AU6179890A - Feeding device for converters, free of harmonic distortion - Google Patents

Description

Feeding device for converters, free of harmonic distortion

The present invention relates to means adapted for forming electronic converters which, fed from the AC mains, are able to take only a perfectly sinusoidal current which is perfectly in phase with the voltage, even if the load applied at the output of these converters has a reactive term or non linear impedance, as is the case for example for a fluorescent discharge lamp.

In known devices of this kind, particularly in so far as the South African patents 82/3819 , 82/3968, 82/7811 and 82/8709, the Australian patent 84/00184 and German patents 31 12281 and 33 12575 are concerned, such constructions have been described.

Such devices, often complex to put into practice, only make it possible to minimize rejection of the harmonic currents in the mains and are far from requiring a perfectly sinusoidal input current, especially when the standardized variations of the mains voltage make themselves felt or when the output load does not belong exactly to the specified type.

Thus, such devices are only a palliative which makes it possible to comply, under very precise and specified conditions, with the less constraining standards which govern rejection of the harmonic currents in the mains. These devices are incapable of accepting any type of load, resistive such as a simple lamp, inductive such as an electric motor, capacitive such as the rectifier circuit of a stabilized power supply and non linear such as a fluorescent lamp, without causing considerable distortion of the current taken from the mains.

The device of the invention overcomes these drawbacks, which restrict the generalized use of such devices, especially when it is a question of installations comprising a large number of units, such as is the case in fluorescent lighting.

This device, simple to construct, eradicates practically completely the harmonic currents which it would be capable of rejecting in the mains, over an input voltage range from -60% to +30% of the rated value, whereas international standards set a range of -10% to +10%. Furthermore, this device accepts at the output any type of load, resistive, inductive, capacitive or non linear without appreciable distortion of the current taken from the mains.

To attain this result, the device according to the device comprises an electronic chopping converter whose biased input is shunted by a buffer circuit formed by at least two capacitors capable of being charged in series by the mains current supply line and capable of being discharged in parallel across the input of said converter, said buffer.-circuit is itself loaded by two rectifier circuits mounted so that the addition of their output signals occt&rs at the level of said buffer circuit and the input of said converter. The input of the first rectifier circuit is fed by the AC mains whereas the input of the second rectifier circuit is fed by a fraction of the high frequency energy delivered at the output of said converter. The common point to one and other rectifier circuit is high frequency decoupled by a capacitor connected between said common point and the input terminals. of said converter.

The. addition of the rectified low frequency mains current and the rectified high frequency current applied o said buffer circuit makes it possible to take from the mains only a strictly sinusoidal current and to deliver at the output . of the converter a high frequency current having only a reduced ripple rate, whatever the type of load connected to the output, to the extent of course that its mean dissipated power does not exceed the conversion power of the device.

In a variant of the device of the invention, illustrated in figure 1, the current taken from the mains is applied between the terminals la and lb which are connected, through the HF filter formed by the two symmetrical inductances 2a and 2b, to the unbiased input of the rectifier bridge whose biased output is connected in series with the biased output of the second rectifier bridge 4, so that a suitably biased current may flow freely through the series circuit thus formed. Said biased series circuit charges the input terminals 9a and 9b of the converter 10, between which a buffer circuit is connected comprising any number of suitably biased capacitors which are capable of being charged in series by said rectifier circuit and discharged in parallel across the input of converter 10. In the variant described, the number of capacitors used is limited to 5 capacitors 5a, 5b, 5c, 5d and 5e. These capacitors are charged in series by means of the rectifier circuits 3 and 4 through the suitably biased diodes 6a, 6b, 6c and 6d which connect together in series, respectively, the above capacitors two by two. So that each of the capacitors thus charged in series may be discharged between terminals 9a and 9b, two series of suitably biased diodes 7a, 7b, 7c, 7d and 8a, 8b, 8c, 8d are used which provide this function. Thus, capacitor 5a may be discharged through diodes 8a, 8b, 8c and 8d mounted in series and suitably biased. Capacitor 5b may be discharged through the series of suitably biased diodes 7a, ^' 8a, 8c and 8d. Capacitor 5c may be discharged through the series of suitably biased diodes 7a, 7b, 8c and 8d. The capacitor 5d may be discharged through the series of suitably biased diodes 7a, 7b, 7c and 8d. As for capacitor 5e, it may be discharged through the series of suitably biased diodes 7a, 7b, 7c and 7d.

