Load-detecting Power Supply

Reduce wasted energy from inefficient AC/DC wall adapters that run even without a load. This design utilizes SMPS technology instead of linear regulators to drastically improve conversion efficiency. Additional circuitry "wakes up" the power supply only when a load is applied, reducing losses to just tens of milliwatts.

Source: National Semiconductor
11/10/2008

Every year, millions of Watts are lost in heat from inefficient linear AC/DC wall adapters. This loss is incurred even when there is no load applied. The cause of this inefficiency is energy lost from leakage elements found in the transformer and electrolytic capacitors. This reference design shows that by using a switch-mode power supply (SMPS) instead of a linear supply, you can drastically reduce the amount of wasted energy. Additional circuitry “wakes up” the power supply only when a load is applied, reducing losses to just tens of milliwatts.


This wake-on-load AC-DC power adapter is a wall mount power supply that converts 100-240 Vac to 5 Vdc at 500 mA to a USB port. The typical application for this type of AC-DC adapter is a cell phone charger. The unique feature of this power adapter is extremely low power consumption when the output is not loaded.

The power adapter uses National’s LM5021 pulse-width modulation controller and has a load detection and shutdown circuit which turns off the PWM integrated circuit at “no load.” It won’t restart until the output is loaded (wake on load), or the output capacitor is discharged to a certain level. National designed the start-up circuit to consume less than 40 uA current at high line input during shutdown period. By doing so, the no-load power consumption of this adapter can be greatly reduced.

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