GB2472038A

GB2472038A - Power adaptor for discharge light source

Info

Publication number: GB2472038A
Application number: GB0912712A
Authority: GB
Inventors: David Thomas Summerland
Original assignee: Holdip Ltd
Current assignee: Holdip Ltd
Priority date: 2009-07-22
Filing date: 2009-07-22
Publication date: 2011-01-26
Also published as: GB0912712D0

Abstract

A power adaptor 20 for a non-solid state light source (50, fig. 3) comprises an input 22 for connection to a dimmer circuit (10, figure 1) having a minimum operating current, and a drive circuit 34, coupled to the input, that provides an output suitable for driving the light source. The power adaptor is configured such that the current drawn from the input by the drive circuit has an approximately square-shape waveform. The drive circuit may include a LCL series-parallel resonant circuit. The non-solid state light source may particularly be a discharge lamp or a compact fluorescent lamp (CFL).

Description

Title -Improvements relating to Lighting Systems This invention relates to lighting systems, and in particular to power adaptors for light sources.

Conventional dimmer circuits for light sources typically include one or more silicon-controlled rectifiers (SCR5), and most usually a TRIAC (triode for alternating current), which is equivalent to two silicon-controlled rectifiers (SCR5) joined in inverse parallel and with their gates connected together. An SCR or TRIAC is an electronic switch that, once triggered, continues to conduct until the current through it drops below a minimum threshold, such as at the end of a half-cycle of alternating current (AC) mains power. Hence, in order to function effectively, a dimmer circuit including a TRIAC requires a minimum current to be drawn by the light source over a substantial proportion of a half-cycle of alternating current (AC) mains power.

For this reason, conventional power adaptors for discharge lamps or compact fluorescent lamps include power factor correction circuits in order to increase the proportion of the mains half-cycle in which the current drawn by the light source is greater than the threshold level of the dimmer circuit.

There has now been devised an improved power adaptor, which overcomes or substantially mitigates the above-mentioned and/or other disadvantages

associated with the prior art.

According to the invention, there is provided a power adaptor for a non-solid state light source, the power adaptor comprising an input for connection to a dimmer circuit having a minimum operating current, and a drive circuit coupled to the input that provides an output suitable for driving the light source, wherein the power adaptor is configured such that the current drawn from the input by the drive circuit has an approximately square-shape waveform.

The power adaptor according to the invention is advantageous principally because the current drawn through the dimmer circuit may be maintained above the minimum operating current, without drawing excessive power, and without any need for a power correction circuit.

In presently preferred embodiments, the drive circuit includes an LCL series-parallel resonant circuit. In particular, an LCL series-parallel resonant circuit may be configured such that, as the voltage at the power adaptor input varies sinusoidally, the current drawn from the input by the LCL series-parallel resonant circuit inherently follows a square shape, ie without any need for active control.

Hence, according to a further aspect of the invention, there is provided a power adaptor for a non-solid state light source, the power adaptor comprising an input for connection to a dimmer circuit having a minimum operating current, and a drive circuit coupled to the input that provides an output suitable for driving the light source, wherein the drive circuit includes an LCL series-parallel resonant circuit.

The non-solid light source may be a discharge lamp, but this invention is particularly advantageous in relation to compact fluorescent lamps (CFL). The power adaptor is preferably configured substantially as described in UK patent no 2449616 in relation to power adaptors for solid-state light sources. However, the current drawn from the input by the resonant circuit preferably has an approximately square-shape waveform, as the voltage at the input varies sinusoidally, and the current and voltage are preferably in phase.

The LCL series-parallel circuit is preferably adapted to have a first resonant frequency that provides, at a given input voltage, a constant current output that is independent of the load, and a second resonant frequency that provides, at a given input voltage, a current that varies with load. Most preferably, the LCL series-parallel circuit will also have a third resonant frequency at 0Hz, ie DC current.

It is known that to provide long life and to ensure soft-starting of a compact fluorescent lamp (CFL), the cathodes of the lamp must be pre-heated so that their hot resistance value is approximately three to four times that of the cold resistance value. When the power adaptor is adapted for use with a compact fluorescent lamp (CFL), the power adaptor may be adapted to drive the LCL series-parallel resonant circuit at the second, variable current, resonant frequency in order to pre-heat the lamp, but then drive the LCL series-parallel resonant circuit at the first, constant current, resonant frequency during normal operation. In particular, during start-up of the lamp, the LCL series-parallel resonant circuit is preferably driven at the second, variable current, resonant frequency in order to provide sufficient current to heat the cathodes of the lamp, at a voltage that is below the strike potential of the lamp. Once the cathodes of the lamp have been sufficiently pre-heated, the LCL series-parallel resonant circuit is preferably driven at the first, constant current, resonant frequency. The voltage across the lamp preferably becomes sufficient, at this stage, to strike the arc, and current is then drawn with an approximately square-shape waveform.

