Continuous Positive Airway Pressure (CPAP)
Continuous positive airway pressure (CPAP) ventilation is a mainstay treatment for obstructive sleep apnea syndrome.
A CPAP machine physically consists of a compact blower that creates slightly positive air pressure. This slightly positive air pressure, when directed into the mouth of a person afflicted with obstructive sleep apnea, can keep the air passage open and reduces or eliminates the symptoms of the apnea.
There are many CPAP designs available on the market. Features vary based on sophistication. Most have adjustable mask pressure. Some have automated pressure levels that change based on the detection of an apnea event; some automatically adjust pressure based on the breathing cycle; the pressure is reduced during exhalation. Some have water tanks and are capable of adding humidity to the air.
A CPAP machine consists of a blower mechanism with a variable air pressure output. The variation is controlled by a microcontroller-based embedded system. The embedded system contains inputs from air monitoring sensors and motor control sensors.
Mask air is monitored by input sensors that measure temperature, humidity, and pressure. Motor-related sensors include motor-winding current input and motor voltage. Additional system sensors include temperature.
System outputs include the motor speed and any audible alarms or spoken voice alarms.
The user interface consists primarily of on/off and pressure control buttons, and pressure indicators. These indicators can be implemented on backlit LCDs for more expensive systems or simply signaling LEDs for less expensive systems.
Some CPAP machines are battery powered and portable, but most are bedside plug-in designs that use line power. The line-powered devices can also have a limited battery backup to provide service during power interruptions.
Air Mask Monitoring Signal Chain
The air line contains sensors for temperature, humidity, and pressure. The pressure reading is fundamental to the operation of the device, but humidity and temperature can be monitored for safety.
The air line sensors can have I2C outputs or they can have analog outputs. Sensors with an I2C output connect directly to the microcontroller. Sensors with analog outputs have either low-level (mV) outputs or a high-level (0 to 5V) output. Typically, these signals are fed through op amps into multichannel ADCs that convert the signals to digital. The digital information is then fed into the microcontroller through a serial interface such as I2C or SPI. See the block diagram for recommended signal chain products for this application.
Both the temperature and pressure ranges are quite narrow in this application. Expected temperatures range from +50°F to +110°F, and expected pressures range from 4 to 20 millibars. Humidity, if used, ranges from 40% to 80%. Because of the narrow ranges and relatively low sampling rates, the analog signal chain can be anchored by low cost multichannel 10- or 12-bit analog- to-digital converters to achieve the desired monitoring performance. Ensure that the ADC that is selected is as accurate as needed by looking at the INL spec. Choose the lowest INL within your price budget that also provides the necessary resolution. If your sensors have millivolt outputs, then also select an op amp that has lower noise characteristics than your ADC.
Refer to the block diagram to view recommended Maxim ICs for implementing the airline monitoring function of this application.
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Motor Control and Monitoring
The air mask pressure sensor input is used to control the blower speed. The blower motor typically runs from 1k RPM up to 30 or 40k RPM to attain the required pressure. This is a very high speed, and for the CPAP application, the speeds need to change very quickly. For this reason, the current in the motor windings must be monitored to ensure the motor is not being dynamically overloaded.
The motor input voltage is usually monitored to ensure that the power supply and battery are healthy. The voltage monitoring can be handled with a voltage monitoring chip or simple comparator.
The current in the motor windings runs from 0A to 2A. Therefore, a 12-bit ADC can typically provide the needed resolution into the millivolt range. The ADC must operate at a fairly high speed to sample at the highest motor speed. The motor current is first fed into a current-sense amplifier to convert the current value to a voltage and to amplify.
Maxim current-sense amplifiers and ADCs are typically used to monitor winding current. Refer to the block diagram to view recommended Maxim ICs for implementing the motor control and monitoring function of this application.
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CPAP devices commonly use line power with battery backup though some strictly battery-powered devices exist.
The motor typically uses a standard DC voltage from 12V to 24V. Therefore, any battery must be a multicell battery, and the battery is typically external to the CPAP enclosure. A multicell Li+ battery must have a battery cell balancer along with a battery charger and fuel gauge.
The typical power supply uses a wall transformer to obtain 12VDC or 24VDC at approximately 2A—enough to run the motor at full speed. The internal power rails are supplied by switching regulators and LDOs to obtain the needed voltage rails. Any battery used in a CPAP application is typically external to the CPAP enclosure.
Maxim step-down switching regulators and LDOs are commonly used to create the IC voltage rails from the input DC voltages. Additionally, Maxim battery chargers, cell balancers, and fuel gauges are used for battery management.
Refer to the block diagram and click on the power supply and battery blocks to view recommended Maxim ICs for this application.
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