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.
Current can be boosted for short durations. Capacitors could be used to supply adequate power for less than the 100% duty cycle. The batteries would be charged in spurts rather than under continuous power.