Feature: Consumer electronics
Changing R1 to 11.47kΩ increases the switchover voltage to 11.8V, decreasing the step size. Te cordless telephone circuit was
much more tolerant of power steps, so R15 comprised a 22kΩ + 10kΩ resistor, yielding a switchover voltage of 5V. Te waveforms can be seen in Figure 4.
Te scope plot is set to 1s/division. Te red trace shows the 12V output from the mains- powered wall cube, and the blue trace shows the car battery voltage (at a constant 12V). Te wall cube is connected at 600ms, causing a step in the red trace from 0-12V. Te green trace shows the AC-coupled
Figure 4: 12V (green trace) is hardly disturbed when the mains (red trace) is disconnected
‘always on’ 12V output from the LTC4416. Te disturbance to the green trace could not be seen when the oscilloscope was DC coupled. Changing to AC coupling shows little disturbance when the wall cube is connected (at 600ms) and disconnected (at 5.8s). Ironically, the noise on this rail was significantly higher when the wall cube was connected, showing that the noise from the wall cube output was higher than that from the LTC3789.
Figure 5: The power supplies and ideal diodes mounted to the side of the UPS box
Modifications To allow the UPS to be inserted in series, the circuits require cables from the wall cubes to be cut. A neater solution would be to generate, say, 340V DC from the car battery, and feed this into an extension socket and then plug the wall cubes into that socket. Since the circuitry inside all wall cubes contains a rectifier, the voltage can be AC or DC. However, the losses incurred in generating 340V from a 12V battery, as well as those incurred in reducing the voltage inside the wall cube, meant that a low-voltage circuit would be far more efficient and simpler, even if it does mean cutting cables. Te LTC4416 evaluation kits include
LEDs to indicate whether the main or backup power supply is being used, and these can easily be brought to the outside casing. Another useful addition would be a pushbutton switch to artificially pull the LTC4416’s enable pins low to test the switchover functionality. In addition to being a simple home UPS
Figure 6: The complete UPS circuit with a battery
that can keep various home appliances alive in the event of a power failure, this circuit can use more powerful MOSFETs and a larger battery for increased output power and longer backup life.
www.electronicsworld.co.uk October 2022 23
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