The power adaptor may also be provided with a switchable low impedance path for the dimmer circuit. The low impedance path for the dimmer circuit is preferably active during at least a proportion of the periods when the drive circuit is inactive.

Most preferably, the low impedance path for the dimmer circuit is active in substantially all of the period in which the drive circuit is inactive, and inactive in substantially all of the period in which the drive circuit is active. In particular, an electronic switch, such as a FET, and a resistor, preferably provide a DC path for the dimmer circuit when the SCR or TRIAC is in its off portion.

The LCL series-parallel resonant circuit may be adapted to provide an output suitable for driving the light source. Alternatively, the LCL series-parallel resonant circuit may be configured as a front-end to a conventional drive circuit for the light source. In this arrangement, the LCL series-parallel resonant circuit would provide high power factor and dimmer circuit compatibility.

As discussed above, the LCL series-parallel resonant circuit being driven at the first, constant current, resonant frequency is preferably adapted to draw current with an approximately square-shape waveform. The current waveform at the input of the power adaptor being a generally square shape enables the power adaptor to maintain the current above the minimum operating current of the dimmer circuit, for a substantial proportion of a half-cycle of alternating current (AC) mains power, without any need for a power factor correction circuit. The power adaptor preferably, therefore, does not include any power factor correction circuit, which would increase cost and size. Furthermore, the approximately square-shaped waveform of the current will substantially reduce the peak current relative to the minimum operating current, and hence substantially reduce the power drawn by the power adaptor.

According to a further aspect of the invention, there is provided a lighting system comprising a dimmer circuit having a minimum operating current, a power adaptor as described above, and a lighting unit including at least one non-solid state light source.

According to a further aspect of the invention, there is provided a lighting unit suitable for direct connection to a mains supply, the lighting unit comprising a power adaptor as described above and one or more non-solid state light sources.

The dimmer circuit preferably includes one or more silicon-controlled rectifiers (SCR5), or a TRIAC. The non-solid light source may be a discharge lamp, but this invention is particularly advantageous in relation to compact fluorescent lamps (CFL).

By "LCL series-parallel resonant circuit" is meant a resonant circuit comprising a first inductor and a first capacitor in series, and a parallel load including a second inductor. The resonant circuit preferably comprises a load connected in parallel across the first capacitor, wherein the load comprises the second inductor and an output for driving the light source, which are connected in series. Any of the first inductor, the first capacitor and the second inductor may comprise a single inductive or capacitive component or a combination of such components.

It has been found that by driving the resonant circuit at a sub-harmonic of the resonant frequency, the power factor and/or efficiency of the power adaptor may be improved. Most preferably, the resonant circuit is driven at a sub-harmonic of 1/x, where x is an odd number, for example, 1/3, 1/5 or 1/7.

Driving the resonant circuit at a sub-harmonic of the resonant frequency has the advantage that the switching frequency and switching losses of the resonance drive circuit may be reduced, thereby improving the efficiency of the power adaptor. In most prior art resonant circuits, driving the circuit at a sub-harmonic would reduce the power. However, the LCL series-parallel resonant circuit is preferably adapted to have one of its resonant frequencies at 0 Hz, as discussed in more detail below, which allows low frequency currents to pass through to the load. Hence, the current passing through the resonant circuit and the power delivered to the load does not change substantially if the circuit is driven at a sub-harmonic of the resonant frequency.

In order to maximise the efficiency of the LCL series-parallel circuit, the reactance XL1 of the first inductor and the reactance XL2 of the second inductor are preferably substantially equal. In particular, the reactance XL1 of the first inductor and the reactance XL2 of the second inductor preferably differ by less than +/-25%.

Furthermore, the value of the first capacitor is preferably chosen such that, at one of the resonant frequencies of the circuit, the reactance X of the first capacitor is substantially equal to the reactance of the first inductor, and most preferably substantially equal to the reactance of the first and second inductors.

When the chosen components satisfy these conditions, at a given input voltage, the current delivered to a load will be constant, independent of the load connected to the power adapter. Furthermore, variation of the input voltage would directly control the magnitude of the constant current delivered to the load. When driving a constant voltage load, the power delivered to the load would therefore be directly proportional to the input voltage, without requiring any feedforward or feedback control.

