Question

as we know that current is directly porportional to current then why in transformer when voltage is increased the current decreases

Answer 1

Did you maybe mean "proportional to voltage"?

Answer 2

yes iia sorry ihave written it current instead of voltage

Answer 3

Total power on the primary side of the transformer is equal to the total power on the secondary side. Which means that \[\ P_1=U_1I_1=P_2=U_2I_2\]
Where P_1- input power, P_2 - output power

So if you have 10v on the primary side and the transformer has a transforming ratio n=2, you will have 20v on the secondary side. Since you increase the voltage on the secondary side you will have to lower the current on the secondary side to keep the power constant (the same as on the primary side).Secondary current will be 2 times smaller then the current on the primary side, so that the power equation can hold.

So total input power must equal total output power. If this were not the case, the great principle of conservation of energy would be false, which we know is true.