The design is not a phone charger, but a stand-by supply for TVs and other consumer appliances that can deliver 1mW to other circuits while idling at 4mW.
“We are moving very close to the ultimate in stand-by power performance,” said Edward Ong, product marketing manager at the firm. “By leveraging recent advances in low-power sensors and processors driven by the mobile-phone market, 1mW of standby power is more than enough to operate functions such as a digital clock, environmental monitoring, wireless signal reception and infra-red activation.”
It is based around a Power Integrations’ LinkZero-LP ICs, and appears in application note DER-417 – a universal-input, 5V 1,600mA fly-back power supply.
LinkZero-LP has a simple control strategy – delivering energy pulses to its output until the output rises to the correct voltage, at which point it stops delivering pulses until the output drops.
If 416 consecutive switching cycles have been skipped, the chip enters power-down mode and, to save power, inhibits switching and shuts off most of its internal circuits. Wake-up occurs when the voltage at C9 (see diagram) drifts down to 3V (from 5.8V). The circuit D14, U6A/U6B etcc discharges C9 quickly for fast turn-on should an output load suddenly appear.
“Power-down mode occurs when the output load has reduced to about 0.3% of full load,” said the firm.
D12 C24 is the main output rectifier and output voltage is regulated here. D13 isolates this from the main output capacitors for reasons not clearly explained in the application note (amongst other anomalies), and adds one diode drop of additional power loss during operation. This Schottky diode, in series with the output, at full 1.6A output will be dissipating perhaps 600mW, so lifetime reductions in wasted power are to be had only when the power supply is on stand-by far more than it is under load.
Edward Ong has quickly replied to Electronics Weekly over anomalies in the application note. If you are trying to work out how this interesting low power design works, he has also added further explanation: “In the schematic C28 was mislabeled as C35. The charge path for this capacitor will be through D14 and D15. D13 functions as a blocking diode that isolates the large value output capacitors C21 and C25 from directly loading the output of the flyback converter. This will allow the flyback converter to go to power-down mode when the output load is just the dummy load R30. R30 represents the system IR sensor or system MCU in stand-by mode.”
TVs, appliances, security and monitoring systems and air-conditioning equipment use power while waiting to be used or while monitoring sensor inputs prior to executing their proper functions, said the firm. “Lawrence Berkeley National Laboratory has estimated that standby power accounts for 5-10% of residential electricity use in developed countries, and is responsible for approximately 400 million tons of global CO2 emissions each year.”