I'm designing a circuit to charge mobile battery. My source is giving a DC output of 15V, 100mA, whereas I need DC 6V, 450mA for charging. Is there any way to boost DC current?
Think of the problem in terms of power. Power = current * voltage. The maximum power input of your system is 15V * 0.1A = 1.5 Watts. Suppose you design a Buck regulator to step the voltage down to 6V. The regulator is not 100% efficient (80% is not an unreasonable number). 80% of 1.5W is 1.2W.
Your desired output is 6V * 0.45A = 2.7W, more than twice what your regulated source can provide. You can't get something from nothing. You're going to need a power source that outputs more current (and/or at a higher voltage) such that the regulator can output at least 2.7W after conversion losses.