High Speed BLDC Motor Control in a Handheld Product

A handheld dryer with a motor spinning at a hundred thousand revolutions per minute is a power electronics problem in a small plastic case. The inverter, the bus, the sensing and the thermal path all have to fit into a handle.

Why the Motor Runs So Fast

The airflow of a fan rises with the speed of the impeller, and the size of the motor falls as the speed rises for the same power. A high speed motor with a small impeller produces the same flow as a large slow one, in a much smaller volume.

The electrical consequence is a motor with a low inductance and a high electrical frequency. At a hundred thousand mechanical revolutions per minute, a two pole machine runs at well over a kilohertz of electrical frequency, and the inverter has to switch fast enough to control it.

The Power Path

The mains input is filtered, rectified and corrected for power factor to produce a high voltage bus. The bus then feeds the inverter, which drives the motor, and a separate stage controls the heating element.

Each of those stages carries the full power of the product, so the copper and the connectors are dimensioned for the current rather than for the average. The trace width calculation is the first sizing step in the layout.

Power Factor Correction

A product of this power has to meet harmonic current limits, so a correction stage is included. A boost converter in critical conduction mode is the usual choice, with an inductor, a switch and a diode operating at a high frequency.

The bus voltage is higher than the mains peak, which gives the inverter headroom, but it also means the inverter devices must be rated for the full bus. The layout of the boost loop is a significant contributor to the emission of the product.

The Inverter

Three half bridges drive the phases of the motor. The devices are chosen for a low on resistance and a low gate charge, because both the conduction and the switching loss matter at this frequency.

An integrated module simplifies the layout and reduces the loop area, at the cost of a fixed thermal path. A discrete design gives more freedom but demands more care in the commutation loop.

Gate Drive at High Frequency

The gate drive has to charge the gate quickly without ringing the gate loop. A high side device needs a bootstrap supply or a charge pump, and the bootstrap capacitor has to be large enough to hold the charge for the duty cycle the modulation demands.

The dead time is set by the driver and the firmware, and at a high electrical frequency it becomes a larger fraction of the period. Too much dead time distorts the current waveform, and too little risks a shoot through that destroys the bridge.

Current Sensing With One Shunt

A single shunt in the bus return measures the current of all three phases, sampled at the moments when the switching pattern makes the measurement meaningful. The method saves two sensors and two amplifiers.

The sampling has to be synchronised with the pulse width modulation, and the window in which the measurement is valid is short at the extremes of the duty cycle. The firmware has to know when the window is too narrow and compensate, or the control loop will see a distorted current.

Sensorless Field Oriented Control

Sensorless control estimates the rotor position from the currents and the applied voltage. At high speed an observer based on the back electromotive force works well, because the signal is large.

At low speed and at standstill the back electromotive force is absent, so a different method is used to start the motor: a forced commutation ramp, or a high frequency injection that detects the saliency of the rotor. The choice affects the audible noise at start up.

Thermal Limits

The motor, the inverter and the heating element all produce heat in a small volume. The airflow that the product exists to produce also cools the electronics, which is why the board is placed in the path of the air.

The thermal design has to be verified in the assembled product, because the airflow in a bare board test is different. A temperature measurement inside the handle at full power is the only evidence that matters.

Heating Element Control

A separate stage controls the element, using a triac with phase control or a switch with a fast chopping action. The control has to be synchronised with the mains, and the current drawn by the element dominates the total power of the product.

The element is a resistive load with a large thermal mass, so the control loop is slow. The measured airflow, the inlet temperature and the set point are the inputs, and the output is limited by the maximum temperature the plastic can tolerate.

Electromagnetic Compatibility

The product contains two switching sources and a mains connection, so the conducted and radiated limits apply at the same time. The input filter and the loop areas in the boost and the inverter are the main levers.

The motor cable is short, which helps, but the inverter still switches a high voltage at a high frequency. The layout follows the same principles as any motor drive and any switching converter.

Layout of the Power Stage

The commutation loop of each half bridge has to be as small as the layout allows, with the bus capacitors placed directly across the devices. The gate loop is kept short as well, because its inductance slows the switching it is meant to control.

The control section is placed away from the switching nodes, with its own reference and a single connection to the power ground. At these currents, a shared ground return is a voltage source in series with the current measurement.

Test and Production

Production testing covers the bus voltage, the inverter switching, the current measurement and the motor start. A functional test at low power proves the electronics, and a full power test proves the thermal design.

The parameters of the motor differ between units, so the control firmware has to identify them at start up rather than assume a nominal value. That identification is part of the production procedure rather than a laboratory exercise.

Process Control and Verification

On a design of this kind, field oriented control is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

Why use a high speed motor? Because airflow scales with speed, so a small impeller at a high speed can replace a large one at a low speed. The product becomes smaller and lighter.

What does the single shunt measurement require? Synchronisation with the switching pattern. The current is only measurable in defined windows, and the firmware has to handle the duty cycles where the window is too short.

How is the motor started without a sensor? By a forced commutation ramp or by high frequency injection. An observer based on the back electromotive force cannot work at standstill.