Motor Driver Circuit Design From Requirement to Board
A motor drive is designed backwards from the mechanical requirement. The torque, the speed and the acceleration profile decide the current, and the current decides the topology, the devices and the copper on the board.
Requirements Before Topology
The first document is a specification, not a schematic. It states the motor type, the supply voltage, the continuous and peak current, the speed range and the environment, together with the efficiency target and the cost.
The peak current is the number that sizes the power stage, and the duration of the peak decides whether the thermal design is set by the continuous rating or by the overload. Both should be written down before a device is chosen.
Choosing the Topology
A brushed motor needs a single bridge, which allows four quadrant operation and regenerative braking. A brushless motor needs a three phase bridge, and a stepper needs two bridges driven with a current controlled sequence.
The topology fixes the number of switches, the gate drive channels and the current sensing arrangement. Changing it later invalidates the layout, so the choice belongs in the specification rather than in the schematic.
The Gate Drive
The gate drive has to charge and discharge the gate capacitance quickly enough to keep the switching loss low, without producing ringing that exceeds the device rating. A series resistor sets the edge rate, and a diode in parallel allows a different rate for turn on and turn off.
The drive current is often underestimated. A device with a large gate charge needs a driver that can supply the peak current the datasheet implies, and a weak driver turns a low loss device into a hot one.
Current Sensing
Current sense is implemented with a shunt, a current transformer or a sense field effect transistor. The shunt is the simplest and the most accurate at low currents, but it dissipates power and needs a Kelvin connection to avoid measuring the resistance of its own pads.
Where the current is large, the shunt loss becomes significant, and an inductive or a magnetic sensor takes over. Each method has a bandwidth, and the bandwidth has to be sufficient for the current loop, which is often faster than the mechanical time constant suggests.
Protection
The bridge needs protection against overcurrent, overvoltage and shoot through. Overcurrent is handled by the current loop and by a comparator that turns the bridge off, while overvoltage appears when the motor decelerates and the energy returns to the supply.
Shoot through is prevented by the dead time in the gate drive, and the dead time has to be long enough to guarantee that both devices are never on together but short enough not to distort the current waveform. It is a design parameter rather than a default.
Thermal Design
The losses in the bridge are conduction loss, switching loss and the loss in the gate drive. Conduction loss scales with the square of the current, so the peak rating of the drive is dominated by it.
The board is part of the heatsink. Copper area, thermal vias under the devices and the attachment of an external heatsink all decide the junction temperature, and the thermal path should be modelled before the layout is fixed rather than measured afterwards.
Layout of the Power Stage
The commutation loop is the area enclosed by the current path when a switch turns off. Minimising it reduces the inductance that produces the voltage spike, and the loop is minimised by placing the devices and the decoupling capacitors as close together as the layout allows.
The gate loop matters for the same reason. A long gate return increases the inductance in the drive path, which slows the switching and couples noise into the control circuit.
Control and Firmware Interface
The controller needs the current, the position or the back electromotive force, and the supply voltage. Those signals should be routed as a group, away from the switching nodes, and the analog measurement should be referenced to a clean ground.
The firmware interface is a hardware constraint. The number of analogue channels, the resolution and the sampling rate the algorithm needs all determine the interface, and they should be agreed before the schematic is complete.
Electromagnetic Compatibility
A motor drive is a switching converter with an inductive load, so it emits both conducted and radiated energy. The measures are familiar: keep the commutation loop small, filter the supply at the entry, and slow the edges to the extent the efficiency allows.
Motor cables are the usual culprit because they carry the switching current out of the enclosure. A shielded cable, bonded at both ends, is often the difference between passing and failing. The techniques overlap with those for any switching converter.
Test and Validation
Testing covers efficiency across the load range, the thermal behaviour at the continuous rating, the current loop response and the behaviour under a fault. Each of these should be measured on the final hardware rather than inferred from a simulation.
The fault tests deserve particular attention, because a bridge that survives normal operation may fail in a way that destroys the motor or the supply. A test that deliberately shorts an output and observes the reaction is worth the cost of a spare board.
Production Considerations
The devices in a motor drive carry large currents through relatively small pads, so the assembly process matters. Solder voids under a power device increase the thermal resistance, and the profile has to be tuned for the copper area the design uses.
The trace width calculation and the ground and power routing plan are the two documents that carry most of the thermal and electrical design into the layout.
Module or Discrete Design
An integrated module contains the bridge, the gate drive and often the protection in one package, which shortens the design and removes the layout risk. Its cost is higher and its thermal path is fixed by the package.
A discrete design gives freedom in the thermal path and in the choice of devices, at the cost of a more demanding layout. The decision usually follows from the volume and from how much thermal margin the application needs.
Sensing Alternatives and Their Bandwidth
Current sense can be resistive, inductive or magnetic, and the choice sets the bandwidth and the loss. A shunt with a fast amplifier gives the widest bandwidth at the cost of dissipation, while an inductive sensor has no loss but a limited low frequency response.
The bandwidth has to be sufficient to close the current loop, which is typically an order of magnitude faster than the mechanical response. A sensor that is fast enough for the torque control may still be too slow to protect the bridge.
Dead Time and Distortion
Dead time prevents both devices in a leg from conducting together, but it also distorts the output voltage at low current, where the direction of the current is uncertain. The resulting waveform distortion appears as torque ripple and as acoustic noise.
The compensation is implemented in the firmware, using the measured current direction to correct the duty cycle. The hardware contribution is to keep the dead time as short as the device and the driver allow.
Commissioning the Drive
Commissioning is a sequence: check the gate drive waveforms without the power stage, verify the current sense scaling, spin the motor at low voltage, then raise the load and the current in steps while monitoring the temperature.
A drive that is commissioned in that order rarely destroys itself. One that is taken straight to full load is being tested with the most expensive instrumentation available, which is the hardware itself.
FAQ
What sizes the power stage? The peak current and its duration. The continuous rating sets the thermal design, but the peak sets the devices and the copper.
Why does the commutation loop matter? Because its inductance produces a voltage spike when the current is interrupted. A smaller loop means a smaller spike and less stress on the devices.
Can a motor drive share a ground with the control circuit? It can share the plane, but the return currents must be kept apart and joined at one point, or the switching current will appear in the measurement.
