GB2468190A

GB2468190A - Protection for lighting using dimmer with load current monitoring and control

Info

Publication number: GB2468190A
Application number: GB201002029A
Authority: GB
Inventors: Novar Ed S Limited; Kanthimathinathan Thirugnanasambandham; Dinesh Kumar; Girish Ramdas Wabale
Original assignee: Novar ED&S Ltd
Priority date: 2009-02-09
Filing date: 2010-02-08
Publication date: 2010-09-01
Also published as: SG164327A1; GB0902127D0; GB201002029D0; CN101801143A

Abstract

A lighting installation comprises a lighting load 60 powered by an AC power supply 50, and a dimmer 5 comprising a semiconductor switch 10, 20 operable to switch the light on and off for a portion of the AC cycle, where the brightness of the light is altered by altering the switching period. The lighting installation is protected by monitoring the load current passing through the switch and, when the monitored current exceeds a threshold value, reducing the switching period, so that power dissipated in the switch is also reduced. The semiconductor switch may be a field effect transistor (FET or MOSFET), a triac, BJT or IGBT. A controller 130 such as a microprocessor may be provided to monitor the load current and control the switching. The lighting load may comprise a plurality or fluorescent or incandescent lamps. The invention may also be used to supply a environmental conditioning device such as a heater or air conditioner.

Description

Control of Environmental Conditioning Devices

Background of the Invention

The present invention relates to the field of

environmental conditioning devices, such as lights, heaters, air-conditioners or the like. More particularly, but not exclusively, this invention relates to the field of dimmer switches for controlling the power supplied to a lighting load (e.g. an incandescent bulb or fluorescent tube) . More particularly, but not exclusively, this invention relates to a method of protecting a lighting installation, the lighting installation comprising a dimmer comprising a semiconductor switch operable to switch one or more lights on and off at least once during each mains AC cycle.

Older designs of dimmer switches used variable resistors to control the voltage delivered across the lighting load. A disadvantage of that approach is that energy not delivered to the lighting load is instead converted into heat in the variable resistor, which results in the switches being inefficient and potentially dangerous.

Modern dimmers work by reducing the mis voltage across the lighting load by switching the lighting load on and off many times per second. Control of the duty cycle in that way is known as phase-controlled dimming. A mains AC circuit provides a voltage that varies sinusoidally from a peak positive voltage through zero voltage to a peak negative vo1tae. Modern dimmers work by chopping up each period of the sine wave of the AC signal into a portion in which the lighting load is on and a portion in which it is off. One approach is to turn the lighting load off whenever the AC current passes through zero current (i.e. whenever the current in the circuit reverses direction), and then to turn it back on at a voltage determined by the setting of a control knob or the like. Another approach is to turn the lighting load on whenever the AC signal passes through OV, and then to turn it off at a voltage determined by the setting of a control knob or the like. In either case, the amount of energy supplied to the light increases or decreases as the light is turned on for more or less of the AC cycle; in other words, the brightness of the light varies with changes in the proportion of the sine wave of the AC supply for which the light is switched on (the duty cycle).

The desired switching behaviour can be achieved for example by providing a knob-controlled variable resistor and a firing capacitor, which in combination control the semiconductor switch, with the semiconductor switch being switched when the voltage across the capacitor exceeds a threshold value, (the time taken to reach the threshold voltage being controlled by the variable resistor) . Thus, for example, in some prior-art dimmers, the semiconductor switch is a triac, and the gate of the triac is connected between the firing capacitor and the variable resistor.

In order to achieve dimming by controlling the duty cycle, it is usually necessary to use a switch that is capable of switching at a frequency similar to the AC mains frequency (e.g. 50Hz or 60Hz) . It is usual to use a semiconductor switch for that purpose. The most common choice of semiconductor switch is a triac, but it is also known to use a field effect transistor (FET), for example a metal-oxide-semiconductor FET (MOSFET) However, there is a finite voltage drop across a semiconductor switch, due to the impedance of the switch.

