Double Conversion UPS systems are so called as they take the AC source, convert this into DC and then reconvert back to AC. This “double conversion” is of course the least efficient of all UPS technologies, but has the distinct advantage of producing a highly stable, high quality power output with no break in output following a transition from AC mains to battery operation.
A common misconception is the assumption that the conversion of AC to DC powers the batteries and then the batteries provide the DC to be converted to AC. What actually happens is the input AC input provides power to two circuits. One is the battery charger, which rectifies the incoming AC and provides the float voltage on the batteries to charge them or keep them topped up. The second is the AC boost circuit, which converts the incoming AC to form the DC “bus”. The bus is supported by high value capacitors which hold the level during transitions. The AC Boost circuit also provides power factor correction on the input (modern systems only). The DC Bus is a high voltage DC rail of aorund 700V for a European model, although this may be ±350V with respect to earth.
In the event of a power cut the unit switches the bus source from the AC boost circuit to the DC boost circuit. This circuit is a DC-DC converter, taking the DC voltage from the battery and converting this to the bus voltage. The bus capacitors ensure that the transition is seamless to the inverter.
The DC bus is used for the inverter. The inverter is the circuit that takes the DC input and converts this to an AC output. There are 2 ways of achieving this. One is termed “low frequency” and the other “high frequency”. Low frequency units use a simple switching inverter and require a transfomer. High frequency units use a special technique called Pulse Width Modulation or PWM to rapidly switch the bus to produce a pulse train output which is then filtered to produce a sine wave output. The latter does not require a transformer and is sometimes referred to as “transformerless”. These are smaller and lighter than their low frequency counterparts.
Double Conversion Uninterruptible Power Supplies are classified as “VFI” according to the international standard on UPS – EN60240. This means that the Voltage and Frequency output are Independent from the input. This gives double conversion UPS the ability to have electronically adjustable output voltage and frequency. They can act as frequency converters taking in a 50Hz input and outputting a 60Hz waveform. The conversion process also allows for a very wide input voltage window with some systems able to cope with voltages half of nominal for low load levels.
Online Double Conversion UPS are considered to provide the best power protection as they can overcome power issues that other technologies cannot. They are also the recommended technology for Extended Run UPS Systems as they produce a sinewave outlet and have improved reliability as the inverters are designed to operate 24/7.
Due to the fact that the inverter is on all the time, forced air cooling is employed. Although fan speed control is incorporated in many systems, based upon the load level, the noise can be a distraction and consideration should be given to this when using double conversion technology in a quiet office or residential setting.