Thus, the AC mains current charges the capacitors cyclically in series, which may be discharged in parallel across the input terminals of converter 10, which may thus begin to operate.

With such operation ensured, converter 10 is able to deliver between its subsidiary output terminals 11a and lib a signal of suitable amplitude, which is applied to the unbiased input of rectifier 4 so that a fraction of the high frequency energy converted by the latter is rectified at the output of rectifier 4 and added to the envelope, itself rectified, of the mains voltage. Thus, by suitably dosing the level of the high frequency energy taken between terminals 11a and lib, almost perfect smoothing may be obtained of the voltage applied between terminals 9a and 9b of converter 10 which allows the latter to deliver between its output terminals 12a and 12b a high frequency current affected by a small low frequency modulation rate, even if the load applied has reactive terms, whether they are inductive, capacitive or more generally non linear.

The purpose of capacitor 13, connected between one of terminals 9a or 9b and the common point to rectifiers 3 and 4, is to provide high frequency decoupling of said common point.

The great advantage of this device is that it makes possible the feeding of converter 10 with a low ripple rate and not charging directly from the mains an electrolytic capacitor of high value, only taking therefrom a perfectly sinusoidal current, over a wide input voltage range and a wide range of variation of the impedance connected at the output of converter 10. Furthermore, this device is compatible with a DC network where it may be advantageous to the extent that the buffer circuit formed by capacitors 5a, 5b, 5c, 5d and 5e represents an excellent means of filtering all the high frequency parasite rejects which might be present between terminals 9a and 9b. In fact, if we consider that a discontinuous or alternating parasite signal is present on one of terminals 9a or 9b, this signal is applied through the small impedance of capacitor 5a or capacitor 5b to a "doubler" rectifier circuit formed respectively by the two diodes 6a and 8a or the two diodes 6d and 7d, which results in transforming said parasite into an increase of the DC charge, respectively of capacitor 5B or capacitor 5d. Such a device therefore eradicates all the switching parasites of capacitor 10, which would otherwise risk being re-introduced, by conduction, into the mains. For this, the reactance of inductances 2a and 2b is large with respect to that of capacitors 5a and 5c, which prevents any high frequency parasite from being re-injected into the mains.

The variant illustrated in the accompanying figure 2 shows a particular embodiment of the device of the invention which is applicable, particularly, to the construction of power supply devices for fluorescent tubes, commonly called "electronic ballasts".

The general structure of the device is similar to that which has just been described except that the number of capacitors forming said buffer circuit has been limited to two capacitors 5a and 5b, the high frequency rectifier circuit is a circuit of "doubler" type comprising diodes 4a and 4b whose input is loaded by capacitor 15, and said diode 6a, charging capacitors 5a and 5b in series, is split into diodes 14a and 14b also connected as a doubler whose input is high frequency loaded by the input 11a of the primary 11 of transformer 16 which, with transistors 17a and 17b, are the only members of said converter 10 shown here. The middle point of diodes 14a and 14b is also connected to capacitor 18 whose value is chosen so that the oscillating circuit which it forms with the primary 11 has a resonance frequency very close to the parallel resonance frequency of the oscillating circuit comprising the secondary 12 associated with capacitor 19. Thus, if the load connected between the output terminals 12a and 12b has its impedance varied significantly, which corresponds to variations of the power drain at the output, these two oscillating circuits being thus tuned, the excess energy is located between the terminals of primary 11 and especially between the terminals of capacitor 18 , which the voltage doubler formed by diodes 14a and 14b charges, which results in re-transforming the high frequency energy into DC energy which may then be re¬ stored respectively in capacitor 5a and 5b, via the suitably biased diodes 7a and 7b.

