CPAP Electronics: Flow Sensing, BLDC & Safety

Objective: maintain a prescribed airway pressure all night—quietly, safely, and efficiently. If the only thing that “snores” is your PWM, we can fix that.

CPAP system overview: blower, sensors, humidifier, UI, power by Ersa

1) System architecture & therapy modes

Core blocks: BLDC blower (pressure source) → flow/pressure sensors → MCU/SoC control → heated humidifier & optional heated tube → UI & alarms → medical AC-DC power. Optional radio (BLE/Wi-Fi) supports compliance data and updates.

CPAP: constant set pressure (e.g., 4–20 cmH₂O). Control tracks mask dynamics and leaks.
APAP: auto-adjusting pressure based on event detection (apnea/hypopnea/snore/flow limitation). Requires robust flow estimation and event classifiers.
Bi-level (S/ST): separate IPAP/EPAP targets; timing logic for spontaneous vs timed breaths; more demanding blower transients.

2) Flow/pressure sensing & AFE design

Differential flow sensing across venturi/orifice with instrumentation AFE by Ersa

Parameter	Typical range	Design targets
Flow	0–120 L/min	<10 ms response; ±(2–3)% FS; temp/altitude compensation
Airway pressure	0–25 cmH₂O	low offset drift; anti-alias RC; 16–24-bit ADC
Temp/Humidity	15–35 °C / 20–100 %RH	condensation-tolerant placement; shielded leads
Filter ΔP (optional)	0–500 Pa	clog detection; service reminders

AFE tips: instrumentation amp + low-noise op-amps; RC anti-alias near sensor; ratiometric ADC if bridge-based flow sensor.
Sampling: 200–1000 Hz for main loop; synchronize flow/pressure reads to minimize phase error.
Hose compensation: store hose length/ID; compensate for added resistance/inertia.

3) Control loops: pressure/flow, leak & ramp logic

Leak estimation and compensation algorithm block diagram by Ersa

Pressure loop: PID with feed-forward; soft ramp in/out; limit dP/dt to avoid arousals.
Leak estimation: compare expected vs measured flow at set pressure; adaptively compensate while bounding integral wind-up.
Ramp/EPR (pressure relief): time-based ramp from comfort floor; exhalation relief (e.g., −1 to −3 cmH₂O) with guard rails to preserve therapeutic pressure.
Event detection (APAP): flow flattening metrics, snore vibration bands, apnea duration windows; hysteresis to reduce false toggling.

Note Use windowed watchdogs; on sensor disagreement, fall back to safe fixed-pressure profile and assert alarm.

4) BLDC blower drive & acoustics

Sinusoidal FOC, current control, stall detect; spread-spectrum PWM by Ersa

Driver: sinusoidal FOC for low torque ripple; current-mode control; stall and over-temp protection.
Acoustics: spread-spectrum PWM to smear tonal peaks; rubber isolation; avoid cavity resonances; qualify bearings for 10k+ hours.
Thermals: blower/driver on dedicated heat path; monitor coil temps; derate at altitude/ambient extremes.

5) Heated humidification & condensation control

Heated plate + heated tube with dew-point controller by Ersa

Heater plate loop: NTC/RTD feedback; PI control with soft-start; watchdog on sensor O/C, thermal cutoff as last resort.
Heated tube: maintain hose wall temperature above dew point; current-limited driver, OCP, open/short detection.
Rainout mitigation: combine ambient/RH data with hose temp; pre-empt condensation during cool downs.

6) Power tree, isolation & safety

Isolation barriers between mains and patient-applied sensors by Ersa

Rail	Loads	Notes
24 V	Blower, heater	inrush control, eFuse, snubbers for inductive transients
12 V / 5 V	Fans, misc. drives	separate from quiet logic rails
3.3 V	MCU, sensors, radio	sequencing, brownout supervisors, event logging

Use medical AC-DC; log brownouts with reason codes; design for safe behavior on power loss (e.g., blower coast-down with alarm).
Applied parts isolation (BF/CF) as needed; respect creepage/clearance; use isolated DC-DC + digital isolators where appropriate.

