1. Field of the invention
The invention concerns an alternating current (AC) electrical power supply circuit principally intended to supply power to data processing equipment and/or systems for acquiring and processing low-level signals, specifically requiring a power supply that does not produce any radiated or conducted interference.
The invention is more particularly directed to a circuit that can be used to provide a power supply of high efficiency and that is stabilized and/or protected to withstand at least short-term interruptions of the AC mains supply.
2. Description of the prior art
It is often necessary to connect an AC electrical power supply device between the AC mains supply and systems including data processing equipment and/or systems for acquiring and processing low-level measurement signals to prevent the operation of such systems being disturbed by the imperfections of the AC mains supply, in particular voltage fluctuations and/or short-term or long-term interruptions. The following capabilities are required of equipment of this kind:
It should not produce any radiated or conducted interference (switching noise) likely to induce in sensors or their connections voltages which will corrupt the low-level signals to be measured and/or processed.
It should be able, as and when required, to stabilize the AC voltage supplied to a certain degree to protect sensitive equipment from fluctuations on the AC mains supply voltage, which are typically .+-.10% relative to the nominal voltage.
It should be able to withstand failure of the AC mains supply, specifically at least short-term (up to 50 ms) interruptions at relatively frequent intervals and, wherever possible, long-term interruptions that are much rarer but can last several minutes or even several hours.
It should where necessary provide galvanic isolation between the AC mains supply and the user circuit.
It should preferably have the highest possible efficiency.
Known systems rarely combine all these qualities.
There are known, for example, devices using "ferro-resonant" type transformers. These procure somewhat mediocre voltage stabilization (typically .+-.5% at the output for a variation of .+-.10% at the input). The energy store guarding against short-term interruptions is provided by a resonant circuit formed by an inductor coupled to the magnetic circuit and a capacitor. Such systems have major defects, for example poor dynamic response to sudden increases in load and major variation in the output voltage due to variations in the AC mains supply frequency. For this reason they cannot usually be employed on AC mains supplies using different frequencies. They are heavy, bulky and obsolete. They also entail the risk of radio frequency interference due to the saturation current of the magnetic component. Finally, and most importantly, they cannot withstand long-term failure of the AC mains supply, necessitating the use of an electrochemical accumulator.
Where a significantly long back-up time is required use is often made of an inverter fed by a rectifier and an electrochemical accumulator of appropriate capacity connected in parallel between the rectifier and the power supply terminals of the inverter. The inverter thus operates from a direct current (DC) voltage filtered by the accumulator itself. The inverter has to produce a sinusoidal voltage from a DC voltage and is often of the pulse width modulation type, using transistors or thyristors, and operating at high frequencies, with the result that its use can be problematical at extreme temperatures. Switching of the semi-conductor devices that inverters comprise often produces electrical interference that makes it difficult to use them in the vicinity of sensitive measuring instruments.
To improve efficiency and to reduce overall size it has also been proposed to use a switching mode inverter with a transformerless connection to the high-voltage supply connected on the input side to a switching mode rectifier connected to the AC mains supply and supplying a high DC voltage and to a DC-DC converter supplied by an electrochemical accumulator which is substituted for the AC mains supply when the latter fails. The converter is to some extent connected in parallel with the output of the rectifier, the accumulator being connected to a charger that needs only to be rated highly enough for recharging it. A system of this kind is not particularly reliable because of the large number of switching mode circuits used. A high level of electrical interference is produced and is difficult to control. This method is difficult to employ when high powers are involved.
Another known approach is to use the AC mains supply normally and to switch over to a standby supply as quickly as possible should it fail. This switching may be effected by solid-state contactors (thyristors) that switch to the load an inverter operating into no load when the AC mains supply is present. The inverter is supplied with power by a battery connected permanently to a charger. With a system of this kind efficiency is high when operating from the AC mains supply and the inverter may be a compact unit since it is used only while the AC mains supply is down. However, interference is inevitably produced when the solid-state contactors change state. There is- usually no isolation from the AC mains supply and it is not possible to stabilize the load voltage. Some of these disadvantages may be circumvented by combining a ferro-resonant inverter with a system of this kind but in this case all the disadvantages previously mentioned apply, for example variation in the output voltage with the frequency and/or the load and poor dynamic response.
The invention combines most of the advantages of the various known systems described above while avoiding their major disadvantages. A first object of the invention is to propose an alternating current electrical power supply circuit combining excellent efficiency with a low level of electrical interference (radiated or conducted) lending itself to effective regulation of the output voltage and/or to the addition of an auxiliary power supply that can be substituted for the AC mains supply should this fail.