Uninterrupted Power Supply


This is the era of information technology and so many sophisticated devices like PCs are used which are very sensitive to the supplies having spikes, power glitches, electrical noise, voltage sags and surges, waveform distortions, frequency variations and brown outs. These are shown in the figure 1.

The Computers needs clean and uninterrupted power. The most sensitive device of a computer is the disk drive which is driven by a motor whose speed depends upon the frequency. Thus frequency variation can cause reading errors, dropping or addition of a few bits which can result in corruption of file of CPU may go into unintended loop with no way to come out. The introduction of semi-conductor memories have made the CPU even more susceptible to the power aberrations. The voltage dropout up to ½ to one cycle (10 ms to 20 ms) may not cause problems with most of the PC’s, as their power supply unit may take care of this due to the presence of filter capacitor at the power stage. Any power cut more than this forces the system to reboot, flushing the data into nowhere land and may cause corruption of disk drives. Also the total harmonics distortion (THD) should not exceed 5 % for most of the PC’s for their satisfactory operation and to avoid failure of vital components. The various options available to get rid of these are:

  1. Frequency stabilizer.
  2. Frequency convertor.
  3. Static inverter.
  4. Servo stabilizer.
  5. CVT (Constant voltage transformer).
  6. CVCF (Constant voltage constant frequency equipment).
  7. Isolation Transformer.
  8. EA Set.
  9. UPS.

Out of these first three first covert AC to DC and then invert it, so the mains frequency does not have any effect on the output frequency. In the frequency stabilizer output frequency is derived from a clock signal by a quarts crystal oscillator, but here all undesirable effect except frequency variation remain and so it is unsuitable.Frequency convertor can have any output frequency but is also suffers from the same defects as frequency stabilizer, so it is unsuitable.Static inverter also have no treatment over the variation over the voltage variation.

Servo stabilizer, controls the voltage but not the frequency. Also it is for low or high voltages but not for irregular waveform. Also its rate of voltage correction is slow and it introduces electrical noise and so is not a solution. CVT is the same as servo stabilizer except that here good normal and common mode noise suppression is there and it can take care of small dropouts for a few tens of millisecond and so is not a solution. CVCF takes care of both the frequency but it fails to provide power at the time of power failure. Isolation transformer can take care of spikes and surges in the voltage but not the variation in the voltage.

The EA set also has the following disadvantages: being a rotary machine it needs more maintenance, MTBF (Mean time between failures) is low, MTTR (Mean time to repair) is high and load and frequency are interrelated and so is sensitive to frequency.

UPS is a combination of a CVCF and a floating battery bank with automatic charger, so it can be used to provide clean power to a computer installation.

The UPS has three basic components: Convertor to convert AC to DC, inverter to convert DC to AC and battery including charging circuit to supply energy to the system when mains is cutoff. The block diagram of an UPS when mains is present and when mains is absent is shown below:

Table of Contents


Three types of UPS systems are available and their comparative features are given below:

  1. OFF LINE: 

    • Inverter is ON only when mains is absent, other times mains is passed to the PC. So the output is as bad as the mains supply.
    • Cheapest technology.
    • Change over time is few milliseconds.
    • Cannot run from weak or unstable mains.

    • It is basically offline with additional circuit to filter the output during mains present. It gives quasi-sine wave
  3. ON LINE: 

    • The output of UPS is always ideal sine wave as the inverter is always ON, even when mains is present.
    • The changeover time is zero.
    • Capable of running from weak or unstable mains.
    • Most reliable energy supply.

Most of the manufactures use PWM (Pulse Width Modulation) technology, where the DC voltage from the battery is switched at a fixed rate by the power device using transistorized circuit, transistorized circuit, MOSFET or IGBT. PWAM technique is also same as PWM with 3 pulses per cycle and step wave synthesis. In case of three phase inverters using PWAM all the three phases are produced and controlled together, as such it results in voltage imbalances in the case of unbalanced loads. Since most of the computer loads are single phase, it is better to use PWM technique.