The current delivered to the lighting load flows through the switch for at least part of each cycle, and the resultant power is dissipated as heat, resulting in an increase in the temperature of the switch. When the load increases beyond the rated value, the temperature of the switch can go beyond safe limits, resulting sooner or later in failure of the switch. Typical dimmers are provided with overload protection such that when the load exceeds a specified value, say 150%, the load is shut off. However, loads over 100% but less than the overload shut-off threshold, will nevertheless stress the switch. Moreover, an overload current of 150% will result in power dissipation of 225% of the power dissipation resulting from operation at the rated current, which is potentially dangerous.

In many installations, only a limited space is available for the dimmer, and consequently there is little room available for provision of further protection circuitry. Additional circuitry would also result in higher costs and lower reliability.

The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved dimmer for controlling a lighting load.

Summary of the Invention

The present invention provides, according to a first aspect, a method of protecting a lighting installation, the lighting installation comprising (a) a lighting load, having a brightness, powered by an AC power supply, said AC power supply having a cycle of period t; and (b) a dimmer comprising a semiconductor switch operable to switch the current to the lighting load on and off; wherein in normal operation the switch switches the lighting load on and switches the lighting load off, and the brightness of the lighting load is altered by altering the switching on or the switching off so that the lighting load is on for a longer or shorter portion x of the period t; the method further comprising: monitoring a load current passing through the switch to the lighting load and, when the monitored current exceeds a first threshold value, reducing the portion x of the period t for which the lighting load is on, so that power dissipated in the switch is also reduced.

Thus, by utilising the control of the duty cycle (the portion x of the period t for which the current is supplied to the load) provided for dimming and brightening, the lighting installation of the invention can keep the dimmer semiconductor switch within safe current (and hence power) limits even for a load that would otherwise significantly overload the switch.

The switch may switch the lighting load on and switch the lighting load off at least once in each cycle. The switch may switch the lighting load on and switch the lighting load off at least once in each half cycle.. The switch may switch the lighting load off at the beginning of every half cycle. The switch may switch the lighting load on at the beginning of every half cycle.

The brightness of the lighting load may be altered by altering the duty cycle, for example by increasing or decreasing a delay between switching the lighting load on/off and switching the lighting load off/on.

The portion x of the period t for which the lighting load is on may be reduced in response to the monitored current exceeding the first threshold value by switching the lighting load on and off more frequently. The portion x of the period t for which the lighting load is on may be reduced in response to the monitored current exceeding the first threshold value by increasing a delay between switching the lighting load off and switching the lighting load on. The portion x of the period t for which the lighting load is on may be reduced in response to the monitored current exceeding the first threshold value by decreasing a delay between switching the lighting load on and switching the lighting load off.

The load current may be monitored by using a current sense resistor to convert the load current into a voltage.

The method may further comprise switching the lighting load off permanently if the load current exceeds a second threshold value. The second threshold value may be for example 150% of the first threshold value. It will be understood that "switching the lighting load off permanently" means switching the lighting load off for a period of time very much greater than the period t, for example more than lOOt. When the second threshold value is exceeded, it may be that the lighting load is switched off until a reset signal is generated within, or supplied to, the dimmer.

The lighting load comprises at least one light, for example a bulb or fluorescent tube. The lighting load may comprise a plurality of lights.

The present invention provides, according to a second aspect, a dimmer for connection to a lighting load, said load having a brightness and powerable by an AC power supply, said AC power supply having a cycle of period t, said dimmer comprising: a semiconductor switch operable to switch the lighting load on and off; wherein in normal operation the switch is arranged to switch the lighting load on and to switch the lighting load off, the switch being operable to alter the brightness of the lighting load by altering the switching on or the switching off so that the lighting load is on for a longer or shorter portion x of the period t; the dimmer further comprising a monitor arranged to monitor a load current passing through the switch to the lighting load and, when the monitored current exceeds a threshold value, to reduce the portion x of the period t for which the lighting load is on, so that power dissipated in the switch is also reduced.

