Device charging system, charging method, and power adapter

A power adapter, comprising:

• a first rectification unit, without an electrolytic capacitor provided after the first rectification unit, configured to rectify an input alternating current and output a voltage with a first pulsating waveform;
• a switch unit, configured to modulate the voltage with the first pulsating waveform according to a control signal;
• a transformer, configured to output a voltage with a second pulsating waveform according to the modulated voltage with the first pulsating waveform;
• a second rectification unit, configured to rectify the voltage with the second pulsating waveform to output a voltage with a third pulsating waveform, wherein the voltage with the third pulsating waveform is configured to be applied to a battery in a device for charging the battery of the device;
• a first charging interface, coupled to the second rectification unit, configured to apply the voltage with the third pulsating waveform to the battery via a second charging interface of the device when the first charging interface is coupled to the second charging interface, in which the second charging interface is coupled to the battery, and the first charging interface comprises:
• a power wire, configured to charge the battery, and a data wire, configured to communicate with the device;
• a control unit, coupled to the switch unit, and configured to output the control signal to the switch unit and to adjust a duty ratio of the control signal such that the voltage with the third pulsating waveform meets a charging requirement of the device, and during a process that the power adaptor charges the battery by outputting the voltage with the third pulsating waveform to the battery, the control unit is further configured to adjust the duty ratio of the control signal to decrease a time length of a valley of the voltage with the third pulsating waveform under a case that a peak value of a voltage of the battery is sampled,
• wherein, the power adapter is configured to charge the device in a second charging mode, and the control unit is configured to send a first instruction to the device when determining that a charging duration of the second charging mode is greater than a predetermined threshold, in which, a charging speed of the first charging mode being faster than that of the second charging mode, the first instruction is configured to query the device whether to enable the first charging mode, and when determining that the charging duration of the second charging mode is greater than the predetermined threshold, the power adapter determines that the device has identified the power adapter, such that the first charging mode is enabled;
• and
• the control unit is configured to receive a reply instruction to the first instruction from the device, in which the reply instruction to the first instruction is configured to indicate that the device agrees to enable the first charging mode.