[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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 sinoidal. 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
1. 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 (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 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, characterized in that 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.
2. A 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 alternating pulse ratio the energy
of the coil (L1) does not have enough time to discharge completely to the charging
capacitor (C1).
3. A 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).
4. A 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.
5. A 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.
6. A 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.
7. A 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.
8. A 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.
9. A 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.