PWM Driver Circuit Design for Solenoids

A solenoid draws a large current to pull its armature in and needs much less to hold it there. A PWM driver exploits that difference: it applies the full supply for the pull in and then switches rapidly to hold the current at a lower value.

Pull In and Hold Current

The difference between the pull in current and the hold current is often a factor of three or more. Holding the full current heats the coil unnecessarily and reduces the force margin by lowering the coil resistance and the available flux.

Reducing the current after the pull in also reduces the power dissipated in the driver, which is often the larger of the two. A driver that holds a solenoid at a third of its pull in current dissipates a third of the heat.

The transition between the two states has to be timed from the mechanical response rather than from a fixed delay. The pull in time varies with the supply voltage and with the temperature, and a delay that is too short drops the armature back out.

How the Current Is Regulated

A PWM driver switches the drive on and off at a frequency well above the mechanical response of the solenoid, and the coil inductance smooths the current. The average current is the duty cycle multiplied by the current that the supply would produce continuously.

The switching frequency has to be high enough that the ripple current is small compared with the average, and low enough that the switching loss in the driver is acceptable. Between ten and thirty kilohertz is the usual range, above the audible band.

The current can also be regulated by measuring it and adjusting the duty cycle in a closed loop. That removes the dependence on the coil resistance, which changes with temperature by almost forty percent over the operating range.

current decay and Recirculation

When the driver switches off, the coil current has to go somewhere. The path it takes determines the decay rate and therefore the ripple and the release time.

A diode across the coil gives a slow decay, because the current circulates through the coil resistance and the diode drop. The current falls slowly, the ripple is small, and the armature releases slowly.

A diode from the switch node to the supply, or a Zener in the same position, gives a fast decay at the cost of a larger voltage on the switch. The choice is between ripple and the release time, and the two cannot both be optimised.

Choosing the Decay Mode

Slow decay is used when the current has to be smooth and the release time does not matter, which is the case for a proportional valve holding a position. The ripple is small and the force is constant.

Fast decay is used when the solenoid has to release quickly, such as a fuel injector or a valve in a fast cycle. The energy returns to the supply and the current falls in a fraction of the time.

A mixed mode with a Zener whose voltage is between the supply and the switch rating gives a decay that is fast enough without stressing the device. The Zener dissipates the energy, and its rating is set from the current and the switching frequency.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pl149813980-oem_fast_turn_custom_pcb_rigid_enig_printed_circuit_board_quick_turn.webp" alt="Solenoid driver MOSFET and flyback path layout on a control PCB” />

dither and Friction

A solenoid valve has friction between the armature and its bore, and the friction is larger than the variation in force that the drive can produce at low currents. The valve then moves in steps rather than proportionally.

dither is a small oscillation superimposed on the drive current, at a frequency of a hundred to two hundred hertz. The oscillation keeps the armature moving slightly so that the average position follows the average current, and the resolution improves markedly.

The dither is generated in the firmware by modulating the duty cycle of the PWM, and its amplitude is a fraction of the hold current. Too little dither leaves the friction in control, and too much produces noise and wear.

Driver Device and Protection

A logic level MOSFET in the low side is the usual switch. Its on resistance has to be low enough that the voltage dropped at the pull in current does not reduce the force, and its thermal path has to carry the switching loss.

The gate drive takes a few microseconds at each transition, and the loss during that time is proportional to the switching frequency. Reducing the frequency lowers the loss and increases the ripple, which is the trade again.

Protection against a shorted coil is part of the design. A solenoid that fails to a short draws the supply current through the driver, and a current limit or a sense circuit that reports the condition saves the driver and the diagnosis.

Supply and Decoupling

The pull in current is several times the hold current and it appears as a step on the supply. A capacitor close to the driver supplies that step and prevents the rail from dropping far enough to reset other circuits.

The capacitor value follows from the charge that has to be supplied during the pull in and from the voltage drop that can be tolerated. A product with several solenoids has to consider the case where they pull in at the same time.

A supply that sags during the pull in can cause a processor reset, and the symptom is a product that resets when a valve operates. The cause is a shared supply rather than a software fault.

Several Solenoids and Timing

A system with several solenoids has to consider the total current and the timing of the pull in events. Staggering them by a few milliseconds spreads the load on the supply and removes the need for a larger and more expensive front end.

The controller should also know when a pull in has failed, because a valve that does not move is a machine fault that has to be reported. Comparing the measured current with the expected profile is enough to detect most failures, and the same sense resistor serves the regulation loop.

Verification and Faults

Verify the driver by measuring the coil current with a current probe and a scope. The pull in peak, the hold level and the ripple are the three numbers to record, and they define whether the mechanical behaviour will be consistent.

Measure the current at both temperature extremes, because the coil resistance changes and the hold current with a fixed duty cycle follows it. A closed loop removes that dependence and is worth the sense resistor.

A valve that buzzes audibly has a dither frequency that is too low or an amplitude that is too large. A valve that sticks has too little dither. The release checks that keep such a driver consistent are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the measures that contain the switching noise in our guide to EMI suppression design principles.

FAQ

Why not just hold the full current? It heats the coil and the driver for no benefit. The hold current can be a third of the pull in current.

What decay mode should I use? Slow decay for a smooth current and a proportional valve, fast decay where the release time matters.

Why does my valve move in steps? Friction. Add dither at a hundred to two hundred hertz and a fraction of the hold current.