As the voltage at the input varies sinusoidally, the current drawn from the input by the resonant circuit will inherently follow a square shape.

As discussed above, the LCL series-parallel circuit is preferably adapted to have a first resonant frequency that provides, at a given input voltage, a constant current output that is independent of the load, and a second resonant frequency that provides, at a given input voltage, a current that varies with load. Most preferably, the LCL series-parallel circuit will also have a third resonant frequency at 0Hz, ie DC current. These resonant frequencies are preferably achieved by selecting the first inductor, the second inductor and the first capacitor, such that the reactances of those components are substantially equal.

The second resonant frequency may be adapted to provide a significantly greater power at the output, relative to the first resonant frequency. A controller of the power adaptor may therefore be adapted to switch between the different resonant frequencies in order to utilise their different characteristics. For example, a controller of the power adaptor may be adapted to switch between the first and second resonant frequencies to compensate for a change of input voltage, eg between 230V and 11OVAC. Furtherexamplesof such control include loading a TRIAC in the lighting system at critical points, and altering the power factor and/or regulation of the power adaptor. In particular, during start-up of the lamp, the LCL series-parallel resonant circuit is preferably driven at the second, variable current, resonant frequency in order to provide sufficient current to heat the cathodes of the lamp, at a voltage that is below the strike potential of the lamp. Once the cathodes of the lamp have been sufficiently pre-heated, the LCL series-parallel resonant circuit is preferably driven at the first, constant current, resonant frequency. The voltage across the lamp preferably becomes sufficient, at this stage, to strike the arc, and current is then drawn with an approximately square-shape waveform.

The resonant circuit is preferably driven by a resonance drive circuit, which provides a resonance drive signal to the resonant circuit. The resonance drive signal is preferably an alternating signal, and is preferably provided by an oscillator that may control two or four electronic switches, eg FETs. The resonance drive signal typically has the form of a square wave. The purpose of the resonance drive circuit is to excite the resonant circuit with an alternating voltage, the alternating voltage typically consisting of blocks of positive and negative voltage.

The electronic switches are typically connected together in the form of a full bridge inverter (4 switches) or a half bridge inverter (2 switches).

As discussed below, the power adaptor may be adapted to control the light output from the light source. In this embodiment, the resonance drive signal is preferably variable, for example by a controller, in order to determine the light output from the light source.

Alternatively, where the power adaptor is configured such that the light output from the light source is only controllable by varying the power available at the input of the power adaptor, the resonance drive signal may be predetermined, preferably to optimise the power factor and/or efficiency of the power adaptor.

The determination of the frequency at which the resonant circuit is driven may be used to calibrate the power adaptor for improved efficiency. Alternatively, the frequency at which the resonant circuit is driven may be varied during use, in order to vary the power being supplied to the light source.

The output for driving the light source may be isolated from the input of the power adaptor, particularly for applications in which users would have access to the light source and/or associated circuitry. In this case, the power adaptor preferably comprises a transformer or one or more isolation capacitors to provide this isolation. Where the power adaptor includes a transformer for providing this isolation, the transformer is preferably a piezoelectric transformer.

The resonant circuit may also include a pair of potential dividing capacitors, to which the first capacitor is connected. Alternatively, where the resonance drive circuit contains four electronic switches (eg FET5) arranged to create two switching legs (eg a "H-bridge"), as a single phase inverter, the pair of capacitors could be replaced by a single capacitor. These capacitors are preferably Y capacitors.

In another embodiment, the resonance drive circuit comprises two electronic switches (eg FET5) connected between the LCL series-parallel resonant circuit and ground, ie two "low-side" switches. These two low-side switches preferably each alternate between ON and OFF, which a first switch being ON whilst a second switch is OFF, and vice versa. This arrangement is particularly advantageous where the switches are driven by a low voltage controller, such as an integrated circuit.

In this embodiment, the first resonant inductor of the LCL series-parallel resonant circuit preferably comprises two inductors, one connected to one end of the first capacitor, and the other connected to the other end of the first capacitor. In this arrangement, one of these two inductors will be active in the positive half cycle of the supply, and the other of these two inductors will be active in the negative half cycle of the supply. In one embodiment, these two inductors are wound about a common core, such that the first resonant inductor of the LCL series-parallel resonant circuit is a three terminal inductor.

Any control circuitry of the power adaptor may be powered by an integrated power supply. Alternatively, the control circuitry of the power adaptor may be powered by a connection to one of the inductors of the resonant circuit, for instance a connection to a winding coupled to that inductor.

