Zero current switching SMPS with common emitter or source
If you have an SMPS that stores energy in an inductor and then releases that energy (per cycle) to an output capacitor then you must &quot;switch&quot; when the inductor current is non-zero because if the current in the inductor is zero then there is zero energy to transfer and the circuit does nothing. Energy stored in an inductor is: -
$$\dfrac{1}{2} LI^2$$
So, no \$I\$ means no energy transfer means no useful product.