The advantages of PWM at 50 kHz are:

  1. Switching losses are extremely low.
  2. High invertor efficiency.
  3. Better dynamic stability (means lesser variation in output with the variation in the load).
  4. Better tolerance for overload and non-linear loads (as inverters are current limited.)
  5. Lesser harmonic distortion. Here harmonic distortion is due to the series inductance of the output transformer.
  6. Better Crest Factor Tolerance (Ration of max. current capability to the full load current).

The device comparison between MOSFET and THRISTOR and UPS comparison based on MOSFET and BJT is given in the tables below:

Majority carrier device.Minority carrier device.
Voltage driven device, hence drive circuitry is simple.Current driven device, so power loss is high and device circuit is complex.
No charge storage effects, so can be used for high frequency applications.Charge stored in base and collector making it unsuitable for high frequency applications.
High switching speed (20 to 100 ns)Low switching speed (500 to 5000 ns)
Absence of second breakdown failure mechanism and hence is more rugged. Prone to second brteakdown.
Predominantly positive temperature co-efficient and hence easy to operate in parallel.Negative temperature coefficient so difficult to operate in parallel.
High thermal stability, no thermal runaway.Large variation in device characteristics with temperature, thermal runaway possible.
No snubbers required.Heavy snubbers required.
Avalanche capability.Usually no avalanche available.
Integral freewheel diode.Usually not available.
MOSFET and BJT UPS Comparison
FeatureMOSFET Based High Frequency UPSBJT / Power Transistor Based UPS
Size of UPSCompact typically 30 – 60 % smaller.Medium to large.
Noise LevelNegiligible.Moderate to high.
Inverter Efficiency80 to 92.5 %.60 to 80 %.
Crest Factor (Surge Handling)Greater than 4:1.Typically 2:1.
Transient response timeLow (less than 30 - 40 ns for 100 % step load change).High.
Reaction time for non-linear loadsLow.High.
Output waveform monitoring and correctionAvailableNot available
Fuseless electronic protection systemAvailableNot available.

Both the MOSFET and IGBT’s have equally good characteristics like high peak current capability, high infront impedance, low turn off time but there are variations in switching losses and conductance depending upon the UPS capacity and DC bus voltage.

MOSFET’s have the highest switching frequencies and so switching losses are less and also circuit diagram is much easier to use and is cost effective and more reliable. Conductance losses are also less for MOSFET compared to IGBT for UPS capacities upto 20 kVA and DC bus voltage upto 180 V.

Also the drive circuit of IGBT is very complicated and mostly are propertiery. Due to this and the secretive nature of their development flexibility in the design is also limited. Also there is cost implication. Also MOSFETS are available from many manufacturers but IGBT’s from few only.

So to summarise, the MOSFET are more suitable for capacities upto 20 kVA and DC voltages upto 180 V, both devices fare equally well in the range of 20 – 30 kVA, but for the capacities higher than 30 kVA UPS based on IGBT is considered desirable.


All devices in sophisticated electronic devices/computers carry a name plate rating, but it should be remembered that their load is non-linear and also there is the problem of high inrush currents and surges. So we have to select high crest factor tolerance, and it is still more important for SMPS Loads. So a judicious load estimation is required.

REQUIREMENT OF BATTERY BACK UP: The capacity of battery is quoted in ampere hours (A⋅h). The capacity required for a particular application in Ah can be calculated by the following formulae:

A⋅h = (System capacity in kVA × 1000 × PF × Backup in hours) / (Total DC voltage of the system × Inverter efficiency × PCU,)

Where PCU is the Percentage Capacity Utilisation and is

60 % for ½ hours backup
70 % for 1 hour backup
75 % for 2 hours backup
83 % for 3 hours backup
85 % for 4 hours backup

and invertor efficiency is considered as 90 %, PF is taken between 0.6 to 0.8.

The two types of batteries are available: Maintenance Batteries and Sealed Maintenance Free Batteries.

