A solenoid is a coil that produces a linear or rotational movement in a mechanical system by applying a current through the coil. There are several types of solenoids, but generally their main use is to displace objects or maintain a specific state or position, much like a traditional relay. These electromechanical solenoids consist of a copper inductive coil wound around a steel or iron armature, sometimes called a “plunger”. The magnetic field of the energized coil pulls on the armature, and the armature transfers a mechanical force to an external mechanism.

Within each application, solenoids and relays are driven in different configurations. Some example solenoid applications include home appliances, printers, HVAC, irrigation systems, engine and transmission control.

This application report categorizes and describes a few types of solenoids, discusses driver configurations, and highlights semiconductor solutions from TI that can simplify solenoid driver solutions.

There are three main categories of solenoids; push/pull, latching/bistable, and proportional.

The first type, push/pull or monostable, is used to displace an object by energizing and de-energizing the coil, or where "in and out" movement is needed. Push/pull solenoid is made up of an iron frame, iron plunger, copper coil, and return spring. Figure 1-1 shows a cross-sectional view of a pull-type solenoid. This type of solenoid can be found in applications such as electronic door locks, valves, and robotics.

The second type is the latching/bistable solenoid. The latching/bistable is similar in use to the push/pull, but the latching solenoid can maintain its position after power is off. When off, the position of latching solenoid is maintained by a permanent magnet, as opposed to a spring for push/pull solenoid. Energizing the coil with a pulse of current will change the position of the solenoid.

Proportional solenoids are solenoids that generate a force proportional to the current flowing through it, as opposed to solenoids changing between two positions or states. By adding a spring, the solenoid can generate a displacement which is proportional to current. In applications such as hydraulics, these solenoids can also be constructed with an air gap, so that fluid pressure does not affect force characteristics of the solenoid. This allows for very fine force and positioning control.

Most systems today use motor drivers to actuate and de-actuate solenoid. The key to driving a solenoid is which FETs to switch on and off, and when to switch them.

There are three basic driver configurations, low-side, high-side, and half-bridge/full-bridge, each with their trade-offs. Choosing which configuration depends on the system requirements, such as switching speed and fault protection. The high-side driver can protect against short to ground fault, whereas a low-side driver protects against short to battery fault.

The typical low-side or high-side driver configuration uses a single MOSFET with enough current handling capability to drive the solenoid. High- and low-side drivers are good choices for push/pull solenoids with a return spring. Figure 2-2 shows the LS/HS configuration, with optional external clamp.

When the MOSFET is enabled, it conducts all the current needed to energize the solenoid. When the MOSFET is disabled, the current in the solenoid must freewheel through a diode, or be allowed to continue flowing or decay to zero, otherwise the MOSFET can see large voltage spikes. The freewheeling diode across the solenoid provides this low impedance path for solenoid current to flow. Figure 2-3 shows a low-side driver.