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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.
The half-bridge driver uses two MOSFETs to control the current through a solenoid; one MOSFET to forward drive the solenoid and the other to recirculate current.
The H-bridge driver uses four MOSFETs, or two half-bridges joined by a load, to control current through a solenoid. With four MOSFETs, bidirectional current control is possible. This makes H-bridge drivers a good choice for single-coil and latching or dual-coil relays.
The H-bridge and half-bridge configurations can be seen in Figure 2-4 and Figure 2-5.
While the half-bridge can only enable slow decay, the half-bridge integrates the freewheeling diode, which is typically an external component. This further reduces the solution size. There is also the benefit of flexibility between driving high- or low-side loads with the half-bridge.
The H-bridge driver can enable both slow and fast decay (coast) by recirculating current with either high- or low-side MOSFETs. Figure 2-6 shows how an H-Bridge can be utilized to drive a conventional solenoid valve with high-side recirculation.
An H-Bridge can also be used as an effective fast discharge circuit. Fast decay can be accomplished by turning off the MOSFETs and allowing current to flow through the body diodes. This results in an opposing voltage to the solenoid current equal to VM plus the forward voltage of the two body diodes. Figure 2-7 shows current flow for fast decay with H-Bridge.
If fast decay and improved system thermal performance are desirable, an H-Bridge configuration could be a good fit.
Because solenoids are inductive loads, they store energy in the magnetic field when current flows through the coil. Whether disabling the solenoid or using PWM to maintain a specific current level, any circuitry used to drive a solenoid must never abruptly stop the flow of current. Doing so will cause a large voltage spike due to the energy leaving the solenoid. This is apparent from the expression that defines the voltage characteristic of inductors: V = L*di/dt.
As mentioned in the previous sections, freewheeling diodes allow for the current to recirculate when the driving FET disables. This keeps the voltage across the solenoid equal to the forward voltage drop of the diode. When disabling a solenoid, the current will recirculate until the energy stored in the inductor dissipates as heat in the series resistances of the diode and solenoid. Because the current decreases slowly, freewheeling diodes should be used in systems when PWM or current regulation schemes are used to implement peak and hold control for power savings.
Some circuits require solenoids to disable quickly to minimize latency in the system for valves or actuators. Clamping circuits may be integrated into the driver or added externally to help dissipate the energy. For instance, adding a Zener diode in series with the freewheeling diode will help to dissipate energy quickly from the solenoid. In this case, the voltage drop across the solenoid when the driving FET is disabled will be equal to the Zener clamping voltage plus the diode drop. Because this voltage is much higher than the freewheeling diode alone, it will dissipate the stored inductive energy much fast.