Two methods :a, try to use thick wire wound secondary b, change to a larger cross-sectional area of the magnetic core transformer. Among them, a is relatively simple, as long as the window allows, try to use a thick line according to the original number of turns can be rewound,b to recalculate and debug, for not very proficient switching power is likely to mess up. In addition, the secondary half-wave rectifier can be replaced by full-bridge rectifier to reduce the DC component in the transformer and reduce the heat output. Also, add fan forced cooling can also try.