Depending on the adapter and the battery’s voltage, the on-board battery charging topology can be a buck or buck-boost type. For a buck charger, one high-side MOSFET and one synchronous FET are required. The charging operation frequency is improved to 700-800KHz. The design size is reduced. The buck charger requires the input voltage to have enough headroom to charge the battery to full. A 4S battery has a typical 16.8V fully charged voltage. The adapter voltage needs to have enough headroom to accommodate the voltage drop on the charging path from the non-designed components and increased contact resistance as shown in Figure 3-1. The BQ24725A is a charger controller widely adopted for the buck charging design when the adapter is available [3].

The buck-boost charger provides the flexibility to charge the different battery configurations from the different input voltages. The benefit is to have the same adapter for different generations or platforms of vacuum cleaning robots with different battery configurations, cell voltages, and capacities. This can help reduce the R&D cost and total system cost. The long-term benefit is that the standardized USB-PD charging adapters can be adopted as the input charging source, so the cost of the total design can be further reduced. Figure 3-2 shows the buck-boost charger controller BQ25730.