Harmonic suppressor for an electronic ballast

Abstract: The invention relates to a suppressor for an electronic ballast in a discharge lamp, which is connected between a ballast and a mains supply, including a supply-voltage rectifier (V1)) and a charging capacitor (C1), at least one energy-storing coil or inductance (L1) and an electronic switch element (V2) connected such that, when said switch element (V2) is conducting, a current path extends from the positive terminal of the rectifier (V1) through said switch and coil to its negative terminal and, when the switch (V2) is non-conducting, a current path extends from the positive terminal of the rectifier through a diode (V3) and the charging capacitor (C1) to its negative terminal. The switch element (V2) is controlled by means of a constant-frequency, permanent pulse-ratio control up to an operating point to be determined in view of a given momentary value of the mains voltage, whereafter the pulse ratio changes.

Description: The present invention relates to a circuit arrangement for use in an electronic ballast included in a discharge lamp for suppressing harmonics included in a current picked up by an appliance from the mains and other electromagnetic interferences generated by the appliance. At present, the design of electronic ballasts used in low-pressure discharge lamps is regulated by international standards IEC 928 and IEC 929. One of the requirements in the latter is e.g. the suppression of harmonics produced in supply wires to a certain level determined in international standard IEC 1000-3-2. There are several prior known solutions for suppressing harmonics in a mains voltage from filter designs consisting of passive components to actively controlled filters operating at a high frequency. One useful group of filter circuits consists of so-called charging-pump or capacitor-pump circuits. These have been described e.g. in the following Patent publications: FI 78807, EP 389847 and DE 3611611. In addition, there are boost type of chopper-filter circuits. A fundamental circuit for an active boost-type mains filter is shown in fig. 1, wherein the section defined by dash lines represents a chopper-filter circuit whose load comprises an actual electronic ballast designated by symbol A2. Operation of the circuit elements is as follows: V1 is a diode bridge for rectifying a mains voltage supplied to mains terminals X1-X2. V2 is an electronic switch component, for example a MOSFET-transistor controlled by a circuit block A1. With V2 conducting, energy is charged in a coil L1. With switch V2 opening, the energy of coil L1 transfers by way of a diode V3 to a charging capacitor C1. The control of switch V2 can be effected in such a manner that the current progressing in terminals X1-X2 is for the most part sinusoidal. Circuit designs based on this principle have been disclosed e.g. in application instruction AN-995 published by International Rectifier ("Electronic Ballasts Using the Cost-Saving IR2155 Driver", International Rectifier, 233 Kansas Street, El Segundo, CA 90245) as well as in Patent publication US 5,001,400. One further evolution is disclosed in Patent application FI 945473. Commercially available are integrated circuits intended for handling the functions of block A1 in fig. 1. The operation of boost-type chopper filters can be broken down to the following modes: Category 1: discontinuous coil-current modes: permanent-frequency and alternating-frequency filters Category 2: continuous coil-current modes: permanent-frequency and hysteresis-control based filters. However, the above-described filter constructions have certain drawbacks and limitations. The current of permanent-frequency filters included in category 1 does not have very good distortion values and a useful result is obtained by supplementing the control with various correcting waveforms. This, of course, makes the configuration more complicated. On the other hand, the alternating-frequency configurations included in category 1 have a small current distortion but those require detection of the breaking of a coil current. In the permanent-frequency choppers of category 2, the current regulation can be implemented in a variety of ways and, as far as the current distortion is concerned, the final result depends on the implementation. In hysteresis regulation, wherein the chopper switch is controlled by sinusoidally varying the rating of a maximum and minimum current, the regulation requires the use of a coil-current measuring and a repeater circuit for achieving a small distortion, i.e. once again a small current distortion requires a complicated circuit configuration. In the cited solutions, the waveform of a mains current is not controlled in the proximity of the zero points of a supply current, either. In addition, all circuit designs are not adaptable in the regulable ballasts of discharge lamps, wherein the dimming of a lamp is desired. An object of this invention is to eliminate the above drawbacks and to achieve a very small current distortion and a high power factor for an appliance by means of a simple circuit design. The invention is based on the controlled switching of a chopper operating mode between a continuous and discontinuous operating mode according to the operating position of the chopper. A circuit of the invention is characterized by what is set forth in the characterizing clause of claim 1 - 9. One benefit gained by a circuit of the invention is the achievement of a very small current distortion and high power factor by means of a simple circuit configuration. In