Where the power adaptor includes an integrated power supply, the integrated power supply preferably draws power directly from the mains power supply, most preferably via the input of the power adaptor. In particular, the integrated power supply is preferably a constant current power supply, such as a switch mode constant current regulator, which preferably does not cause excessive inrush and is low in cost.

A preferred embodiment of the invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is a schematic diagram of a lighting system according to the invention; Figure 2 is a schematic diagram of a power adaptor according to the invention that forms part of the lighting system of Figure 1; and Figure 3 is a schematic diagram of a resonant circuit, including a resonance controller and a resonance drive circuit, that forms part of the power adaptor of Figure 2.

Figure 1 shows a lighting system according to the invention. The lighting system is connected to a mains circuit including a mains supply L,N and a TRIAC dimmer circuit 10, and comprises a power adaptor 20 and a compact fluorescent lamp (CFL) 50. The power adaptor 20 is supplied with electrical power from the mains circuit, and is adapted to provide electrical power to the compact fluorescent lamp (CFL) 50.

Referring now to Figure 2, the power adaptor 20 comprises an input 22 for drawing electrical power from the mains circuit, and an output 24 for providing electrical power to the compact fluorescent lamp (CFL) 50. The power adaptor 20 includes a filtering and rectifying circuit 30 at the input 22, such that the AC voltage waveform drawn from the mains circuit is supplied to the remainder of the power adaptor circuitry as a full-wave rectified waveform (DC+).

The power adaptor 20 also includes a low power, auxiliary power supply 32, and a resonant circuit 34 including a resonance drive circuit 42, which is described in more detail below with reference to Figure 3. The low power, auxiliary power supply 32 provides a low power DC output (+V) for powering the integrated circuits of the resonance drive circuit 42. This provides a stable power supply to the integrated circuits of the power adaptor to ensure stable functioning of those circuits. It is noted that in other embodiments, the integrated circuits of the power adaptor are powered by connections to additional windings coupled to one of the inductors of the resonant circuit, and hence the auxiliary power supply 32 is omitted.

The resonant circuit 34, including the resonance drive circuit 42, is shown in Figure 3. The resonance drive circuit 42 includes a controller adapted to control the output of the resonance drive circuit 42. It is noted that in other embodiments, the resonance drive circuit 42 is self-oscillating, and the controller is omitted altogether.

The resonant circuit 34 has the form of an LCL series-parallel resonant circuit (Li, Ci and L2). The resonance drive circuit 42 is adapted to drive the LCL series-parallel resonant circuit with a square wave driving signal. This square wave signal is generated by two electronic switches, eg FETs, connected to a first end of the resonant circuit, and associated drive circuitry 44. The capacitors 02 and 03 create a connection point for the second end of the resonant circuit, substantially midway in voltage between DC+ and OV.

Alternatively, the resonance drive circuit 42 contains four electronic switches (eg FET5) arranged to create two switching legs (in a "H-bridge"), as a single phase inverter. In this embodiment, the capacitors 02 and 03 are replaced by a single capacitor (02) connected between DC+ and OV. The circuit cannot operate with no capacitance across the DC supply, as a small amount of capacitance is required to protect the switches from overvoltage damage during switching transients.

The LCL series-parallel resonant circuit is configured such that at a chosen frequency, the reactance of Li (XL1), the reactance of Ci (Xci) and the reactance of L2 (XL2) are substantially equal. In this configuration, the LCL series-parallel resonant circuit has two non-zero resonant frequencies. The frequency at which the reactances are equivalent will be one of the two non-zero resonant frequencies. When driving the resonant circuit at this frequency, the resonant circuit supplies a constant current to the output, and hence to the CFL 50, regardless of the load. The magnitude of the constant current is proportional to the input voltage. This resonant frequency is 1 (2)

VLSCP

The resonance drive circuit 42 is therefore adapted to excite the LCL series-parallel resonant circuit close to this resonant frequency, w. As a consequence of driving the resonant circuit close to the resonant frequency, the switching losses in the electronic switches are reduced, and hence the efficiency of the circuit is improved. Further advantages include the reduction of conducted and radiated electromagnetic interference, and hence the reduction of the expense of necessary filtering and screening components.

The dimmer circuit 10 includes a TRIAC, which is an electronic switch that, once triggered, continues to conduct until the current through it drops below a minimum threshold, such as at the end of a half-cycle of alternating current (AC) mains power. Hence, in order to function effectively, a dimmer circuit including a TRIAC requires a minimum current to be drawn by the light source over a substantial proportion of a half-cycle of alternating current (AC) mains power. The current drawn from the input by the resonant circuit therefore has an approximately square-shape waveform, as the voltage at the input varies sinusoidally, and the current and voltage are in phase.

