A sudden loss of power will disrupt most business operations, it is not only total mains failures or ‘blackouts’ which can trigger devastating effects. Many electrical loads, for example computer systems, are equally susceptible to power sags, brown-outs, black-outs, power spikes and surges, noise and radio frequency interference, and supply frequency changes. Such loads are often referred to as ‘critical loads’, partly because their continuous operation is fundamental to the functioning of the business, and also because they require a more stable and reliable power source than that generally offered by the utility mains supply in order to guarantee their correct function. Critical Load Applications The numbers and types of load falling into the ‘critical’ category are rapidly expanding as an ever increasing range of microprocessor-based equipment enters both the industrial and commercial marketplaces. This is typified by the growth of on-line transaction processing and Ecommerce where 24 hour trading demands absolute power quality with zero downtime. Among typical critical loads are: • Computers – e.g. data processing and control systems. • Industrial process equipment – e.g. precision manufacturing. • Medical equipment – e.g. life support and monitoring systems. • Telecommunications network equipment – e.g. PABX. • Point of sales (POS) terminals – e.g retailing environment. • On-line business transactions – e.g. internet shopping. The effects of an inadequate supply to a critical load can include: • Cessation of the business process – i.e. a total inability to trade and/or communicate • data loss or corruption due to software crashing • Expensive hardware failure including component damage – e.g. due to power sags, spikes etc. • Production loss due to incorrect operation of a manufacturing process and possible production equipment damage • Inappropriate control system operation • Lost business due to failed POS or telecommunications equipment • Possible time penalty paid to repair/reset affected systems

2.  Applications
◦ Standby source for “critical” loads (computer)
◦ Primary source when normal ac not available
 Uninterrupted Power Supply (UPS)
◦ 2 common configurations

4.  Normal condition
◦ Main supply applied to the load
◦ Rectifier keeps batteries charged
 Main supply failure
◦ Inverter supplies power to the load
◦ When interruption is over, main supply is applied
to the load

6.  Normal condition
◦ Inverter operates continuously, powering the load
◦ Rectifier maintains the charge on the batteries
◦ Inverter conditions the supply to the load
 Inverter failure
◦ Main supply powers the load

8.  Main supply failure
◦ Battery supplies the inverter
◦ Inverter powers the load
 Normal conditions
◦ Inverter operates as a bridge rectifier
◦ Main supply powers the load

9.  Switched-mode
 Resonant
 Bidirectional

11.  Two inverters
◦ Input side: operates at high-frequency
◦ Reduces size of transformer and DC filter
components at the input of the output-side
inverter

12.  Two inverters continued
◦ Output side: operates at output frequency

14.  Two inverters
◦ Input side is a resonant inverter
◦ Output side operates with PWM control at the
output frequency

16.  Output inverter and Rectifier are combined
into a cycloconverter (converts one
frequency ac into another frequency ac)
 Power flow in either direction

18.  Four conversions
 AC-DC – DC-AC – AC-DC – DC-AC
 Ultimately an AC – AC Conversion