Many of you would be using your computer every
day, which is connected to an un-interruptible Power supply, also known as UPS rather than
a wall socket. But, have you ever wondered how does a UPS
work? In this video, we will explore all the components,
their working, understand the working of the circuit on the motherboard, types of UPS with
their workings, and some variations in UPS designs. Before that, We would recommend you to watch
these 4 videos on how these circuits work, and you can watch this video to understand
how a MOSFET work. Now, as you have watched them, let's open
this UPS. As you can see, there is a battery, transformer,
and a Motherboard, there is also a daughterboard for the front panel. Let's look at the components and their wiring. This is a 12V 7.2 Amp-hour Lead-acid battery,
with a fast-charging voltage of 14.5 to 14.9 volts, and a standby charge of 13.6 to 13.8
volts. This is the charge cycle graph of a Lead Acid
battery. The first phase is the constant current phase. Thus the initial current is less than or equal
to the maximum charge current, that should be supplied to the battery. Then, during the fast charge phase, it uses
the cycle use voltage to charge the battery. And, when the battery reaches a certain charge,
then it is charged slowly, the voltage across its terminals is at the floating use voltage,
and it is kept at that voltage even if the battery is fully charged. The battery is connected to the motherboard
between the MOSFETs. The MOSFETs Drains are connected to the secondary
winding of the transformer. And, These 4 wires are from the primary winding
of the transformer. The black wire is common and other wires are
different taps. We will explore them later in the video. These 4 wires are connected to the Motherboard
through this pin. Then we have these two wires, they are connected
to the output sockets. And the input is connected to the board through
this pin. Now, let's look at the components of the board. This red connector is the input from the mains
AC. This is the safety and filter section for
the input. Here we have a MOV, or varistor , safety capacitor,
filter capacitor, coupled inductor, and fuse. There are multiple of these components, for
simplicity one of each is shown. This is a current sensing transformer, and
voltage is sensed by a voltage divider below the PCB. This is a snubber network for the primary
winding of the transformer. Next, we have 4 relays, the first relay is
used to connect the live wire to the circuit. Hence, when the UPS is shut down, or the UPS
is in backup mode, the live wire is disconnected from the rest of the circuit. These two relays are used for switching the
taps in the transformer's primary winding. We will discuss them later in the video. This relay is used to connect to the output
sockets. Hence, when the UPS is turned off, the UPS
can still charge the battery. Moving on, we have a familiar-looking circuit. This is the charger circuit from our first
video. This is used to charge the battery and to
keep 13.6 to 13.8-volt continuously at its terminals. This is a full-bridge rectifier IC. Here we have a battery monitoring IC, and
a charger control IC. And this is the snubber capacitor for the
transformer. The diode and resistor are below the PCB as
SMD components. Now, these blocks contain the main controls
of the board. They are used to monitor and control all the
parts of the circuit. They are digital ICs and not analog circuits. Hence, we will explore them in another video. But, for basics, they monitor the voltages
at different parts of the board and react to the changes in the voltage by changing
their output signals. They are used to control the relays, depending
on the mains voltage. They monitor the charging circuit and somewhat
control them. They monitor the current, temperature of the
circuit, and control the next section, which we will explore. Between them, is the power supply for controllers, It can be another small transformer with a rectifier and a filter. Or it can be a voltage regulator. As the controllers use very low current, voltage
regulators can be used. Or any other type of power supply can be used
depending on the manufacturer. This is a buzzer, It produces a sound to indicate
the state of the UPS. Now, the last part of the board contains an
Inverter. This is used to convert DC from the battery
to AC for the output. These are the connection for battery terminals,
and these are the terminals for the load. The back of the power MOSFETs is connected
to its drain terminal, hence we use the heatsink as a terminal, and connect it to the transformer's
secondary winding. The current flowing through the MOSFETs is
high. For example, we have a 13.6 volts peak voltage,
