Patent Application
20130033177
The invention relates to an electronic ballast for lighting applications, and to a method for controlling current drawn by an electronic ballast for lighting applications from a power source.
Dimming of electric lighting, in particular domestic lighting, is typically performed by a TRIAC-based controller, which is usually mounted in place of an ordinary light switch. The TRIAC-based controller allows a user to select the level of illumination required by adjustment of a control.
A TRIAC-based dimmer operates by conducting over only a part of the alternating current mains cycle, which is known as phase angle control. During a positive half-cycle, the TRIAC is triggered by a timing circuit in the dimmer, which can be adjusted by a user. The TRIAC continues to conduct until the current flowing through the TRIAC falls below a holding current, typically in the range of 10 to 30 mA. The TRIAC is then ready to be triggered again by the timing circuit during the negative half-cycle. Other dimmers are based on field-effect transistors (FETs) and these also require a continuous holding current to flow through them to maintain conduction.
TRIACs work particularly well in dimming conventional incandescent lamps, which are linear resistive loads because the AC mains current and voltage will remain in phase. This ensures that the current flowing through the TRIAC falls below the holding current very nearly at the end of each half-cycle. Thus, the TRIAC can accurately cut off part of the leading edge of each half-cycle and maintain conduction for the remainder of the half-cycle to allow a desired amount of power to reach the lamp.
On the other hand, with non-linear loads it is possible that the current flowing through the TRIAC will fall below the holding current prematurely or not at all. One such non-linear load is a compact fluorescent lamp (CFL). These offer a much higher lifespan and efficiency than conventional incandescent lamps, but they do not work well with dimmers as the electronic ballast used with CFLs does not draw a current from the mains that is higher than the holding current continuously over a half-cycle; instead the current is drawn in spikes. This leads to flickering (typically at the lower dimmer settings) and multiple firing (typically at the higher dimmer settings), which can cause buzzing and even damage to the dimmer. Despite the benefits mentioned above that CFLs present, their uptake has been affected by this problem as consumers wish to be able to dim the lights in various areas of a house, such as bedrooms and living rooms.
There have been various attempts to overcome this problem. One way is to incorporate full power factor correction into the ballast. However, this is complicated and costly. Furthermore, it requires larger components to handle the increased power, and this is incompatible with the requirement to house the electronic ballast in the lamp base or a luminaire.
US2008/0211417 discloses a dimmable ballast, which measures the conduction angle of the dimmer and adjusts the switching frequency of the lamp to ensure that the power factor and luminous intensity of the lamp are in accordance with the conduction angle. This is however, a complicated arrangement.
WO98/46050 discloses another complicated arrangement, in which a power feedback circuit is used to ensure that sufficient current is drawn from the dimmer to maintain conduction of a TRIAC.
Other ballasts use a charge pump, which uses the lamp voltage swing to pump current from the AC mains to an electrolytic storage capacitor in the ballast. An inverter in the ballast uses the energy stored in the storage capacitor to generate high voltage AC to drive the CFL. With these charge pump circuits, the current drawn from the dimmer is given by the following equation:
|iin|=Cinƒs(|νin|+2Vα−VB)
where:
As can be seen, from this equation the current drawn is dependent on the peak lamp voltage and the voltage across the electrolytic storage capacitor, and it is possible for this to fall below the holding current and even to zero (if the lamp voltage is less than half the voltage across the electrolytic storage capacitor) irrespective of the switching frequency and value of the input capacitor. The problems mentioned above (i.e. buzzing and flickering) can therefore be manifest in the charge pump style of ballast as well, especially at low dimming levels.
Furthermore, the current drawn from the mains will fall below the holding current of a TRIAC if the value of the charge pump input capacitor is too low. Using a larger capacitor could solve this problem (albeit with additional expense and bulk), but introduces another problem. That is that the resonant frequency of the inverter in the ballast changes when the TRIAC switches on and off (because the resonant frequency is a function of the mains voltage; thus when the TRIAC turns on the mains voltage and resonant frequency change rapidly). This change in resonant frequency is greater if the value of the charge pump input capacitor is increased. The change is even more pronounced in 230V applications since the charge pump input capacitor typically has a similar value to the resonant capacitor across the lamp in the inverter.