The semiconductor switch may for example be a field-effect transistor (e.g. a MOSFET), or a triac.

The dimmer may further comprise a controller, which may be a microprocessor, which may monitor the load current, and which may control the switching of the switch.

The present invention provides, according to a third aspect, a lighting installation including a dimmer according to the second aspect of the invention.

The present invention provides, according to a fourth aspect, a method of protecting an environmental-conditioning installation, the installation comprising (a) a load, having an output level, powered by an AC power supply, said AC power supply having a cycle of period t; and (b) a control comprising a semiconductor switch operable to switch the

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current to the load on and off; wherein in normal operation the switch switches the load on and switches the load off, and the output level of the load is altered by altering the switching on or the switching off so that the load is on for a longer or shorter portion x of the period t; the method further comprising: monitoring a load current passing through the switch to the load and, when the monitored current exceeds a first threshold value, reducing the portion x of the period t for which the load is on, so that power dissipated in the switch is also reduced.

The present invention provides, according to a fifth aspect, a control for connection to a load for an environmental-conditioning installation, said load having an output level and powerable by an AC power supply, said AC power supply having a cycle of period t, said control comprising: a semiconductor switch operable to switch the load on and off; wherein in normal operation the switch is arranged to switch the load on and to switch the load off, the switch being operable to alter the output level of the load by altering the switching on or the switching off so that the load is on for a longer or shorter portion x of the period t; the control further comprising a monitor arranged to monitor a load current passing through the switch to the load and, when the monitored current exceeds a threshold value, to reduce the portion x of the period t for which the load is on, so that power dissipated in the switch is also reduced.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 is a block diagram of a dimmer from an example embodiment of a lighting installation according to the invention; Figure 2 shows a flow chart illustrating a method of controlling the dimmer of Fig. 1 according to an example embodiment of the method of the invention; and Figure 3 is a plot of current through a load and corresponding switch voltages for (a) a 200W load with a 60% duty cycle, operating in a normal mode of operation, and (b) a 400W over-load.

Detailed Description

A lighting installation according to an example embodiment of the invention includes (Fig. 1) dimmer 5 including a board 15, upon which are mounted components forming a number of functional blocks.

At the heart of the dimmer 5 are two MOSFETs 10, 20, connected in series between AC IN 50 and a light (load 60) controlled by the dimmer 5; the MOSFETs 10, 20 thereby control the current supplied to the load 60. The MOSFETs 10, 20 are switched on and off by control signals from -9-.

driver 30. Two MOSFETs 10, 20 are provided to enable switching in both the positive and negative halves of the cycle.

Controller 130 turns on dimmer MOSFETs 10, 20 by supplying MCU driving signals (Ml and M2) through MOSFET driver 30. To detect the point at which the AC cycle of the AC mains supply crosses zero, the MOSFETs are turned "OFF" for a short while just before a zero cross is expected (from a measured AC frequency) A link power supply unit (PSU) 70 is also provided and works in a "power-stealing" mode: it includes a capacitor which is charged whilst the MOSFET5 10, 20 are off, and then the power stored in the capacitor is used to power components of the dimmer 5 whilst the MOSFETs 10, 20 are turned on. It generates two isolated outputs, a +15 V output for the MOSFET driver 30 and a +5 V for display LEDs, the controller 130 and a RF system associated with the dimmer (for sending and receiving signals from remote elements of the lighting installation) Initially during normal operation of the dimmer 5, both MOSFET5 are "OFF". A zero-cross detector (ZCD) circuit 90 senses the voltage across MOSFET5 10, 20 and if MOSFET5 10, are OFF, ZCD 90 generates a square-wave pulse. With the resulting ZCD information, software running in controller 130 will determine the zero cross and the frequency of the of AC mains.

