Power supply device, organic light emitting display device having the same, and method of supplying power

A power supply device comprising:

• a switching mode power supply (SMPS) circuit unit including an electric transformer having a first stage and a second stage, the SMPS circuit unit configured to generate a second stage output voltage by using the electric transformer;
• a power switching unit configured to receive the second stage output voltage, and further configured to switch the second output voltage to generate a first power voltage that has a logic high level and a logic low level in one frame;
• and
• a pulse width modulation (PWM) control unit configured to control a voltage level of the second stage output voltage by providing a control signal to the first stage of the electric transformer,
• wherein the first power voltage is simultaneously provided to a plurality of pixels included in a display panel, and wherein a magnitude of a logic low level of the first power voltage is determined according to a magnitude of an image load, wherein the magnitude of the image load is determined by a color and a gradation level of an image to be displayed in a next frame.