In this particular embodiment, the high frequency energy for smoothing the rectified voltage present between terminals 9a and 9b by addition to the rectified mains voltage is taken between terminals 11a and lib of primary 11. For this, the output lib is suitably situated on the primary winding 11 so that capacitor 15, connected between the output lib and the input of the voltage doubler formed by diodes 4a and 4b, derives a voltage in addition to the mains voltage exactly adapted to smoothing of the voltage present at terminals 9a and 9b. It is also possible to dose the amplitude of said additional voltage re-injected at the input by choosing a suitable value of capacitor 15, whose high frequency reactance considered ensures the adjustment.

The rest of the circuit conforms to what has been described above. Capacitors 5a and 5b are charged in series, via the suitably biased diodes 4a and 4b, by means of the biased output of the mains rectifier 3 which is mounted in series with the biased output of the doubler bridge comprising diodes 4a and 4b. Parallel discharge across the terminals 9a and 9b of capacitor 5a and capacitor 5b takes place, respectively, by means of diode 8a and diode 7a.

The useful converted energy is available between the output terminals 12 a and 12b representing the secondary of transformer 16.

It is obvious that in this embodiment the doubler bridge comprising diodes 4a and 4b could be replaced .by a pump rectifier comprising any number of stages or by a single rectifier stage, and the winding corresponding to outputs 11a and lib could be independent of the primary 11. Similarly, the secondary 12 could be not separate from primary 11, the transformer 16 then being of the self- transformer type.

The device of the invention may be used in association with most converter structures. This device makes it possible to supply such a converter, from an AC network, with a rectified current having only a low ripple rate, without altering the power factor of the current taken from the mains or causing it to undergo an appreciable harmonic distortion. Furthermore, within the limit of the mean power converted by converter 10, this device makes it possible to connect any type of load, inductive, capacitive or more generally non linear to the output thereof, without appreciably altering the characteristics of the current taken from the mains.

The device of the invention may be used in all cases where it is desired to provide an economic and reliable power supply for any electronic converter for electronic ballast, electronic transformer, stabilized power supply use etc...

As is obvious and as it follows moreover from what has gone before, the invention is in no wise limited to the modes of application and embodiments which have been more especially envisaged; it embraces, on the contrary, all variants thereof.

Claims (4)

8 -CLAIMS

1. Device for supplying an electronic converter 10 from a distribution network, permitting the converter to take from said network only a substantially sinusoidal current in phase with the voltage, whatever the intrinsic characteristics of the suitable mean impedance load which is connected* at the output thereof, characterized by the fact that between the inputs 9a and 9b of converter 10 is connected a buffer circuit comprising at least two capacitors 5a and 5b whose series charge is provided from the rectified mains via a rectifier means 6a preventing direct discharge: of the capacitors, such discharge being ensured in parallel for each of said capacitors across the input terminals 9a and 9b, through suitably biased rectifier means 7a and 8a, charging of said capacitors, after start up of the converter, being completed by a fraction of the high frequency energy taken from the output thereof and added after rectification by the rectifier means 4 to the mains voltage rectified by the rectifier means 3.

2. Device according to claim 1, characterized by the fact that the biased outputs of the rectifier means 3 and 4 are connepted in series, a high frequency decoupling capacitor .13 being connected between the common point of said series and one of the terminals 9a or 9b.

3. Device according to claim 2, characterized by the fact that the rectifier means 4 is of a voltage multiplier type comprising at least two suitably biased diodes 4a and 4b, its unbiased input being fed by means of the capacitor 15 connected to a tapping lib of the primary of capacitor 15 connected to a tapping 11a of the output transformer 16.

4. Device according to claims 1, 2 and 4, characterized by the fact^' that said rectifier means 6a is replaced by two rectifier means 14a and 14b mounted in series and suitably biased whose common point is connected both to capacitor 18 and to the input of the primary 11, these latter forming an oscillating circuit whose resonance frequency is close to that of the oscillating circuit formed by the secondary 12 associated with capacitor 19, so that the impedance variations of the load applied at the output result in recovery of DC current by the capacitors 5a and 5b, with good efficiency.