Fast charging has become one of the must-have features of the new generation Android phones. Where previous smartphone generations charged with powers between 15W~45W, the latest high-end phones charge with peak powers of 100W or more, achieving a full charge within 20 minutes! So how do these phones achieve the high power charging without high losses in both charger system and the battery? In our latest fasting charging video, we will explain how high power charging works, and how to a design a high efficiency charging system using dual-cell battery technology.
High power charging in single cell battery systems will quickly lead to very high battery charging current, which increases losses in the charger and battery. A way to increase charging power without increasing battery current is to use two batteries in series for higher total battery voltage. A single high voltage capacitive divider like RT9758 is then used to achieve a high input bus voltage with relatively low bus current. This system can achieve around 42W charging with 97.2% efficiency. In dual cell systems, cell balancing is needed to avoid exceeding the maximum cell voltage. In the video, we will explain how to design a dual-cell battery charging system with a buck-boost switching charger RT9490, a capacitive divider RT9758 and cell-balancing.
You may have heard about smartphones that can charge with 100W or more and can be fully charged within 20 minutes. How is this accomplished?
These phones use the dual cell charging system with two high voltage capacitive divider circuits like the RT9758 in parallel, thereby doubling the output charge current capability. In the video, we will show you how to design and test the RT9758 in parallel configuration to achieve 87W high power charging in USB type C PD PPS 5A mode. Some smartphones use special adapters that can provide 6A current, and then the charging power can be more than 100W. Where lower power systems charge batteries with around 2C, the high power systems charge each battery with more than 5C, which makes it possible to achieve a full charge within 20 minutes.
You might wonder whether these battery cells can handle such high charging currents. We can assume that these cells are specially designed to have extremely low internal resistance, down to 1.2mΩ, so they don’t heat up so much under high charge or discharge currents.
In most high power charging systems, the maximum power is only used at the beginning of the charge cycle, to quickly top-off the battery charge. The charging power will gradually reduce for the remaining part of the charging. This will reduce the temperature rise within the phone and will help to get a more optimally charged battery, at the expense of slightly longer charging time.
In the future, you’ll see not only high-power USB PD travel adapters, but also the new generation cars will have high power USB ports which are powered from the car battery like the RTQ7880-QT and our new generation RTQ7882-QT buck-boost car charger solutions that can achieve charging powers up to 100W. High power charging systems can be used not only to charge your phone, but also the newer tablets and Notebook PCs that include USB type C ports.