Feature: Consumer electronics
Building an uninterruptible power
supply for home devices By Simon Bramble, Staff Engineer, Analog Devices
so here we describe an uninterruptible power supply (UPS) design (Figure 1) for the home to keep alive the most essential services.
A
UPS for the home With the energy supply reaching a crisis point this year, this UPS design will keep Wi-Fi alive in a power outage. Since it takes over two minutes for the average Wi-Fi router to reboot, it’s important to have a steady energy supply. Tis UPS provides 12V/5A for the Wi-Fi access point, as well as other electronics, and an additional 6.5V/1.5A for a cordless telephone – enough to maintain communication with the outside world. Te backup power source for the circuit
in Figure 1 is a used car battery, purchased from a scrapyard for £20; see Figures 5 and 6. Te LTC3789 is a four-switch buck-boost converter that provides a constant 12V supply, from an input that can be above or below this
s the world becomes more advanced, our dependence on electricity grows. Yet, power outages can reduce the most sophisticated homes to primitive ones,
voltage. Its evaluation kit provides 12V at 5A from a 5V to 36V input voltage. Since the Wi-Fi router only needs 1A, this evaluation kit can be used to power other applications that require 12V. Te cordless telephone typically needs
6.5V at about 600mA, so LT8608 from Analog Devices was chosen to provide this rail with 2.5μA quiescent current. Te LT8608 and LTC3789 have maximum input voltages of 42V and 38V respectively, so they were connected directly to the car battery for highest circuit efficiency. Some lower-cost battery chargers can generate high voltages if they are not connected to the battery correctly, since the battery does not adequately absorb the charging current. Terefore, if the charger has a good connection to the circuit but a poor connection to the battery, voltage can be generated that might damage the electronics. Tis circuit can be operated either with
the charger permanently connected or not. Note that the safety aspect of keeping the battery charger permanently connected in an unventilated room depends on the type of battery and charger used.
In this circuit, the LTC4416 provides a dual ideal
diode for switching between the mains voltage and backup supplies. Te LTC4416 contains a precision comparator that detects when the mains supply has failed and switches over to its backup supply using four external P-channel MOSFETs (PFETs).
Simpler circuits A simpler form of this circuit is a dual diode OR configuration, where the cathodes of two diodes are joined and the mains and backup supplies are connected to the anodes. However, this circuit only feeds the highest of the two supplies to the output at the cathode and incurs a loss of 0.6V across the diode. A more efficient circuit can be designed using
PFETs, to replace the diodes. Te voltage drop across the body diode of the PFET is measured and, if a certain threshold is exceeded, the FET is switched on, thus shorting out the body diode. If this voltage drop goes negative, the drive to
the PFET is removed and the body diode blocks the reverse current; thus, an ideal diode has been created that has a low forward voltage drop and blocks in the reverse direction; see Figure 2. With our UPS circuit, the body diode of each PFET points from the input to the output, so if
www.electronicsworld.co.uk October 2022 21
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