AUTOMATIC BATTERY CHARGERS: These are the sophisticated variety of Battery Chargers using the latest components like SCR’s, IC’s etc. These have two modes of operation: FLOAT and BOOST. In the FLOAT mode (Constant voltage mode) the voltage remains within ±1 % or 0.05 V/cell. Here output is sensed by a resistor and is compared with a Zener reference and the error voltage is used to vary the firing angle of rectifiers. The output here can be adjusted from 2 V/cell to 2.3 V/cell.

In the BOOST mode (Constant current mode), the current remains constant within ±5 % of the set value when voltage varies from 1.7 V/cell to 2.4 V/cell. In the AUTO mode switching between FLOAT and BOOST is done automatically, according to the status of the battery. When the battery voltage is low, it remains in the boost condition to quickly charge the batteries. When the battery voltage reaches 2.4 V/cell, it switches over to FLOAT mode. Further in this mode if current increases by more than 9 to 11 % of set current, the charger switches back to the BOOST mode.


  1. BYPASS: 

    • STATIC BYPASS: Here when UPS fails to give the output, static bypass takes acre to supply AC power to load from auxilary source or Mains without delay.

    • HOT STANDBY: Another UPS is ready to take over in case of failure of the main UPS.

    • PARALLEL REDUNDANT: Both UPS give a synchronized output to load connected to both the UPS systems.

      The features of static switch are:

      1. Make before break.
      2. Provides overload facility.
      3. Provision to monitor UPS and input power supply frequency.
      4. Electronically coupled to load.

  2. UPS CONNECTIVITY SOFTWARE: This software gives added protection to the system. It tells the user the status of the UPS. It also has auto shutdown facility in case of very low battery voltage. Also if many systems are connected to many UPS systems it can take power from the healthy UPS by the methodology of server-client. For this purpose Novell Netware / Windows NT / SCO UNIX can be used. Here we can use simple connection or RS 232 connection.


