Say goodbye to power outages with a home-grade UPS

question:

How to use Wi-Fi and other home devices during a power outage?

Answer:

A car battery can be used as a backup power source to design a home uninterruptible power supply (UPS). This power supply is connected to a buck-boost converter to generate a stable 12V/5A power supply for powering a Wi-Fi router and to a 6.5V/1.5A buck converter to power a cordless phone.

Introduction

As technology advances, humans become more dependent on electricity. Once there is no electricity, a luxury house can become a bare house in seconds. This article introduces how to design circuits to prevent home devices from losing power, so as to ensure that the most important service at home - Wi-Fi - remains unimpeded.

Home Uninterruptible Power Supply (UPS)

In 2022, world peace is on the brink of collapse and an energy crisis is imminent. The circuit in Figure 1 is designed to keep Wi-Fi running at home even in the event of a power outage. It takes an average of over 2 minutes for a Wi-Fi router to reboot, which might be considered a "first world problem" (trivial), but if the power goes out in the middle of a conference call, that wait can feel very long. Even a slight voltage drop can cause major problems. This home uninterruptible power supply design can provide 12V/5A power to a Wi-Fi access point (and any other electronic devices) and an additional 6.5V/1.5A power to a cordless phone. This is enough to keep most laptops communicating with the outside world.

Figure 1. Schematic diagram of an uninterruptible power supply (UPS)

The backup power source for the circuit design in Figure 1 can be a second-hand car battery bought from a scrapyard for £20. The ADI LTC3789 is a four-switch buck-boost converter that can provide a constant 12V supply with very high efficiency, and its input voltage can be higher or lower than this voltage. Its evaluation kit can provide a 12V, 5A power supply output under the condition of 5V to 36V input voltage, so it can be used directly without modification. The Wi-Fi router only needs 1A of current, so the evaluation kit can be used to power many other applications that require 12V.

The cordless phone requires a 6.5V, approximately 600mA supply, so the LT8608 was chosen to provide a low noise, high efficiency supply for this rail with a very low quiescent current (2.5μA). The maximum input voltages for the LT8608 and LTC3789 are 42V and 38V, respectively, so they are connected directly to the car battery for maximum circuit efficiency. Some lower cost battery chargers can generate high voltages if not properly connected to the battery, causing the battery to be unable to fully absorb the charging current. Therefore, if the charger has a good connection to the circuit but a poor connection to the battery, the voltage generated can damage the electronic device. The wide input voltage range of the LTC3789 and LT8608 alleviates concerns about high voltages generated when the battery charger is connected. The circuit can operate with or without the battery charger permanently connected. However, the safety aspects of leaving the battery charger permanently connected in an unventilated room depend on the type of battery and charger used.

The clever part of this circuit is provided by the LTC4416. This is a dual ideal diode that is responsible for switching the main supply voltage and the backup supply. The LTC4416 contains a precision comparator that detects a failure in the main supply and uses four external P-channel MOSFETs (PFETs) to switch to the backup supply.

A simpler form of this circuit is a dual-channel diode OR configuration with the cathodes of the two diodes connected together and the primary and backup supplies connected to the anodes. However, this circuit only feeds the highest of the two supplies to the output at the cathode and results in a 0.6V loss on the diode. A more efficient circuit can be designed by replacing the diode with a PFET. The voltage drop across the PFET body diode is measured and if it exceeds a certain threshold, the FET turns on, shorting the body diode. If this voltage drop is negative, the drive to the PFET is removed and the body diode blocks reverse current. This creates an ideal diode with a low forward voltage drop and reverse blocking capabilities, as shown in Figure 2.

Figure 2. Ideal diode implementation of a diode OR circuit.

In this circuit, the body diode of each PFET points from input to output, so if one input voltage is more than 600mV higher than the other, the body diode will conduct. Therefore, if the backup supply happens to be higher than the main supply, the load will be powered by the backup supply, which is undesirable. Reversing the PFETs can solve this problem, but if the output voltage is more than 600mV higher than the input voltage, the body diode will conduct and current will flow backwards.

A more elegant solution is to add an additional PFET for each path, as shown in Figure 3. In this circuit, the two body diodes face each other, so when the FET is off, the circuit provides a bidirectional open circuit and isolates each channel regardless of the input or output voltage.

Figure 3. Diode OR circuit with bidirectional disconnect function

For the 12V circuit, the evaluation kit for the LTC4416 (DC1059A) was modified to provide a switching voltage of 11.17V, using a 100kΩ resistor for R3 and 10kΩ + 2.2kΩ for R1. This worked well, but ADI found that the Wi-Fi access point requires a precise 12V supply, and sometimes it would restart when the 12V main power supply was switched back again. This was due to the voltage step (from 11.17V to 12V) disrupting the router electronics. Changing R1 to 11.47kΩ increased the switching voltage to 11.8V, reducing the size of the voltage step.

The cordless phone circuit is more sensitive to power supply steps, so R15 consists of 22kΩ + 10kΩ resistors to provide a switching voltage of 5V.

The waveforms are shown in Figure 4. The green trace shows the always-on 12V output of the LTC4416, the red trace shows the 12V output of the wall adapter (main power), and the blue trace shows the car battery voltage. When the oscilloscope is DC-coupled, no interference is seen on the green trace. After changing to AC coupling, very little interference can be seen when the main 12V power supply is connected (600ms) and disconnected (5.8s). Ironically, the noise on this rail is significantly higher when the 12V main power supply is connected, indicating that the wall adapter output is noisier than the LTC3789.

Figure 4. When the main power supply (red trace) is disconnected, the 12V (green trace) is almost undisturbed

A photograph of the UPS electronics is shown in Figure 5, and the complete circuit is shown in Figure 6.

Figure 5. Power supply and ideal diode mounted on the side of the UPS box

Figure 6. Complete circuit with battery

Future Improvements

The previously mentioned circuit requires cutting the cable from the wall adapter to allow the UPS to be plugged in in series. A cleaner solution is to generate 340V DC from the car battery and feed it into the extension socket, then plug the wall adapter into the extension socket. Since all wall adapters contain a rectifier in their internal circuitry, it doesn't matter whether this voltage is AC or DC. However, there are losses in generating 340V from a 12V battery, and there are also losses in reducing that voltage within the wall adapter, which means that a low-voltage circuit will be more efficient and simpler, even if it requires cutting the wall adapter cable.

The LTC4416 evaluation kit includes LEDs to indicate whether the main or backup power is being used, and these LEDs are easily placed on the case. Another useful addition is a push button switch for artificially pulling the LTC4416's enable pin low to test the switching function.

This circuit has been extensively tested and performs well. For higher currents, an N-channel ideal diode can be used. The LTC4416 is one of a variety of ideal diodes and hot swap devices available from Analog Devices.

in conclusion

The circuit described in this article shows the design of a simple domestic uninterruptible power supply that can keep various household appliances running during a power outage. This circuit can of course be modified to provide higher output power and longer backup time using more powerful MOSFETs and larger batteries.