To maintain an even brightness and a constant charge pump function, it is necessary for the feedback control of the inverter to respond rapidly to this change of resonant frequency. However, it is difficult to design a feedback control circuit for the inverter that can maintain adequate operation at deep dimming levels and cope with the large signal frequency changes (which can be higher than 10 kHz) as the TRIAC turns on and off.
According to the invention, there is provided an electronic ballast for lighting applications, the electronic ballast comprising a first charge pump having an input capacitor charged with a supply current drawn from a power source by application of a charging voltage to the input capacitor, the magnitude of the supply current being proportional to the magnitude of the charging voltage; and a voltage booster for generating a boost voltage, which is used to augment the charging voltage, thereby increasing the current drawn from the power source.
Hence, by augmenting the charging voltage for the input capacitor, the current drawn from the power source is increased and the conduction of a TRIAC in a dimmer will be maintained as desired. The problems of flickering and buzzing mentioned above are therefore overcome. It is also possible to reduce the size of the input capacitor, which means the resonant frequency change in the inverter is reduced and the feedback network can be designed more easily as the small signal requirements dominate.
The power source is typically an AC power source, such as a 120V or 230V mains power source. In some countries, 100V or 200V mains power sources are used.
Typically, the electronic ballast is coupled to the power source by a bridge rectifier, which produces a supply voltage for the electronic ballast.
In one embodiment, a first terminal of the input capacitor is coupled to the bridge rectifier such that the charging voltage increases with the supply voltage.
The first terminal of the input capacitor is normally coupled to the bridge rectifier via one or more diodes as will be explained in detail below.
The electronic ballast preferably further comprises an electromagnetic interference (EMI) filter coupling the power source to the bridge rectifier.
The EMI filter may comprise a pair of filter capacitors in series between input terminals of the bridge rectifier, and a first terminal of the input capacitor may be coupled to the junction of the filter capacitors.
The input capacitor of the first charge pump is normally coupled via a diode to a reservoir capacitor. The input capacitor pumps current from the power source to the reservoir capacitor. The structure of the first charge pump will be explained in detail below.
Typically, a second terminal of the input capacitor is coupled to a source of alternating voltage generated within the ballast. In some embodiment, this source of alternating voltage is generated for driving a lamp. Thus, the lamp voltage may be used to drive the charge pump, or in other words to cause the input capacitor to pump current from the power source to the reservoir capacitor. The alternating voltage is typically oscillating at high frequency.
Preferably, the source of alternating voltage is an inverter. The inverter will usually have a resonant circuit driven by a pair of electronic switches in a half-bridge arrangement, the pair of electronic switches switching alternately. The pair of electronic switches may be coupled across the reservoir capacitor mentioned above, which then provides a source of DC for the inverter. The rapid switching of the electronic switches causes the resonant circuit to oscillate. Typically, the resonant circuit comprises a coil and capacitor in series, the source of alternating voltage being at the junction of the coil and capacitor.
In a preferred embodiment, the lamp comprises a compact fluorescent lamp (CFL) or an assembly of LEDs in series.
However, the invention may be used with other types of gas discharge lamp, such as fluorescent tube lights.
If an assembly of LEDs in series is used as the lamp then they are usually coupled to the ballast by way of a bridge rectifier. This will rectify the AC from the source of alternating voltage to produce a DC voltage for the LEDs. The bridge rectifier may be isolated from the source of alternating voltage by way of a transformer.
In one embodiment, the voltage booster comprises a secondary winding of a transformer that generates the boost voltage.
Preferably, the primary winding of the transformer is driven by the source of alternating voltage. To achieve this, the primary winding may either be the coil in the resonant circuit or a separate coil coupled from the source of alternating voltage to a ground terminal.
The second terminal of the input capacitor may be coupled to the source of alternating voltage via the secondary winding of the transformer, the primary winding being driven by the alternating voltage. In this case, the secondary winding may be coupled directly to the transformer or via another secondary winding of the transformer, such as one used to energise a lamp. In this case, the boost voltage is used to increase the voltage at the second terminal to increase the charging voltage.
In another embodiment, the electronic ballast further comprises a second charge pump adapted to increase the voltage at a first terminal of the input capacitor. This therefore increases the charging potential. The second charge pump typically comprises a second charge pump input capacitor that couples the boost voltage to one of many points in the electronic ballast suitable to raise the voltage at the first terminal of the input capacitor. These points will be explained in detail below.