addition, this circuit is adaptable in regulable ballasts, wherein it is desirable to adjust a lamp-produced light output to a level determined by an external control signal. The invention will now be described in more detail with reference made to the drawings fig. 2 and fig. 3, which illustrate one circuit configuration of the invention and its operation. In the drawings, the actual ballast is depicted in a simplified form by replacing these components with a block A2 since, from the standpoint of a filter or suppressor, the ballast comprises a substantially constant-power load. A control block A1 for power switches can be a separate control circuit or more preferably an oscillator included in the ballast section A2 anyway. A diode bridge V1 serves as a full-wave rectifier. The circuit has a two-stage operation. In the first stage, a switch V2 is conducting and, thus, the current induced by a supply voltage increases in a coil L1 in proportion to the strength of supply current. The passage of current in the first operating stage is depicted in fig. 2, in which the darkened solid line represents a current path. When the transistor V2 is opened in stage 2, the energy tied up in the coil L1 forces the current to continue in the coil. Now, it has no other route but to flow by way of a diode V3 to a charging capacitor C1 for charging the same and this condition is depicted in fig. 3 with a darkened line. The amount of energy tied up in the coil during a single cycle is dependent on the strength of the supply voltage and the switch-on period of V2. In a circuit of the invention, the above-described circuit is controlled such that the operating mode of a chopper is switched at a certain point of operation. Here, the permanent frequency is the same as the frequency of a lamp circuit included in the block A2 of figs. 1-3. For about a half of the total duration of a mains cycle, the chopper of the invention operates at a permanent 50 % pulse ratio. When the mains voltage is near its peak value, the pulse ratio is in turn adjusted lower in the control block A1. The regulating parameters include the maximum current of a chopper switch V2 and the output voltage of a chopper. Operation of the regulating circuit is quite simple: the value of a current limit is adjusted by means of a feedback received from the output voltage. The chopper coil is dimensioned such that the energy charged in the coil L1 during a permanent pulse ratio discharges entirely to the charging capacitor C1, i.e. the question is about a discontinuous coil-current operating mode. In proximity of the voltage peak value, the energy of coil L1 does not have enough time to discharge completely to the charging capacitor C1, i.e. the question is thus about a continuous coil-current operating mode. By suitably selecting the operating-mode switching point, the current picked up from the mains can be made quite accurately sinusoidal (total harmonic distortion less than 7 % and circuit power factor 0,98). The selection of an operating-mode switching point is affected by the magnitude of a load, the ratio between an output voltage and supply voltage, the operating frequency and the inductance of a coil. The described operating mode is illustrated in a string of drawing figures 4-11, in which fig. 4 depicts the voltage waveform at the top end (terminal point of circuit elements L1 and V3) of a switch V2 fig. 5 depicts the current waveform of the switch V2 in the discontinuous operating condition of a chopper fig. 6 depicts the current waveform of the switch V2 in the continuous operating condition of a chopper fig. 7 depicts the current waveform of a coil L1 at the moment of switching the operating condition fig. 8 depicts the current waveform of the coil L2 in the discontinuous operating condition fig. 9 depicts the current waveform of the coil L2 in the continuous operating condition fig. 10 depicts the current waveform of a diode V3 in the continuous operating condition fig. 11 depicts the mains-current waveform of a filter or suppressor in a terminal X1-X2. The simplicity of a solution of the invention is due to the fact that the coil does not require additional winding for the detection of a current interruption, the regulating circuit does not require a repeater stage and the adjustment of current limit does not require a sinusoidal or any other type of correction affecting the waveform. Thus, the design is highly preferable also in terms of implementing costs. A circuit of the invention is useful also in such regulable ballasts of discharge lamps, wherein the lamp intensity is adjusted lower by increasing the supply frequency. Thus, the nominal effect of a chopper also decreases and the distortion values remain good. A circuit system of the invention can be supplemented by connecting an inductive circuit element between the mains-supply terminal X1 and the rectifier V1, a capacitive circuit element between the terminals X1 and X2 or a capacitive circuit element across the positive and negative terminal of the rectifier V1. An object of these circuit elements is to suppress radio-frequency electromagnetic disturbances. A circuit arrangement of the invention is particularly useful in electronic ballasts included in low-pressure discharge lamps with basic designs prior known e.g. from Patent FI 65,524. However, it is obvious for a skilled person that the invention is not limited to just the above-described application but it is just as well applicable in the electronic ballasts of high-pressure lamps and in other ballasts operating on the chopper principle.

