A method of controlling a power converter (1; 2) having a first stage converter (10; 20) and a second stage converter (11; 21), comprising: converting an input voltage (V in) into an intermediate voltage (V mid); converting the intermediate voltage (V mid) into an output voltage (V out) and providing the output voltage (V out) to a load; detecting a loading amount of the load; characterized in that if the loading amount of the load is larger than a first threshold value, the load is entered into a heavy load condition and the power converter adjusts the intermediate voltage (V mid) so that the intermediate voltage (V mid) has the maximum value at the moment when the load enters the heavy load condition, and a change of the intermediate voltage (V mid) is in a negative correlation with a change of the loading amount of the load as the loading amount of the load is continuously increased from the entry into the heavy load condition; and if the loading amount of the load is smaller than a second threshold value, the load entering into a light load condition and the power converter keeping the intermediate voltage (V mid) unchanged or adjusting the intermediate voltage (V mid), so that the change of the intermediate voltage (V mid) is in a positive correlation with the change of the loading amount of the load within at least a loading change range, wherein the first threshold value is larger than the second threshold value; wherein if the loading amount of the load is in a range between the first threshold value and the second threshold value, the intermediate voltage (Vmid) is adjusted to increase at a predetermined increase ratio according to the increasing loading amount of the load, wherein during an operation of the second stage converter (11; 21), conduction loss, switching loss, driving loss and core loss occur, and wherein under the heavy load condition the working mode of the first stage converter is set as a continuous working mode, wherein if the first stage converter is a resonant circuit topology, the switching frequency of the first stage converter is adjusted and if the first stage converter is a pulse width modulation circuit topology, the duty ration of the pulse width modulation signal is adjusted. converting an input voltage (V in) into an intermediate voltage (V mid); converting the intermediate voltage (V mid) into an output voltage (V out) and providing the output voltage (V out) to a load; detecting a loading amount of the load; characterized in that if the loading amount of the load is larger than a first threshold value, the load is entered into a heavy load condition and the power converter adjusts the intermediate voltage (V mid) so that the intermediate voltage (V mid) has the maximum value at the moment when the load enters the heavy load condition, and a change of the intermediate voltage (V mid) is in a negative correlation with a change of the loading amount of the load as the loading amount of the load is continuously increased from the entry into the heavy load condition; and if the loading amount of the load is smaller than a second threshold value, the load entering into a light load condition and the power converter keeping the intermediate voltage (V mid) unchanged or adjusting the intermediate voltage (V mid), so that the change of the intermediate voltage (V mid) is in a positive correlation with the change of the loading amount of the load within at least a loading change range, wherein the first threshold value is larger than the second threshold value; wherein if the loading amount of the load is in a range between the first threshold value and the second threshold value, the intermediate voltage (Vmid) is adjusted to increase at a predetermined increase ratio according to the increasing loading amount of the load, wherein during an operation of the second stage converter (11; 21), conduction loss, switching loss, driving loss and core loss occur, and wherein under the heavy load condition the working mode of the first stage converter is set as a continuous working mode, wherein if the first stage converter is a resonant circuit topology, the switching frequency of the first stage converter is adjusted and if the first stage converter is a pulse width modulation circuit topology, the duty ration of the pulse width modulation signal is adjusted.