3 Battery Powered Appliance System Overview

Battery Charger

Many tools used around the home these days market themselves as cordless, enabling free usage wherever needed. To provide this feature, appliances implement battery packs to source the required power. These battery packs need to be charged from the wall, and once connected to the appliance, the power must be delivered to the motor or attached components. The following sections discuss these subsystems and why a gate drive is utilized.

Figure 3-1 Battery Charger

AC voltage standards of America or Europe alike (100-120VAC, 220-240VAC respectively) are often rectified and boosted to 400V. The rectification is achieved via a Power Factor Correction (PFC) circuit. Shown is a typical boost PFC circuit, employing a low-side gate driver. This is a simple and cost-efficient approach, important to keep end-system costs low.

A DC/DC conversion is then needed to charge the battery pack at the proper voltage level. This is employed for a range of electrified appliances, from power tools to battery-powered riding mowers. A viable way to drive the half-bridge for this DC/DC conversion is with a push-pull circuit.

Figure 3-2 Pulse Transformer for DC/DC

This implementation can be seen fully illustrated in the following reference design and explained in detail in the following application note.

Battery Disconnect Switch

Figure 3-3 Battery Disconnect Switch

Some configurations have this 7^th MOSFET on the high-side of the circuit, rather than where it is shown above. See this application brief for a more detailed explanation of the advantages and differences of designing with high-side versus low-side configurations. If a high-side implementation is deemed preferable, this application report explains how this can be achieved with a half-bridge gate driver.

Motor Drive

Gate drivers are used for each winding on the motor, most often employing three total to support the 3-phases typical of a brushless DC motor.

Figure 3-4 Three-Phase Motor Drive

To select a gate driver for motor drive systems, it is important to consider the voltage level of the system. Smaller products such as power tools or vacuums typically operate on 48V or less, making a 120V half-bridge optimal. For larger equipment such as professional tools or riding lawn mowers, multiple batteries can be stacked to operate at 80V or greater, meaning a 220V half-bridge or even 700V can be required to provide sufficient headroom for noisy signals.