermal vias under hot components. Demod issues appear when high di/dt switching nodes couple into sense traces, or when the coil return and analog ground are not partitioned cleanly. Keep the rectifier loop tight, route demod/sense away from switch nodes, and add clear return paths with controlled impedance and shielding where needed.
What protections are mandatory on the RX power-path when wired input is present?
The RX path should prevent reverse current into the pad and avoid back-powering the wired port. Ideal-diode or power-mux behavior is recommended, with reverse blocking, OCP, and thermal limiting on the selected source. Add source priority and debounce to avoid oscillation. Validation should include hot-plug, brownout, and cable transients while monitoring reverse current and VSYS stability.
How can a design prove “safe behavior” during foreign object heating scenarios?
Safe behavior requires rapid power reduction plus clear reporting. A controlled test places a known metal target at defined pad offsets, then verifies the RX reduces requested power quickly and does not re-ramp repeatedly without user action. Thermal sensing should corroborate the decision, and fault flags should be logged for service diagnostics. The pass criteria should include response time, maximum temperature rise, and recovery policy.

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