1. Technical Field
The present invention relates to a charging unit. In particular, although not exclusively, the present invention relates to a charging unit comprising a switched mode power supply.
2. Description of Related Art
Portable devices are often powered by a rechargeable battery. Most commonly, the rechargeable battery is charged by a charger including an AC to DC converter and being connected to an AC mains power supply. In addition, DC chargers such as vehicle chargers can be used to recharge batteries. Such chargers typically can provide charging currents of around 1 A.
Some portable devices have a USB port for connection to a host device. The USB connection can be used for data transfer. For example, a mobile phone may be connected to a computer for synchronising their respective address books.
The USB specification also provides that a USB connection is able to supply power to a device. However, the power that can be supplied is limited. The USB specification recites a DC voltage of 4.75 to 5.25V, and the current that can be provided through a USB connection is limited to around 0.5 A. Thus, the time taken to charge a portable device using only USB charging can be significantly longer than is found when using AC mains charging.
Portable devices that allow both USB charging and AC mains charging have been proposed. One example is described in US 2005/0141208. This document describes a USB device that can be connected simultaneously to an AC adapter and to a USB port on a host device by a USB connection. When the AC adapter is connected, the USB device operates as a self-powered USB device, i.e. it is not supplied with power from the host device.