Abstract: A transformer has two magnetic cores, at least one primary winding unit mounted in the magnetic cores, at least one secondary winding unit mounted in the magnetic cores and two rectifying circuit boards externally mounted beside the magnetic cores. An AC voltage output from the secondary winding unit is transmitted to and rectified by the rectifying circuit board. Therefore, the size of the transformer is compact, and heat energy generated by electronic elements mounted on the rectifying circuit board is effectively dissipated to maintain normal operation of the transformer. Further, since the transmission path from the secondary winding unit to the rectifying circuit board is short, energy loss is reasonably reduced when the transformer is operated under a high frequency situation or a larger current mode.