and the output requires 340 volts peak voltage. The voltage is increased by 25 times. Thus we need 25 times the output current at
the input. Hence, you can see two or more MOSFETs in
parallel to distribute the current. This high current also generates high heat,
hence we use the heatsink to dissipate it. These are the snubber network capacitors of
the MOSFETs, resistors are below the PCB. These 4 MOSFETs are used to control the MOSFETs
of H-Bridge. They are not used as signal inverters, that
were used in the Inverter video. The signal to these MOSFETs is provided by
the controller, Thus the controller can provide an inverted signal also. They are used to provide high voltage, to
the gate of H-Bridge MOSFETs, from the low voltage signal of the controller. Also, you can see there are more three MOSFETs,
these are the temperature sensors, they are used to monitor the temperature of the MOSFETs. Now, let's explore why the transformer has
4 wires. Let's say this is the primary winding of the
transformer, and it is made of 36 turns. Also, here we will refer to the peak voltage
of the mains, that is, 340 volts, the 240 volts is the RMS voltage. Now, let's apply 360 volts to the winding,
thus the windings will have 10 volts per turn. That is, there is a voltage difference of
10 volts between each turn. If we measure the voltage across neutral,
and second turn, we will get 350 volts, as 10 volts drop each turn. Similarly, if we measure between neutral,
and third turn, we will get 340 volts. Now, let's apply 320 volts between neutral,
and third turn. The voltage per turn is 9.4. But let's assume it as 10 volts per turn for
simplicity. As reducing a turn dropped the voltage by
10, thus, if we add a turn the voltage will increase by 10 volts. Hence, if we measure between the neutral,
and second turn, we get 330 volts. Similarly, the voltage between the neutral,
and first turn will be 340 volts. The increase in voltage happens because, the
changing voltage in 34 turns of the windings, induces a changing magnetic field, which induces
the changing voltage in all the 36 turns of the windings. It's like a transformer that has 34 turns
in the primary winding, and 35 or 36 turns in the secondary winding. And here we have only one winding with different
taps, but the same principle follows. This is how an autotransformer works. The output terminals are moved across the
winding to change the output voltage. Similarly, this transformer is used to change
the voltage for desired output. And the desired output is 340 volts. Hence, whenever the voltage is between 340,
and 360, it steps down the voltage to near 340 volts. And, whenever the voltage is between 340,
and 320 volts, it steps up to near 340 volts. Thus we get near 340 volts if the input is
between 320, and 360 volts. As there are only three connections, a voltage
of 343, or 338 will remain unchanged, as the minimum drop, or the minimum rise is also
10 volts. This switching of the connections is done
by relays. These are SPDT, or single pole double thrust
relays. These relays can switch between two terminals
depending on the voltage applied to them. This is a normally closed terminal and this
is a normally open terminal. Hence, when no voltage is applied, the common
is connected to the normally closed terminal, thus the name normally closed. And when the voltage is applied the electromagnet
turns on, and the common connects to the normally open terminal. This is how a relay works. And this is how they are connected. The live wire is connected to the common of
relay R1, and the two output terminals are at the first, and third turn. And, UPS Output is connected to the common
of the relay R2, with output terminals at the first, and the second turn. This configuration only supports voltage range
from 320, to 350 volts. Let's look at all of them. If the input is 320 volts. Relay R1 switches to the third turn, and relay
R2 to the first turn. If the input is 330 volts, R1 at third, and
R2 will switch to second. If the input is 340 volts, R1 and R2 will
connect to their common terminal, that is, first. And If the input is 350 volts, R1 to first,
and R2 to second. For 360 volts, R2 cannot connect to the third
turn, Hence, it can only be decreased to 350 volts. These relays are controlled by the controller. Depending on the mains AC voltage, the controller
switches the relays. The mains AC is filtered in the first block,
then the current and voltage are measured. Depending on the voltage, the relays switch
to different taps of the primary winding of the transformer. Thus the modified voltage is given to the
output. Also, the mains AC is passed to the battery
charger block, which can output different voltage and current, depending upon the battery's
