UPS (Uninterrupted Power Supply)

 Block diagram and explanation of UPS 

–         A computer requires a regulated, stabilized power supply.

–        Fluctuations or sudden power failures may cause damage to computers.

–        Thus, there is a need of uninterrupted power supply i.e. UPS, to protect computer from these damages.

–        After, the A.C. mains failure, UPS provides some backup time, to shut down computer system properly.

–        There are two types of UPS, ON-line Ups and OFF-line UPS.

 

·         Schematic block diagram of ON-lineUPS  ( Uninterrupted Power Supply) Online UPS

 

ON- line UPS is also called as true UPS. The block diagram of ON-line UPS is shown above.

It has two power sources and transfer switch between them. Here, Battery is the primary power source and utility power is the secondary power source.

Under normal operation, the ON-line UPS is always running off the battery using inverter, while the line power runs the battery charger. Therefore, it is sometimes also called a double-conversion ON-line UPS.  That means there is no transfer time in the event of a power failure.

A computer powered by an ON-line UPS responds to a power failure, keeps working without interruption. The secondary power path provides backup in the event that, the inverter fails or stutters due to some sort of internal problem.

The most advantage of ON-line UPS is, the double-conversion process totally isolates the output power from the input power.  These UPS are available in the range of 5000VA upto hundreds of thousands of VA.

The disadvantage of ON-line UPS are: They are costlier than that of OFF-line UPS, and it’s inefficiency. Because all the power going to the loads is converted from A.C. to D.C. and back to A.C., which means much of the power is dissipated as heat and it is occurring every time.

To overcome this problem, a new design called a delta conversion ON-line UPS are designed (delta means difference here). In this design, the battery charger is replaced with a delta converter.

Instead of providing all the output from the battery under normal circumstances, some of it is provided directly by the delta converter from the input line power.

In case of power failure, the delta converter stops operating and the unit acts like a regular double-conversion ON-line UPS, since the inverter is also running off the battery all the time.

Simplified block schematic diagram of a delta conversion ON-line UPS is as shown below-

The converter and inverter both handle D.C. and A.C. current, providing the two power sources of the UPS. Bypassing the battery for part of the battery during normal operation reduces power consumption.

This new design is available in large UPS (over 5000VA.)

 

• Off Line UPS

OFF line UPS is actually a standby power supply (SPS)

For the lower end of budget scale mostly such UPS are used.

Here, the primary power source is line power from the utility, and secondary power source is the battery. It is called standby UPS because the battery and inverter are normally not supplying power to the equipment.

The battery charger is using line power to charge the battery and the battery with inverter are waiting “on standby” until they are needed. When the mains A.C. power goes out, the transfer switch changes to the secondary power source.

When line power is restored, the UPS switches back.

The block schematic diagram of OFF-line UPS is as shown in following fig.

The primary power source is filtered and surge-suppressed to protect against line noise and other problems that would not cause a switch to battery power.

The issue with standby UPS is that, when the line power goes out the switch to battery power happens very quickly, but not instantly. There is a delay of a fraction of a second while the switch occurs, which is called the switch time or transfer time of UPS. It is the time required for the UPS to switch from line to battery power.

For a true ON-line UPS this will be zero. But for Standby or OFF-line UPS it will be normally few milliseconds. Hence, the Hold or hold-up time becomes important here.

It is the amount of time the power supply can handle having its input cut-off before being interrupted.

If the transfer time is equal or larger than the hold time, then the UPS is not useful.

The hold up time is the amount of time the power supply will keep producing its output, if it loses its input. Typical value is 20 mSec.

Standby UPS are available in the range of 1000VA.

 

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