Electronic – SMPS Efficiency Paradox


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.


simulate this circuit – Schematic created using CircuitLab

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?

Best Answer

Current != energy.

While current is flowing to ground through the switch, the energy represented by that current flow is being stored in the inductor.

When the switch opens, that energy is delivered to the load.