Typically, however, the second charge pump input capacitor will be coupled to the bridge rectifier, via one or more diodes. It may be coupled to a second reservoir capacitor through a diode. The second charge pump capacitor preferably is coupled to the secondary winding of the transformer that generates the boost voltage.
In another aspect of the invention, there is provided a method for controlling current drawn by an electronic ballast for lighting applications from a power source, the method comprising charging an input capacitor in a first charge pump with a supply current drawn from a power source by application of a charging voltage to the input capacitor, the magnitude of the supply current being proportional to the magnitude of the charging voltage; and generating a boost voltage, which is used to augment the charging voltage, thereby increasing the current drawn from the power source.
The boost voltage is preferably generated by a secondary winding of a transformer, the primary winding of which is energised by a source of alternating voltage generated within the electronic ballast for driving a lamp.
The boost voltage may be used to augment the charging voltage by increasing the voltage at either a first or a second terminal of the input capacitor.
Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:
In the first embodiment, shown in
In parallel with capacitor 7, there are two series transistor switches 8, 9, which are arranged to switch alternately at a high frequency (typically 40 to 70 kHz). A D.C. blocking capacitor 10 couples the junction between these transistor switches 8, 9 to a resonant circuit made up of an inductor 11 and a capacitor 12. The inductor 11 is the primary winding in a transformer.
The junction between inductor 11 and capacitor 12 is coupled to the junction between diodes 5, 6 by a charge pump input capacitor 13. It is also coupled to a terminal of a compact fluorescent lamp 14. Two secondary windings 15a, 15b generate the voltage required to illuminate lamp 14. The resistor 20 is used to monitor the current flowing through the lamp and is not directly relevant to this invention.
A third secondary winding 16 generates a boost voltage to augment the voltage received from the bridge rectifier via diode 4. The boost voltage is coupled to the junction between diodes 4, 5 by charge pump capacitor 17. The third secondary winding 16 and capacitor form a second charge pump that acts as an input voltage booster. The resistors 18, 19 across the third secondary winding 16 are used to monitor for an end-of-life condition and are not directly relevant to this invention. The use of a second charge pump also reduces ringing in the EMI filter formed from coil 2 and capacitors 3a, 3b. This ringing can occur if the peak mains voltage is almost equal to the voltage across reservoir capacitor 7. In this case, diodes 4, 5 and 6 will cease to conduct, leading to ringing in the EMI filter due to the energy stored in it. This ringing can cause the current drawn through the TRIAC to drop below the hold current. However, the second charge pump prevents this by continuing to draw a small current from the mains. Previously, additional circuitry has been used to prevent this ringing.
The operation of the circuit shown in
Due to the influence of the resonant circuit formed by inductor 11 and capacitor 12 (in parallel with capacitor 13 when diodes 4 and 5 are conducting), the voltage across lamp 14 is sinusoidal. The charge pump input capacitor 13 may therefore be considered to be in series with a high-frequency voltage source to pump energy from the A.C. mains and discharge it into the reservoir capacitor 7.
When the lamp voltage is at a positive peak, it will begin to decrease with a sinusoidal form. Because the voltage on charge pump input capacitor 13 cannot change rapidly, diode 6 becomes reverse biased and the voltage at the junction of diodes 5 and 6 decreases, following the sinusoidal waveform of the lamp voltage. The voltage across charge pump input capacitor 13 because no current flows through it as both diodes 5 and 6 are reverse biased. This continues until the voltage at the junction of diodes 5 and 6 equals the voltage provided from bridge rectifier 1. At this point, diode 5 becomes forward biased and the voltage at the junction of diodes 5 and 6 is clamped to the voltage provided from bridge rectifier 1. The lamp voltage continues to decrease and therefore the voltage across charge pump input capacitor 13 increases. The charge pump input capacitor 13 is absorbing energy from the A.C. mains via the bridge rectifier 1, and the voltage across it peaks at a value equal to the lamp voltage plus the voltage provided from bridge rectifier 1. This coincides with the lamp voltage reaching the negative peak of its sinusoidal waveform.
