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ON-LINE
UPS
Design of power
electronic circuits
Siddharth Bhowmik -07000418
Kumar Utkarsh -07000419
Chandan Kumar -07000420
Smriti Agarwal -07000421
Sharath M -07000422
N Viswanath -07000424
BTech IIIElectrical Engineering
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IntroductionThis reference design describes the design of a single phase on-line uninterruptable power supply (UPS).
UPSs are used to protect sensitive electrical equipment such as computers, workstations, servers, and
Other power-sensitive systems.
The online ups or a true ups is totally opposite to the conventional standby UPS. Both of these are
similar in that they have the same two power source and a transfer switch that selects between them.
The online UPS is exactly opposite from the standby UPS because here the primary power source is the
UPSs battery and utility power source is the secondary source.
Under normal operation, the online UPS is always running off the battery using its inverter while the line
power runs battery charger. Thus, it is also called a double conversion online UPS. If the power goes out,
the load continues to get power through the inverter and only the battery charging fails. Thus, there is
no transfer time in the event of power failure, only the battery starts to rundown.
If the primary equipment fails, then the transfer switch is used to switch to the utility power source.
Here transfer time comes into play. Another key advantage to having the equipment running off the
battery most of the time is that the double conversion process completely isolates the load from the line
power.
On-line UPS works on double conversion topology. This name arises from the operating principle of an
on-line UPS. When the UPS works in normal operation mode, while the mains line (or the power cord for
the ac line) is available, the input voltage is rectified to the dc bus. The output inverter converts the dc
bus voltage back to a pure sinusoidal voltage. The dc/dc converter is connected to the dc bus and
converts the battery voltage to the dc bus level. The converter is activated during a power failure, and
delivers the energy stored in the batteries to the dc bus. The dc bus voltage is again converted to a pure
sine voltage. A battery charger is used to charge the batteries. The charger can be powered from the
mains line or from the dc bus.
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Primary Circuit
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Various parts of online UPS1. Rectifier
2. Battery charger
3. Battery
4. Buck converter
5. Boost converter
6. Inverter
7. Transfer switch
Battery charger consists of rectifier, buck converter and power BJT switch for pulsed charging.
Rectifier
We use a full bridge rectifier with an output filter in our circuit. This converts the output to a rectified
DC. The schematic and values of LC filter are as follows:
L = 0.03682H and C = 0.00039307F.
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Battery Charger
Types of battery charging methods:
Constant Voltage Constant Current Pulsed Charging Taper Current Burp Charging IVI Charging Trickle Charging Float Charging Random Charging
The battery charger is actually a buck converter circuit designed to produce the voltage levels necessary
to meet the battery specification. Pulse charging technique is implemented to charge the battery. Pulse
chargers feed the charge current to the battery in pulses. The charging rate can be precisely controlled
by varying the width of pulses, typically about 1s. During the charging process, short rest periods of 20
to 30ms between pulses allow the chemical actions in the battery to stabilize by equalizing the reactions
throughout the bulk of the electrode before recommencing the charge. This enables the chemical
reaction to keep pace with the rate of inputting electric energy. This method can reduce unwanted
chemical reactions at electrode surface.
Operation of battery charger:
The battery voltage is monitored in a loop, it is compared to V th,lower and Vth,upper using the
comparator. As the battery charges to a voltage level beyond upper threshold the buck converter is
turned off. Similarly a drop in battery charge to a voltage level below lower threshold the converter is
turned on. Thus the battery voltage is maintained regulated well within the lower and upper threshold
limits neglecting the small ripple which is inevitable.
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Buck converter
0
R1
0.025k
2
1
+
-
+
-
S1
S
VON = 1.0V
VOFF = 0.0V
V1
1Vdc
1.000V
10.00V
C1
470u
1
2
V2
207.07Vdc
207.5V
V3
TD = 0
TF = 1n
PW = 0.017341m
PER = 0.05m
V1 = 10
TR = 1n
V2 = -1
L1
1mH
1 2
0
D2
UT268
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Boost converter:
V4
72Vdc
L1
10mH
1 2D2
UT268
C1
470u
1
2
0
R1
0.25k
2
1
V2
TD = 0
TF = 10ns
PW = 41u
PER = 50u
V1 = -1
TR = 10ns
V2 = 8
+ -
+- S1
S
VON = 1V
VOFF = 0.0V
V3
1Vdc
0
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Inverter using switch mode PWM:
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PWM output pulses:
Discrepancy in PWM pulses:
Inverter output:
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Base drive circuit:
Output of the base drive circuit:
BATTERY SPECIFICATION
The battery used in our design has the following specifications:
1.) Battery voltage rating -:72 volts2.) Battery capacity -:7 ampere-hour
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Input Specifications
Input frequency:- 50Hz Input voltage:-230V rms Single phase
Functional Modes
On-line: If the input power is available, the UPS supplies a load and eliminates all possibledefects on the line (online double conversion)
Battery: If the input power is not available, the UPS supplies a load from batteries. The backuptime is given by battery capacity.
Bypass: The UPS directly connects its output and input, so the load is directly connected to theinput line. The transition to this mode is set manually or automatically during overload or fault.