Solenoid valves are a key component of many respiratory applications, such as oxygen concentrators, and are used to accomplish multiple functions. Oxygen concentrators use a compressor to separate air and oxygen into separate canisters. Solenoid valves are then activated to release specified amounts of each gas into a chamber, which then uses a separate valve to supply the mixture to the patient through a mask. Solenoid valves can also be used to stop the flow of gas to the mask if a fault is detected. These valves are similarly useful in anesthesia delivery systems where multiple gases are typically mixed together before delivery to the patient. Oxygen concentrators tend to have 4 – 6 such valves while anesthesia delivery systems tend to have more than 15.

Solenoid valves have different configurations for how they can control the flow of fluid or gas. The simplest configuration is a 2-way valve that connects two pipes or tubes together, but there are also options for 3-way and 4-way junctions that can simple on/off, binary/latching or dithering valves. Each different valve type requires a different control method, but this reference design focused only on controlling the simplest, the on/off valves. On/off valves have a natural, un-energized position and are operated by applying current to energize the solenoid, which changes the position of the valve. In a naturally closed 2-way valve, the valve will be closed and prevent the flow of fluid or gas until the solenoid is energized, which will open the valve to flow. As an alternative example, in a 3-way junction valve, the natural position will connect two of the three pipes together and will connect two different pipes when the valve is energized.

The control for on/off valves can be straight forward implementation in which current needs to be applied to the solenoid to energize the valve. Some on/off valves are unidirectional, in that they can only accept current in a single direction to be energized, while others are bidirectional. The amount of current required depends on the specific valve, but the current waveform for solenoid valves is consistent. A peak current is required to energize the valve initially while a lower amount of current is required to maintain the energized state. Regardless of directionality, these valves can be minimally controlled by a low-side or high-side MOSFET and valve driver that can receive input from an MCU to energize the valve. While simple, this control scheme can be inefficient because it will maintain current flow at the peak current rather than reducing to maintain the energized state. By adding a current feedback path it is possible to implement peak-and-hold control which backs off the current but maintains the energized state to increase system efficiency. Some valve drivers integrate the peak-and-hold control while others will require the MCU to read the current and implement that control.

For this reference design, the DRV8847, an 18-V, 2-A dual H-bridge motor driver, was selected to drive the solenoid valves. Each of the half bridges in the device can be independently controlled, which means that the DRV8847 is capable of driving up to four unidirectional valves or two bidirectional valves. This design has two DRV8847 ICs, one for unidirectional control and one for bidirectional control. The DRV8847 is capable of driving 1 A through each half bridge and the integrated MOSFETs have a 1-Ω R_DS(on). The device also integrates a current sense amplifier to do overcurrent protection; however, the driver does not incorporate options for peak-and-hold control, so this would have to be implemented by the MCU. Each half-bridge of the DRV8847 is driven by PWM from a local MCU, which was selected as the MSP430FR2155. This MCU was selected because it has all of the required peripherals, is physically small, and is cost-effective.

Pumps are also an integral part of ventilation system that helps with pumping air and medicine to the lungs. These pumps often need precise and fine motorized control that can operate electrically. Stepper motor and dual H Bridge drivers are favored with high microstepping like DRV8825 and DRV8886AT.

As system requirements can vary, solenoids/valves and pumps may be connected to a 12-V, 24-V, 36-V or even 48-V rail with varying current requirement. Some other key devices to be considered based on system requirements are as follows: