This is one of those things that I have always had a really hard time wrapping my head around.
In the horrendously simplified ideal step-up SMPS model below, I have set it up so when at 50% duty cycle, the output voltage is double the input voltage. For the sake of easy math, the current in the inductor is maintained at ~1A.
As you can see, the power transfer is 1W so it operates at 100% efficiency.
But here is the kicker… only half the current taken from the supply ever reaches the load…
Intuitively one's head is telling one it can only be 50% efficient.
Granted an ideal SMPS would not be 100% efficient, but numbers in the 90 – 98% efficiencies in a real world SMPS are not unusual…
What is going on?