In every cycle, dimmer MOSFETs 10, 20 are turned "ON" only if after the input capacitor of the link PSU 70 has charged sufficiently; a signal representing that voltage goes to the controller 130 MCU from a 130-volt detection -10 -unit 100, as an indicator of the state of the power supply 70.

A short-circuit protection block 40 generates a shut down signal (active high), whenever current through the MOSFET (and hence through the load) exceeds approximately 13A. The shut-down signal disables the MOSFET driving signals Ml, M2 so that MOSFETS 10, 20 are disabled. This circuit is very fast acting one; it is for protecting the devices against short circuit and is not for overloads. The level-shifted current sense amplifier 110, amplifies the voltage across current sense resistor 80. The current sensed is AC and the ADC used is unipolar. Suitable offset and gain are provided so that the maximum positive voltage is about 1.1 V and minimum voltage is about 0 V. Thus, the dimmer 5 contains all hardware necessary to monitor the current flowing to the load 60, and to convert the monitored current information to digital data. That hardware is utilised to implement an example embodiment of a method according to the invention. The example current-overload protection mechanism will now be described.

The current-sense resistor 80 converts the current passing through the MOSFETs 10, 20 into a proportional voltage. A low-cost operational amplifier 110 amplifies that voltage with suitable offset. The output of the amplifier 110 is set at about 0.55 V for no current and varies between OV and 1.1 V (for the maximum current to which the unit is being designed) . This signal is processed by the controller 130 to obtain the rms current value.

When the current exceeds the rated value, the duty cycle of MOSFET5 10, 20 is reduced by the controller 130, altering the timing of signals Ml, M2, to keep the rms

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-11 -current drawn through the load within 5 % of the maximum permissible load.

Potential errors in the system, include: -Analogue-digital conversion error (since the unit is working under highly noisy environmental conditions arising from the switching of the MOSFETs 10, 20, other switches, link power supply 70 etc. The error count can be as high as 4 bits in a 10-bit ADC.) -Offset error (op amp offset as well as the intentional bias to shift the negative current-sense signal) -Difference between average and rms at various duty cycles.

Those are addressed in this example as follows: The first two problems are solved by adopting over sampling. The current signal is sampled at the rate of one per millisecond (for 50Hz AC); i.e., the current is sampled at an interval equal to 1/20 of a period. The data on each of these twenty samples are accumulated and averaged for 64 cycles to get 20 averaged samples. The averaged samples represent the data for twenty equally spaced instants in a typical cycle. The averaged samples are accumulated and divided by 20 to get the current DC offset. From these averaged samples the rectified average data is obtained by the following method.

When a given averaged sample value is greater than the DC offset, the DC offset is subtracted from the averaged sample value; when it is less than the DC offset, the averaged sample value is subtracted from the DC offset; thus, effectively, the modulus of the difference between the -12 -averaged sample value and the DC offset is calculated. By taking the average of the rectified samples, the rectified average value for the whole cycle is calculated, which is the average value of the rectified current. The Rt'4S current is obtained by the multiplying the average value of the rectified current by a form factor. The form factor is a function of the duty cycle. The RMS of a full wave rectified sine wave is equal to 1.11 times its average value. When the conduction angle is less than 100% (full 180 degrees), the ratio of rms to average value increases as the conduction angle decreases. The following table gives the data in steps of 10 degrees: Rms value! Duty Average Cycle Value 10/180 4.90 3.46 2.82 2.43 2.16 1.96 1.81 1.68 1.57 1.48 1.40 1.33 1.27 1.22 1.17 1.14 1.12 1.11 The conduction angle information is calculated as -13 -GateDriveDuration *180 HalJPeriod The average rectified value computed above is multiplied by the corresponding factor to find the rms value.

