A power adapter compri sing: a first rectifier configured to rectify an input alternating current and output a first voltage with a first ripple waveform; a switch unit configured to modulate the first voltage according to a control signal and output a modulated first voltage; a transformer having a primary winding and a secondary winding, and configured to output a second voltage with a second ripple waveform according to the modulated first voltage, wherein a first end of the primary winding is coupled to a first output end of the first rectifier without electrolytic condensers and a second end of the primary winding is coupled to the switch unit; a second rectifier coupled to the secondary winding, and configured to rectify the second voltage to output a third voltage with a third ripple waveform, wherein the third voltage is configured to be introduced into a terminal to charge a battery in the terminal when the power adapter is coupled to the terminal; a sampling unit configured to sample voltage and/or current at the primary winding to obtain a voltage sampling value and/or a current sampling value, wherein the sampling unit is configured to sample the voltage at the primary winding by sampling the voltage across two ends of the primary winding, and to sample the current at the primary winding by sampling voltage on a resistor coupled to the second end of the primary winding; a controllable switch and a filtering unit coupled in series to a first output end of the second rectifier; and a control unit coupled to the sampling unit and the switch unit respectively, and configured to output the control signal to the switch unit, and to change an output of the second rectifier by adjusting a duty ratio of the control signal according to the current sampling value and/or the voltage sampling value such that the third voltage meets a charging requirement of the battery of the terminal when the power adapter is coupled to the terminal to be charged, wherein the control unit is further configured to: communicate with the terminal to determine a charging mode; control the controllable switch to switch on when determining the charging mode as a first charging mode so as to introduce the filtering unit to perform a filtering function on the output of the second rectifier to realize direct current charging of the terminal when the power adapter is coupled to the terminal, and control the controllable switch to switch off when determining the charging mode as a second charging mode; in which a charging speed of the second charging mode is faster than that of the first charging mode. a first rectifier configured to rectify an input alternating current and output a first voltage with a first ripple waveform; a switch unit configured to modulate the first voltage according to a control signal and output a modulated first voltage; a transformer having a primary winding and a secondary winding, and configured to output a second voltage with a second ripple waveform according to the modulated first voltage, wherein a first end of the primary winding is coupled to a first output end of the first rectifier without electrolytic condensers and a second end of the primary winding is coupled to the switch unit; a second rectifier coupled to the secondary winding, and configured to rectify the second voltage to output a third voltage with a third ripple waveform, wherein the third voltage is configured to be introduced into a terminal to charge a battery in the terminal when the power adapter is coupled to the terminal; a sampling unit configured to sample voltage and/or current at the primary winding to obtain a voltage sampling value and/or a current sampling value, wherein the sampling unit is configured to sample the voltage at the primary winding by sampling the voltage across two ends of the primary winding, and to sample the current at the primary winding by sampling voltage on a resistor coupled to the second end of the primary winding; a controllable switch and a filtering unit coupled in series to a first output end of the second rectifier; and a control unit coupled to the sampling unit and the switch unit respectively, and configured to output the control signal to the switch unit, and to change an output of the second rectifier by adjusting a duty ratio of the control signal according to the current sampling value and/or the voltage sampling value such that the third voltage meets a charging requirement of the battery of the terminal when the power adapter is coupled to the terminal to be charged, wherein the control unit is further configured to: communicate with the terminal to determine a charging mode; control the controllable switch to switch on when determining the charging mode as a first charging mode so as to introduce the filtering unit to perform a filtering function on the output of the second rectifier to realize direct current charging of the terminal when the power adapter is coupled to the terminal, and control the controllable switch to switch off when determining the charging mode as a second charging mode; communicate with the terminal to determine a charging mode; control the controllable switch to switch on when determining the charging mode as a first charging mode so as to introduce the filtering unit to perform a filtering function on the output of the second rectifier to realize direct current charging of the terminal when the power adapter is coupled to the terminal, and control the controllable switch to switch off when determining the charging mode as a second charging mode; in which a charging speed of the second charging mode is faster than that of the first charging mode.