[Edit: Steve/Craig, sorry, mistakenly posted to Main forum, please move to Chat forum]

Hi all,

I have a device, whose electronics require about 500mW of power @ 5Vdc to run. The device should be directly connected to 230Vrms AC mains, 50Hz. One of the requirements is, the power consumed by this product from the AC mains should be less than 1W, when measured at 230Vrms AC input.

A better device to do this job is a Buck regulator - LNK302 from Power Integrations, which is at least 60% efficient, so that for an output power of 500mW, the input power would be 800mW. The circuit is simple (extracted from Power Integrations website):

Though 800mW is less than 1W @ 230Vrms, one more requirement is the RMS line current drawn from the AC mains should not exceed 4mA RMS measured at 230Vrms AC input.

For all direct switching power supplies(isolated or non isolated), where the input section is a rectifier and capacitor filter, the current is drawn from AC mains is in peaks. Due to the nature of this waveform, the RMS to average ratio of input current drawn is much higher (compared to a sinusoidal current waveform). However, the power consumed by the circuit is still less than a Watt.

How does the approval agencies(UL, FCC, CE) look at the power consumed by units such as adaptors, cellphone chargers, which are dominantly buck switchers? How are these power supplies rated for "Green specs"?

Thanks and Regards,
Vignesh

Simulation waveforms:

1. Circuit used - R2 is to measure line current, R1 is the Buck switcher(dissipating about 850mW power)

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2. Top plot: DC voltage across C1, and power dissipated by R1
Bottom plot: AC mains voltage - 230Vrms, and current drawn from the AC mains

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3. RMS current drawn from AC mains: Note that the time intreval is between 9ms to 100ms, not from 0ms to 100ms. Because the current drawn for the very first cycle is large, as the capacitor has to charge from 0V.

RMS current drawn from the AC mains is about 11mA

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4. Power consumed from AC mains


5. Average power consumed from AC mains (first cycle is neglected as capacitor has drawn heavy currents)
Average power consumed is about 900mW