charge cycle, to store the energy in the battery. When there is a power cut, the voltage and
current suddenly drops to zero, this is measured by the controller, and it turns off the charger,
and turns on the inverter block. Thus the energy stored in the battery is used
to supply the current at the output. This is how a Line-Interactive Un-interruptible
Power Supply works. Now you know how each component and block works. Now, there are two more types of UPS available. They are offline, and online UPS. Let's look at the offline UPS. In this UPS, there is no active voltage control,
all other working remains the same. Mains AC powers the output and charges the
battery, when there is a power cut, a relay switches from mains to transformers output. And, In online UPS, the Mains AC is rectified
to DC, then it is used to run the inverter to power the output, and simultaneously charge
the battery, when there is a power cut, a relay switches from the rectifier circuit
to the battery. In offline UPS, any voltage variation is passed
directly to the output. In online UPS, always a pure waveform is generated. The line-interactive UPS is a compromise between
these two types, where output can somewhat be controlled. These are other comparison points about them. In the online UPS, some more components are
used. First a PFC or power factor correction circuit. As you know a full bridge rectifier converts
AC to DC, and the capacitor filters the voltage to create pure DC. But the capacitor does not filter the current. When the rectifier output voltage rises above
the capacitor voltage, the capacitor charges, and when the voltage falls, the capacitor
provides the current. This causes a distorted waveform of the current,
hence to avoid this, and draw a sinusoidal waveform of the current, we use an active
power factor correction circuit, or PFC in short. A PFC is just a boost converter, It is used
to push the current in the capacitor, so the circuit draws the current in the sinusoidal
waveform. This is controlled by this IC. It measures the current through the circuit
and switches on and off the MOSFET, to create one of these two waveforms of the current. In this waveform, the current is increased
and decreased with reference to a sine wave. And, In this waveform, the MOSFET turns on
when the current is zero, and turns off when it reaches the desired value, which is referenced
by a sine wave. Now the average current in a cycle is sinusoidal
in nature. The current waveform is not purely sinusoidal,
hence the power factor is below 1. PFC is used in high load circuits, for light
load circuits, the power factor can be ignored. As the PFC is a boost converter, the voltage
at the output is higher than the input. Now, this high voltage DC is supplied to the
inverter block. When there is a power cut, the battery is
used to supply the power, but the voltage required at the inverter is high, and the
battery can supply only low voltage. Hence, we use a boost converter again, to
boost the voltage of the battery to a high voltage, and supply it to the inverter block. This is how all three major types of UPS works. Now, there can be another two major variations
in all these UPS. If you have ever opened a UPS, rather than three transformers,
you may have seen only one big transformer In these types of UPS, the H-bridge is used
as the battery charger. You may know that a MOSFET has an internal
diode. Hence, in these types of UPS, the voltage
induced in secondary, due to the voltage of primary, is rectified by these diodes and
filtered by the capacitor, to charge the battery. You may have seen a capacitor near the H-bridge,
It is this filter capacitor. This process is not this simple, there much
more to it, and we are not exploring it in this video. At last, there can be variation in the inverter
sections, that is, rather than an H-bridge, there can be a push-pull inverter configuration. This configuration uses two switches rather
than four. Here the transformer used is center-tapped,
and there are three wires from it. When the switch s1 is closed, the current
flows in this loop, and in this direction in the transformer. And, when the switch s2 is closed, the current
flows in another loop, and in the opposite direction in the transformer. Thus creating an alternating voltage at the
windings of the transformer. This is how a UPS works. To summarise,
The UPS is mainly of three types, Offline, Line-Interactive, Online, depending upon the
topology with two or more variations. First, charging the battery can be done by
a charger or by the inverter. And Second, the inverter section can be made
of H-bridge or push-pull configuration. Now you know how a UPS works. Thank you for watching.
