Portable Fan Circuit Design: Motor, Battery and Noise

A portable fan is a battery, a small brushless motor and a charging input in a housing that the user holds. The electronics are simple, and almost every design decision is about noise, run time and cost.

What the Product Does

The motor drives an impeller, and the user selects a speed. The airflow rises with the speed, and so does the noise, so the product is judged on the balance it strikes rather than on its maximum.

Power arrives from a USB supply, either through a cable or through a charging base. The battery gives the run time, and the charging time is part of the specification the customer sees.

The Motor and the Impeller

A small brushless motor with a low inductance runs at a few thousand revolutions per minute, and the impeller is designed for that speed. The combination sets the airflow at a given electrical power.

The motor is the dominant source of noise below the airflow limit, because its commutation and its bearings produce a tonal output. A motor with a lower cogging torque and a smoother drive produces less of it.

The Drive Stage

A three phase inverter with a sensorless algorithm drives the motor, or a single phase bridge drives a two phase winding in the simplest products. The drive is integrated into a controller in most designs.

The switching frequency is chosen above the audible band where possible, or it is randomised to spread the tone. A frequency that sits exactly on a mechanical resonance of the housing will be audible at a low speed.

Speed Control

The speed is set by the user and held by the control loop as the battery voltage falls. Without that regulation the fan slows down as the battery discharges, which the user notices immediately.

The measurement needed by the loop is the motor current or the back electromotive force. A sensorless controller estimates the speed from the electrical signals, and the estimation has to remain accurate at low speed where the signal is small.

Battery and Run Time

The battery is a single cell or a small pack, and the run time is the capacity divided by the current at the selected speed. The relationship is nearly linear at a moderate speed and worse at the highest setting.

The voltage sag at a high current is the limiting factor rather than the capacity. A cell with a low internal resistance delivers more of its energy before the controller reaches its cut off voltage.

Charging

The product charges from a USB port, so the input may be five volts or a negotiated higher voltage. A buck or a buck boost charger adapts the input to the battery, and the negotiation is handled by a controller if a fast charge protocol is used.

The charging path is also the interface for a firmware update in some products. The connector and its protection are shared, so the entry circuit is designed for both functions rather than for the charger alone.

Thermal Behaviour

The motor and the drive are the heat sources, and the airflow that the product produces cools them. That makes the thermal design dependent on the operating point in a way that is unusual.

At the lowest speed the airflow is small and the cooling is poor, so a product that runs cool at full speed may be hotter at a low setting where the motor is less efficient. The measurement should cover the whole range rather than one point.

Noise Reduction

The sources are the motor, the impeller, the airflow and the structure. Reducing the motor noise means a smoother drive, while reducing the structural noise means a stiffer housing and a better mounting for the motor.

The electrical contribution is the commutation. A sinusoidal drive produces a smoother torque than a six step drive, at a small cost in switching loss, and the change is audible on a quiet product.

User Interface and Features

The interface is a set of buttons, an indicator for the battery and sometimes an oscillation motor that moves the head of the fan. Each feature adds a load and a control requirement.

The oscillation function is a second motor driven at a low speed, and its interaction with the main motor is a power budget item rather than a mechanical detail.

Mechanical and Electrical Integration

The board is mounted in the airflow, which cools it and also exposes it to dust. A conformal coating protects the surface, and the connector for the battery has to be retained against the vibration of the motor.

The battery is the heaviest component, and its position sets the balance of the product in the hand. The layout of the board often follows from that mechanical decision rather than leading it.

Testing

Testing covers the airflow and the noise at each setting, the run time, the charging behaviour and the temperature across the range. The noise test is done in the assembled product, because the housing is part of the acoustic design.

Where the product is sold in a market with an energy label, the measurement conditions are defined by the regulation. The test must follow them, because a result obtained under other conditions is not comparable.

Cost and Component Choice

The controller, the battery and the motor dominate the cost. An integrated controller with the gate drive and the sensing inside reduces both the component count and the board area.

The trade is less freedom in the layout and a fixed thermal path. For a product of this kind the integration usually wins, and the design effort moves to the impeller and the housing.

Design Choices That Matter

The motor and the impeller are the two components that decide the performance, and they should be selected together rather than sequentially. A controller that can hold the speed across the battery range is the third.

The remaining decisions are ordinary: a trace width that carries the motor current, a clean motor drive layout and a charging path that survives the connector being plugged in repeatedly.

Process Control and Verification

On a design of this kind, noise is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Why does the fan slow down as the battery discharges? Because the drive is holding a duty cycle rather than a speed. A closed loop controller corrects the speed as the voltage falls.

Is a lower speed always quieter? Not necessarily. The airflow noise falls, but a low speed can excite a structural resonance and the motor may be less efficient.

What sets the run time? The capacity of the cell, its internal resistance and the speed setting. The voltage sag is often the limit rather than the stored energy.