First, AC12V to AC24V can be boosted by a boost transformer according to 1:2:
Second, AC12V to DC24V can be boosted by the booster transformer according to 1:2 after the rectifier filter voltage regulation:
Third, DC12V to AC24V can be achieved by the way of oscillation-boost.
Fourth, DC12V to DC24V can be actually boosted by a series of circuits such as oscillation - boost - rectification - filter - voltage regulation.