Electric Toothbrush Circuit Design: Motor and Charger

An electric toothbrush contains a motor, a battery, a radio and a charger inside a sealed handle about twenty millimetres across. The circuit is small, and every choice is dominated by the space and the sealing.

What Fits Inside the Handle

The board is narrow and often no more than four millimetres of usable height, which rules out tall components and forces the parts to be distributed along the length rather than stacked. The battery occupies most of the volume.

The motor sits at one end, the charging coil at the other, and the electronics between them. That arrangement is a mechanical decision that the layout has to follow, and it constrains where the noisy and sensitive circuits can go.

The Motor and Its Drive

A sonic toothbrush uses a two phase motor driven with an alternating waveform at a few hundred hertz. The drive is an bridge with two channels, and the current is shaped to produce the oscillation rather than a rotation.

The drive waveform is what the user perceives as the brushing action, so its amplitude and frequency are product parameters rather than engineering details. The firmware holds them, and the hardware has to be able to produce them from a battery voltage that falls as the battery discharges.

Waveform and Amplitude Control

The motor current has to be controlled rather than the voltage, because the mechanical load changes with the pressure the user applies. A current controlled drive keeps the amplitude constant, which is what makes the product feel consistent.

Sensing the current in a bridge that is driven in both directions requires either a shunt in the bridge return or two shunts. The measurement is used by the control loop and by the stall detection that protects the motor.

Power Management

The battery is a single lithium cell, and the drive needs a supply that is stable as the cell discharges. A boost or a buck boost converter provides it, with the quiescent current of the whole system set by the standby requirement.

Standby current in the microamp range is achieved by disabling everything that is not needed and by choosing a converter with a low quiescent current. The measurement of that current is part of the product specification rather than a detail.

Wireless Charging

The handle has no contacts, so the battery is charged through an inductive link. The coil, the rectifier and the charger are the same as in any small wireless charging design, scaled to a few watts and a small coil.

The coil is also a mechanical component. It has to fit inside the handle, survive the sealing and not interfere with the motor, and its position affects the coupling and the temperature of the battery beside it.

Battery Management

A single cell needs protection against overcharge, over discharge and overcurrent, and a temperature input for the charge profile. The charge current is limited by the thermal path, because the cell is wrapped in a sealed handle with no airflow.

The charge termination and the temperature fold back are implemented in the charger. The fuel gauge, where it exists, has to be calibrated so that the reported percentage does not jump when the load changes.

Radio and App Interface

Where the product reports its usage to a phone, a low energy radio is included. The antenna is small and close to the battery and the motor, so its efficiency is limited and its matching has to be verified with the product assembled.

The radio is operated on a duty cycle. A link that is maintained continuously would consume more than the motor, so the reporting interval is a design parameter that trades usefulness against battery life.

Sensors

The pressure sensor detects how hard the user is pressing and warns when the pressure is excessive. An inertial sensor detects the orientation and the movement pattern and can estimate whether each zone has been brushed.

Both sensors are small and both sit close to the motor, which is a vibration source. Their mounting and their filtering have to accept that environment, and the algorithms have to distinguish the brushing signal from the motor noise.

User Interface

The interface is an indicator, a button and sometimes a small display. The button has to be sealed, which means either a membrane or a capacitive sensor behind the plastic.

Feedback is provided by the motor itself, by a light or by a vibration pattern. The choice is a user experience decision, and each option has a different cost in power and in board area.

Sealing and Water

The handle is sealed, so the board is protected from water but not from condensation. The interior is small and the temperature changes, so moisture can condense on the electronics unless the design allows for it.

Conformal coating is the usual protection, and the assembly process has to be controlled so that the coating reaches the areas behind the components. A charging coil worked into the same space makes the process more difficult.

Thermal Design

The heat sources are the motor, the charger and the battery, all inside a sealed tube. The heat leaves through the plastic, which is a poor conductor, so the duty cycle of the product is limited by the temperature.

The charge current is limited by the same thermal path, which is why a full charge takes several hours. Increasing the current would raise the temperature of the cell beyond the limit the design allows.

Assembly and Test

The board is small and the assembly is dense, so the placement and the stencil are designed together with the panel. Testing is performed before the handle is closed, because a unit that cannot be opened afterwards has to be verified completely at that point.

The test covers the charging link, the motor drive, the sensors and the radio. The motor drive test verifies the current control, and the charge test verifies the coil coupling and the temperature fold back.

Certification

The product is certified for electrical safety, for the wireless charging function and for the radio. Because it is used in the mouth, the materials in contact are also subject to a biocompatibility requirement that constrains the choice of plastic.

The certification samples have to match the production article, including the charging coil and the sealing. A change of coil or of enclosure after approval invalidates the test, and the re-test is part of the cost of the change.

Process Control and Verification

On a design of this kind, motor drive is the item that decides how the rest of the board is arranged. 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. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

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

FAQ

Why is the motor current controlled rather than the voltage? Because the mechanical load changes with the pressure applied. Controlling the current keeps the brushing amplitude constant.

Why does charging take several hours? Because the cell is sealed with no airflow, so the charge current is limited by the temperature rise rather than by the charger.

How is the antenna matched? With the product assembled and the battery fitted, because both affect the tuning. A match measured on the bare board is not the final result.