At this point, diode 5 is reverse biased again. Diode 6 is also reverse biased because the voltage at the junction of diodes 5 and 6 is lower than the voltage on reservoir capacitor 7. Therefore, no current flows through charge pump input capacitor 13, and the voltage across it remains constant. However, the voltage at the junction of diodes 5 and 6 is continuously increasing as the lamp voltage has begun to increase again, having passed the negative peak.
Eventually, the voltage at the junction of diodes 5 and 6 reaches the same voltage as the reservoir capacitor 7 and diode 6 is forward biased. The voltage at the junction of diodes 5 and 6 is then clamped to the voltage on the reservoir capacitor 7. Charge pump input capacitor 13 is caused to discharge its stored energy into reservoir capacitor 7 due to the increasing lamp voltage. This continues until the lamp voltage reaches a positive peak again when the diode 6 is reverse biased again and the next cycle proceeds as described above.
The effect of reintroducing third secondary winding 16, capacitor 17 and diode 4 will now be described. Since third secondary winding 16 forms a transformer with inductor 11, the current flowing through lamp 14 will cause a voltage to be generated across third secondary winding 16. This voltage is used to charge up charge pump capacitor 17 and causes the potential at the junction between diodes 4 and 5 to increase. In effect, this augments the voltage provided by the bridge rectifier 1, and the charge pump input capacitor 13 is charged by a charging voltage that is higher than the voltage provided by the bridge rectifier 1 alone and that increases with the augmented voltage. Thus, the current drawn from the A.C. mains through the bridge rectifier 1 will be increased as the augmented voltage increases.
It is quite common in electronic ballasts for CFLs to provide a third secondary winding for the purpose of detecting an end-of-life condition of the lamp, and this invention can make use of this winding as described above.
It is preferable if the voltage generated by the third secondary winding 16 is in phase or exactly out of phase (or at least as close as possible to either of these conditions) with the voltage across the lamp. If they are in phase then additional current is drawn by the two capacitors 13 and 17 acting in parallel, which helps to mitigate the ringing mentioned above. If they are out of phase then the voltage across capacitor 13 is enhanced.
In this embodiment, the inductor 2 turns the spikes of current pumped through capacitor 13 into a steady DC current. As the lamp voltage increases, the current pumped through capacitor 13 can only pass through the diodes of bridge rectifier 1 towards diode 6 and reservoir capacitor 7. Preferably, the values of capacitors 21, 22 are higher than the value of capacitor 13.
The fourth embodiment of
In this embodiment, capacitor 23 pumps current from the third secondary winding 16 through the diodes of bridge rectifier 1 into reservoir capacitor 31. Preferably, the amount of current pumped by capacitor 23 should be at least as large as the current drawn by charge pump capacitor 13.
In
In a variant of this embodiment, the capacitor 13 is coupled to the junction between diodes 4 and 5 and capacitor 17 is coupled to the junction between diodes 25 and 6. This variant should be used if capacitor 13 will draw more current than capacitor 17; otherwise, the circuit of
A zener diode 30 is coupled across reservoir capacitor 7 to prevent the voltage across this rising too high in the event of “overpumping”, which can occur when high levels of dimming are applied. This “overpumping” can occur when used with arrays of LEDs (unlike CFLs, which always require a small amount of power to heat the electrodes even at very deep dimming levels), and a bleeder resistor can be used to dissipate the excess energy as heat.
It is possible to remove capacitor 12 from the resonant circuit as it is no longer necessary to generate the high voltages required to ignite a CFL. However, it is beneficial to retain capacitor 12 to assist with pumping current from the mains using capacitor 13, especially if an array of LEDs with a high combined forward voltage are used.
In a variant of this embodiment, the capacitor 13 is connected to the junction between inductor 11 and primary winding 32a rather than to the junction between capacitor 10 and inductor 11. This has the advantage of reducing the capacitive load on the half-bridge formed by transistors 8, 9, but does reduce the voltage swing available across primary winding 32a.
The charge pump principle described in the above embodiments can also be used with other types of converter, such as flyback and buck converters. In these cases, the charge pump capacitors are driven by secondary windings on the transformers within such converters. These are particularly beneficial when used with the LED lamp embodiments of
Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11154493.8 | Feb 2011 | EP | regional |