Claims: A harmonic suppressor for an electronic ballast in a discharge lamp, which is connected between a ballast (A2) and a mains supply, including a supply-voltage rectifier (V1) and a charging capacitor (C1), at least one energy-storing coil (L1) and an electronic switch (V2) connected such that, when said switch (V2) is conducting, a current path extends from the positive terminal of the rectifier (V1) through said switch and inductance to its negative terminal and, when the switch (V2) is non-conducting, a current path extends from the positive terminal of the rectifier through a diode (V3) and the charging capacitor (C1) to its negative terminal, the switch (V2) being controlled by means of a control block (A1) producing a constant-frequency, permanent pulse-ratio control up to an operating point to be determined in view of a given momentary value of the mains voltage, whereafter the pulse ratio changes, characterized in that the control block (A1) of the switch (V2) is an oscillator included in the ballast (A2). A harmonic suppressor as set forth in claim 1, characterized in that during the permanent pulse ratio the energy charged in the coil (L1) discharges entirely to a charging capacitor and during the alternative pulse ratio the energy of the coil (L1) does not have enough time to discharge completely to the charging capacitor (C1). A harmonic suppressor as set forth in claim 2, characterized in that said operating mode is achieved by means of a suitable dimensioning of the inductance of said coil (L1). A harmonic suppressor as set forth in claim 1, 2 or 3, characterized in that the regulation of a pulse ratio is achieved by adjusting the current limit value of the switch element (V2) by means of a feedback signal to be conducted from the suppressor output voltage. A harmonic suppressor as set forth in claim 1, 2, 3 or 4, characterized in that the operating point determining the operating-mode switch of said switch element (V2) relative to the momentary value of the mains voltage is selected in such a manner that the current picked up from the mains has a total harmonic distortion as small as possible. A harmonic suppressor as set forth in claim 1, 2, 3, 4 or 5, characterized in that the operating point determining the operating-mode switch of said switch element (V2) relative to the momentary value of the mains voltage is selected in such a manner that the assembly consisting of a filter circuit and a load circuit (A2) has a power factor as high as possible. A harmonic suppressor as set forth in any of claims 1, 2, 3, 4, 5 or 6, characterized in that between the mains supply and the rectifier is connected at least one capacitive or inductive circuit element. A harmonic suppressor as set forth in any of claims 1, 2, 3, 4, 5, 6 or 7, characterized in that between the positive and negative terminal of the rectifier is connected a capacitive circuit element. A harmonic suppressor as set forth in any of claims 1, 2, 3, 4, 5, 6, 7 or 8, characterized in that the electronic switch element (V2) is a duct transistor or a bipolar transistor fitted with an insulated grid or a bipolar transistor.

Inventors: AHONEN ERKKI, AHONEN, ERKKI
Applicants: HELVAR OY AB
Application year: 1996
Main classification: H05B41
IPC codes: H05B41