Motor Drivers and Protection

A controller pin decides what should happen. A motor driver supplies the current that makes it happen. That driver also forms a protection boundary between low-energy logic and high-current inductive loads that can create voltage spikes, heat, electrical noise, unexpected motion, and wiring hazards.

Learning Objectives

By the end of this lesson, you should be able to:

choose a driver topology for DC motors, steppers, servos, relays, and solenoids;
explain flyback, recirculation, current limiting, and shoot-through;
estimate stall current and MOSFET conduction loss;
add fuses, reverse-polarity protection, TVS clamps, fault inputs, and emergency-stop behavior;
verify driver temperature, supply droop, wiring, grounding, and safe fault response.

Driver Types

Low-side MOSFET: good for one-direction DC loads, relays, and solenoids; check flyback path, ground switching limits, and logic-level gate drive.
High-side switch: good for protected load switching; check supply rating, current limit, and fault reporting.
H-bridge: good for reversible brushed DC motors; check peak current, dead time, and braking mode.
Stepper chopper driver: good for bipolar steppers; check phase current, decay mode, and cooling.
Servo drive: good for closed-loop motors; check feedback, following error, brake handling, and safe torque off if needed.
Relay or contactor: good for slow isolation or mains switching; check contact rating, coil suppression, arc control, and safety rules.

Do not choose a driver only by average running current. Motor startup, stall, reversal, and jam conditions define the stressful cases.

H-Bridge Basics

flowchart LR MCU["MCU PWM, DIR, EN"] --> DRV["H-bridge driver"] SUP["Motor supply"] --> FUSE["Fuse or protected input"] --> DRV DRV --> M["DC motor"] SHUNT["Current sense"] --> DRV DRV --> FAULT["Fault output"] FAULT --> MCU TVS["TVS and bulk cap"] --> DRV

An H-bridge reverses motor polarity by turning on diagonal switch pairs. Turning on the high-side and low-side switch in the same leg creates shoot-through, a direct supply short. Good drivers include dead time, gate-drive sequencing, current sensing, undervoltage lockout, and thermal shutdown.

Inductive Energy and Flyback

A motor winding resists sudden current change:

$$
v = L\frac{di}{dt}
$$

When current is interrupted, the winding forces voltage to whatever value is needed to keep current flowing unless a safe path exists. That path may be a diode, MOSFET body diode, synchronous recirculation path, TVS clamp, snubber, or an integrated driver structure.

For a relay or solenoid, a simple flyback diode protects the switch but slows release. For a motor, an H-bridge must handle current flow during PWM off-time, braking, and reversal.

Current Limit and Heat

Stall current for a brushed DC motor is:

$$
I_\text{stall} = \frac{V}{R_\text{winding}}
$$

MOSFET conduction loss is commonly estimated by:

$$
P_\text{cond} \approx I^2R_{DS(on)}
$$

If a driver path has 80 milliohm total on-resistance and motor current is 3 A:

$$
P_\text{cond} \approx 3^2 \times 0.08 = 0.72\ W
$$

That heat may be acceptable on a good PCB with airflow and copper area, or unacceptable in a small sealed enclosure. Switching loss, diode loss, ambient temperature, and neighboring heat sources add to the real thermal burden.

Protection Architecture

flowchart LR PWR["Supply input"] --> FUSE["Fuse or eFuse"] FUSE --> REV["Reverse polarity protection"] REV --> BULK["Bulk capacitor and TVS"] BULK --> DRIVER["Motor driver"] DRIVER --> MOTOR["Motor or actuator"] DRIVER --> FAULT["Fault to MCU"] LIMIT["Limit switch or guard"] --> MCU["Controller"] ESTOP["Emergency stop"] --> SAFE["Remove torque or power"] SAFE --> DRIVER

Protection should be layered. Firmware is useful, but hardware protection must handle shorts, reversed connectors, stalled loads, and unsafe motion even when software is late or wrong.

Layout and Wiring Rules

Keep high-current loops short and wide.
Place bulk capacitance close to the driver supply pins.
Route current-sense traces away from switching nodes and motor leads.
Separate motor returns from ADC and sensor references; join grounds deliberately.
Twist motor leads when practical to reduce radiated noise.
Use connectors rated for expected current, vibration, and service handling.
Add strain relief where moving cables can fatigue.

Fault Handling

Driver fault pins should be connected to firmware and tested. A useful fault response usually includes stopping PWM, disabling the driver, recording the fault, and requiring deliberate restart after inspection. For hazardous motion, use a hardware safety chain, guarded limit switches, emergency stop, safe torque off, or contactor-based power removal according to the machine risk.

Worked Example

A 24 V brushed DC motor has winding resistance 2.4 ohm. The selected H-bridge has 120 milliohm total high-side plus low-side on-resistance in the active path and a 5 A peak rating.

Stall current:

$$
I_\text{stall} = \frac{24}{2.4} = 10\ A
$$

The 5 A peak driver is not acceptable unless it has a current limit below 5 A and the motor torque requirement is still met at that limit. At 4 A, conduction loss is:

$$
P_\text{cond} \approx 4^2 \times 0.12 = 1.92\ W
$$

That is a serious thermal design question. The driver may need more copper area, a lower-resistance driver, a heat sink, airflow, or a lower current limit.

Verification Checklist

Confirm motor supply polarity with the driver disconnected.
Verify fuse or eFuse rating against wire gauge and expected fault energy.
Measure motor winding resistance and estimate stall current.
Power the driver from a current-limited supply for first motion.
Confirm PWM, direction, enable, and brake modes at low duty.
Trigger limit switch, overcurrent, thermal, and emergency-stop paths where available.
Measure supply droop during acceleration and reversal.
Measure driver temperature after repeated duty cycles.
Inspect motor-lead noise effects on sensors and communication links.

Common Mistakes

Selecting a driver from average current rather than startup, stall, and thermal limits.
Forgetting flyback or recirculation paths for inductive loads.
Ignoring heat sinking, PCB copper, enclosure temperature, and airflow.
Driving H-bridge switches with unsafe firmware states.
Sharing motor return current with precision sensor ground.
Leaving driver fault pins unconnected or untested.
Treating emergency stop as only a software button.

Summary

A motor driver is a power stage, signal translator, and protection boundary. Select the topology for the actuator, size it for peak and continuous current, provide inductive current paths, protect wiring and supply inputs, separate noisy returns, and verify fault behavior before connecting the final mechanism.

Mind Map

mindmap root((Motor Drivers)) Core concept Logic controls power Motor is inductive Driver protects boundary Faults must be tested Formulas v equals L di dt Istall equals V over Rwinding Pcond equals I squared RDSon Heat rises with current squared Applications DC H bridge Stepper chopper Servo drive Relay or solenoid Design rules Size peak current Add flyback path Prevent shoot through Fuse the supply Separate noisy returns Practical checks Current limited startup Driver temperature Fault pin action Supply droop E stop behavior Common mistakes Average current only No clamp path Poor cooling Untested fault Software only safety