Capacity:.. KVA Single/Two/Three Phase.
Technology:MOSFET / IGBT based PWM technology with switching frequency not less than 20 kHz.
UPS Type:Online.
Bypass (optional):Manual / Static bypass switch (biderctional).
AC input voltage:230 V (+20 %, -25 %.) (Single Phase)
Input frequency:50 Hz ± 3 Hz.
Harmonic distortion:Less than 2 %.
Output voltage:230 V ± 1%.
Duty Rating:Continuous.
Output frequency:50 Hz ± 0.1 %.
DC Battery voltage:0.5 kVA & 1 kVA: 48 / 72 V DC.
2 KVA & 3 kVA: 72 / 120 V DC.
5 KVA & 6 kVA: 120 / 144 V DC.
Invertor efficiency:Better than 88 %.
Overall efficiency:Better than 80 %.
Output waveform:Pure sinewave.
Voltage adjustability:+ 5 % (To be kept under cover).
Overload capacity:120 % for 10 minutes.
Transient voltage:4 % maximum (recovery within 2 variation cycles for 10 minutes.)
DC component in output:Nil.
Crest factor tolerance:Not less than 3:1.
Static bypass switch:(By using TRIAC/SCR): Less than 8 ms from invertor to mains and vice-versa.
Charger:SCR module controlled constant voltage/current charger.
Protection:Suitable current limiting circuit to be provided in battery charging circuit to limit the current while in boost charges to safe limits. Suitable protection to be provided in respect of:
a. AC Input:Single/Three pole MCB.
b. DC fuse:Fuse in battery path.
c. Output Overload:Invertor trip (electronic sensing).
d. Battery low voltage:Invertor trip (electronic sensing).
e. Battey under current:Current limit.
f. Short circuit:Invertor trip (electronic sensing).
Soft Start:Both on charger and invertor.
Control & User Connection:
a. Input Thro:3 way terminal 3 core cable.
b. Output Thro:15 A output socket/terminal.
c. Battery Connection:2 way terminal.
d. Charger ON/OFF:Rocker switch.
e. Invertor ON/OFF:Rocker switch.
f. Trip facility:For overload.
  For short circuit.
  For battery low.
  For output over voltage.
Alarms (Audio):1. Input voltage out of range.
2. Overload.
3. High temperature.
4. Battery low.
5. Battery on discharge.
6. Load on battery.
7. AC failure.
Indications:Indication shall be provided by means of LED/Lamps. Mimic display indication status of the status of system operation shall be provided.
1. Mains ON (Lamp).
2. Charger ON (Lamp).
3. Inverter ON (LED).
4. Battery Under Voltage.
5. Battery on Discharge.
6. Inverter trip.
7. Output Overload (LED).
8. Bypass ON (LED).
Meters:1. Input Voltage.
2. Output Voltage.
3. Input frequency.
4. Output frequency.
5. Output load (Ammeter).
Connectivity Software (optional):UPS should have compatibility for Novell Netware for auto shutdown, battery status indication, AC power analysis.
Operating temperature:0 to 45 °C ambient.
Relative humidity:0 – 90 %.
Noise:Less than 50 dB.
Battery:Stationary Tubular (Exide / Amco / Southern)
Lead Acid Automotive: (Exide / Amco / Southern)
Sealed Maintenance Free: (Exide / HitachiI / Yuasa / Panasonic)
Battery Capacity:As per the chart given below:
UPS CapacityApproximate Backup
1 Hour2 Hour3 Hour
500 VA12 V, 15 A⋅h – 6 Nos.12 V, 24 A⋅h – 6 Nos.12 V, 38 A⋅h – 6 Nos.
1 kVA12 V, 24 A⋅h – 6 Nos.12 V, 38 A⋅h – 6 Nos.12 V, 65 A⋅h – 6 Nos.
2 kVA12 V, 24 A⋅h – 10 Nos.12 V, 65 A⋅h – 6 Nos.12 V, 65 A⋅h – 10 Nos.
3 kVA12 V, 38 A⋅h – 10 Nos.12 V, 65 A⋅h – 10 Nos.6 V, 80 A⋅h – 20 Nos.
5 kVA12 V, 65 A⋅h – 12 Nos.6 V, 120 A⋅h – 20 Nos. 
UPS CapacityApproximate Backup
1 Hour2 Hour3 Hour
500 VA 12 V, 40 A⋅h – 4 Nos.12 V, 40 A⋅h – 6 Nos.
1 kVA12 V, 40 A⋅h – 4 Nos.12 V, 40 A⋅h – 6 Nos.12 V, 75 A⋅h – 6 Nos.
2 kVA12 V, 40 A⋅h – 6 Nos.12 V, 60 A⋅h – 6 Nos.12 V, 75 A⋅h – 10 Nos.
3 kVA12 V, 40 A⋅h – 10 Nos.12 V, 75 A⋅h – 10 Nos.12 V, 100 A⋅h– 10 Nos.
5 kVA12 V, 60 A⋅h – 12 Nos.12 V, 100 A⋅h– 12 Nos.6 V, 180 A⋅h – 20 Nos.
UPS CapacityApproximate Backup
1 Hour2 Hour3 Hour
500 VA 12 V, 40 A⋅h – 4 Nos.12 V, 40 A⋅h – 6 Nos.
1 kVA12 V, 40 A⋅h – 4 Nos.12 V, 40 A⋅h – 6 Nos.12 V, 75 A⋅h – 6 Nos.
2 kVA12 V, 40 A⋅h – 6 Nos.12 V, 60 A⋅h – 6 Nos.12 V, 75 A⋅h – 10 Nos.
3 kVA12 V, 40 A⋅h – 10 Nos.12 V, 75 A⋅h – 10 Nos.12 V, 100 A⋅h– 10 Nos.
5 kVA12 V, 60 A⋅h – 12 Nos.12 V, 100 A⋅h– 12 Nos.12 V, 180 A⋅h– 10 Nos.


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