7) PCB layout & EMC practices

EMC layout zones: AFEs, logic, power, high dI/dt drives by Ersa

Partition AFEs away from motor/heater PWM; define return paths; guard around high-impedance nodes.
Filters/TVS at cable entries; common-mode chokes on USB/Wi-Fi antennas as needed; chassis bonding plan documented.
Validate immunity with worst-case harness lengths and mask hose attached; measure recovery behavior per test plan.

8) Firmware, connectivity & cybersecurity

RTOS tasks: sensing, control, humidifier, HMI, logger, comms by Ersa

Tasks: sensing (5–10 ms), control (1–5 ms), humidifier (10–50 ms), HMI, comms, logger.
Data: nightly compliance, leak/usage stats, fault codes; transactional writes to FRAM/Flash; secure timestamping.
Security: secure boot, signed firmware, role-based service menu; encrypt PHI in transit/at rest.

9) Compliance mapping (ISO/IEC)

Topic	Standard	Engineering artifact
Sleep apnoea therapy	ISO 80601-2-70	essential performance, pressure accuracy, alarms, labeling
Basic safety	IEC 60601-1	schematics, creepage/clearance, leakage tests
EMC	IEC 60601-1-2	filters/shields, immunity recovery plan, test matrix
Alarms	IEC 60601-1-8	priority tones, visual behavior, usability validation
Software	IEC 62304	software safety class, SRS, verification evidence
Usability	IEC 62366	use-related risk files, formative/summative studies
Risk management	ISO 14971	hazard analysis, FMEA/FMEDA, residual risk eval
QMS	ISO 13485	DHF/DMR, traceability, change control

10) Risk analysis (hazards → mitigations)

Hazard	Cause	Mitigation
Pressure overshoot/undershoot	loop instability, leak step	PID + feed-forward, anti-windup, leak estimator, safe fallback profile
Heater runaway	sensor O/C, firmware fault	dual sensors, OCP, thermal cutoff, independent watchdog
Condensation “rainout”	cold hose, high RH	heated tube control, dew-point tracking, user prompts
EMC upset	immunity failure	filters/shields, watchdog, brownout logging, safe state
Unauthorized firmware	tampering	secure boot, signed updates, role-based access
Data loss	power fail mid-write	transactional writes, supervisors, hold-up energy

11) Manufacturing QA, calibration & service

ICT/FCT: sensor loopback, blower run-in, heater safety tests by Ersa

ICT/FCT: sensor loopback; ADC linearity; blower run-in; heater open/short; alarm path; UI/knob/buttons.
Calibration: pressure zero/span; flow calibration with traceable reference; hose compensation tables; store to NVM with CRC.
Acoustics: SPL at 1 m in quiet room; accept per spec (e.g., ≤30 dBA class target—set your own spec).
Serviceability: filter ΔP logs; blower hour counter; heater cycles; easy access for cleaning and filter swap.

12) Sample BOM highlights

Function	Component class	Selection cues
Flow/pressure sense	diff-pressure + gauge sensors	low drift, fast response, medical collateral
AFE/ADC	inst. amp, low-noise op-amps, 16–24-bit ADC	CMRR, input noise, anti-alias filters
Blower drive	BLDC driver + MOSFETs	FOC support, stall detect, thermal pad package
Heater control	current driver, NTC/RTD, cutoff	OCP, watchdog, safety approvals
MCU/Security	safety-capable MCU + secure element	watchdogs, secure boot, OTA capability
Power	medical AC-DC, supervisors, eFuse	inrush, brownout logging, derating
Connectivity	BLE/Wi-Fi module	EMC, coexistence, security features
Storage	FRAM/Flash	endurance for nightly logs, power-fail safety
HMI	display, encoder/keys, beeper	glove usability, loudness profile