In my dc to dc buck converter(0 to 17V - 12v), when i connect a load ,converter output voltage decreases to 1V
Your power source can source (dc converter or the power supply feeding it) can supply a limited power (P=V*I).

So in example if a power supply is set on 1W then at 10V it can source 100mA. If a 1k resistor is placed it consumes Pload = V²/R = 0.1W which is less than the power supply can provide and you get all the current you need for it.

Now if you have a 1 ohm resistor instead, Pload = 100W which is more than the source can provide. In that case the power source will operate at is limit by lowering the voltage to a level that Pload = Psupply. 
For this example the supply will drop the voltage to around 1V so the total Pload is 1W.