The LCL series-parallel circuit is adapted to have a first resonant frequency that provides, at a given input voltage, a constant current output that is independent of the load, and a second resonant frequency that provides, at a given input voltage, a current that varies with load. The LCL series-parallel circuit also has a third resonant frequency at 0Hz, ie DC current.

It is known that to provide long life and to ensure soft-starting of a compact fluorescent lamp (CFL), the cathodes of the lamp must be pre-heated so that their hot resistance value is approximately three to four times that of the cold resistance value. The power adaptor is therefore adapted to drive the LCL series-parallel resonant circuit at the second, variable current, resonant frequency in order to pre-heat the lamp, but then drive the LCL series-parallel resonant circuit at the first, constant current, resonant frequency during normal operation. In particular, during start-up of the lamp, the LCL series-parallel resonant circuit is driven by the resonance drive circuit 42 at the second, variable current, resonant frequency in order to provide sufficient current to heat the cathodes of the lamp, at a voltage that is below the strike potential of the lamp. Once the cathodes of the lamp have been sufficiently pre-heated, the LCL series-parallel resonant circuit is driven by the resonance drive circuit 42 at the first, constant current, resonant frequency.

The voltage across the lamp becomes sufficient, at this stage, to strike the arc, and current is then drawn with an approximately square-shape waveform.

The current waveform at the input of the power adaptor being a generally square shape enables the power adaptor to maintain the current above the minimum operating current of the dimmer circuit, for a substantial proportion of a half-cycle of alternating current (AC) mains power, without any need for a power factor correction circuit. Furthermore, the approximately square-shaped waveform of the current will substantially reduce the peak current relative to the minimum operating current, and hence substantially reduce the power drawn by the power adaptor.

Claims

Claims 1. A power adaptor for a non-solid state light source, the power adaptor comprising an input for connection to a dimmer circuit having a minimum operating current, and a drive circuit coupled to the input that provides an output suitable for driving the light source, wherein the power adaptor is configured such that the current drawn from the input by the drive circuit has an approximately square-shape waveform.
2. A power adaptor as claimed in Claim 1, wherein the drive circuit includes an LCL series-parallel resonant circuit.
3. A power adaptor as claimed in Claim 1 or Claim 2, wherein the non-solid state light source is a discharge lamp or a compact fluorescent lamp (CFL).
4. A power adaptor as claimed in Claim 2, wherein the LCL series-parallel circuit is adapted to have a first resonant frequency that provides, at a given input voltage, a constant current output that is independent of the load, and a second resonant frequency that provides, at a given input voltage, a current that varies with load.
5. A power adaptor as claimed in Claim 4, wherein the power adaptor is adapted for use with a compact fluorescent lamp (CFL), and the power adaptor is adapted to drive the LCL series-parallel resonant circuit at the second, variable current, resonant frequency in order to pre-heat the lamp, but then drive the LCL series-parallel resonant circuit at the first, constant current, resonant frequency during normal operation.
6. A power adaptor as claimed in any preceding claim, wherein the power adaptor includes a switchable low impedance path for the dimmer circuit.
7. A power adaptor as claimed in Claim 6, wherein the low impedance path for the dimmer circuit is active during at least a proportion of the periods when the drive circuit is inactive.
8. A power adaptor as claimed in Claim 7, wherein the low impedance path for the dimmer circuit is active in substantially all of the period in which the drive circuit is inactive, and inactive in substantially all of the period in which the drive circuit is active.
9. A power adaptor as claimed in any preceding claim, wherein an LCL series-parallel resonant circuit is provided as a front-end to a conventional drive circuit for the light source.
10. A lighting system comprising a dimmer circuit having a minimum operating current, a power adaptor as claimed in any preceding claim, and a lighting unit including at least one non-solid state light source.
11. A lighting system as claimed in Claim 10, wherein the dimmer circuit includes one or more silicon-controlled rectifiers (SCR5), a TRIAC or a MOSFET.
12. A lighting unit suitable for direct connection to a mains supply including a dimmer circuit, the lighting unit comprising a power adaptor as claimed in any preceding claim and one or more non-solid state light sources.
13. A lighting system as claimed in Claim 10 or Claim 11, or a lighting unit as claimed in Claim 12, wherein the non-solid state light source is a discharge lamp or a compact fluorescent lamp (CFL).