Thus, the controller (NCU) 130 calculates the rms value of the current flowing through the MOSFET switches. The maximum expected rms current for a 1KW load = (1000/240) 4.167 Amps. If the computed rms current exceeds the rated value, the duty cycle is decreased by the controller 130, by changing the timing of signals Ml, M2. The new duty cycle will be computed as the old duty cycle * (4.167/measured rms) . The process of rms computation continues as a new datum is received every 1.28 sec (64 * 20 milli sec) . The initial reduction in duty cycle will be a step change, preferably a small step change, and the subsequent changes will be in finer steps, so that user does not see abrupt changes in the intensity of the light; the duty cycle is gradually fine tuned, until the rms current is within 5 % of the rated value.

Fig. 2 surnmarises the steps of operation that protect dimmer 5 from overload currents.

In a first step, the voltage signal 150 corresponding to the load current sensed in current sensor 80 is monitored.

If the signal 150 does not exceed the value corresponding to the rated current of MOSFET5 10, 20, the monitoring continues.

-14 -If the signal 150 exceeds the value corresponding to the rated current of MOSFETs 10, 20, the drive signals Ml and M2 to MOSFET driver 30 are altered to reduce the duty cycle of MOSFETs 10, 20.

If the peak value of signal 150 does not exceed 150% of the value corresponding to the rated current of MOSFETs 10, 20, the monitoring continues.

If the peak value of the signal 150 exceeds 150% of the value corresponding to the rated current of MOSFETs 10, 20, the drive signals Ml and M2 to MOSFET driver 30 are altered to switch off MOSFETs 10, 20 permanently. MOSFETS 10, 20 then remain switched off until the controller 130 user asks for a restart of the dimmer.

The reduction in duty cycle in an overload mode of operation is shown by the plots in Fig. 3. In Fig. 3(a), the current 200 is on for 6 ms of the 10 ms 50Hz AC half-cycle, i.e. a duty cycle of 60%. voltage plots 2lOa,b show the MOO signals that control the MOSFETs. There is an inversion in the driver. The MOSFETs are switched ON for a full negative half cycle when the body diode is forward biased and during the initial part of the other (positive) half cycle. These signals cause the current to be controlled. The overall rms current is 730mA, corresponding to a delivered power of about 170W. As discussed above, dimming is achieved by changing the delay between the start of each cycle and the switching high of the voltages 210a,b (when 210 a, b go high the corresponding MOSFET is turned OFF) Fig. 3(b) illustrates the behaviour in an overload situation. Note that here current 200 is rising much more rapidly than in Fig. 3(b). The overload protection causes -15 -the voltages 210a,b (corresponding to MOSFETS 10, 20) to be made high (resulting in the turn OFF of the MOSFET) sooner in the cycle than in the case shown in Fig. 3(a): the current 200 is cut after only 4.l2ms (a duty cycle of 41%) Although the (over-)load is 400W, the power actually delivered to the load is only about 270W (230V rms * 1.175A rms) Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

Where in the foregoing description, integers or

elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

-16 -Claims 1. A method of protecting a lighting installation, the lighting installation comprising (a) a lighting load, having a brightness, powered by an AC power supply, said AC power supply having a cycle of period t; and (b) a dimmer comprising a semiconductor switch operable to switch the current to the lighting load on and off; wherein in normal operation the switch switches the lighting load on and switches the lighting load off, and the brightness of the lighting load is altered by altering the switching on or the switching off so that the lighting load is on for a longer or shorter portion x of the period t; the method further comprising: monitoring a load current passing through the switch to the lighting load and, when the monitored current exceeds a first threshold value, reducing the portion x of the period t for which the lighting load is on, so that power dissipated in the switch is also reduced.
2. A method as claimed in claim 1, in which the portion x is reduced, when the monitored current exceeds the first threshold value, in steps that are sufficiently small for a user not to see abrupt changes in the intensity of the light from the lighting load.
3. A method as claimed in claim 1 or claim 2, in which the portion x of the period t for which the lighting load is on is reduced in response to the monitored current exceeding the first threshold value by increasing a delay between switching the lighting load off and switching the lighting load on.

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4. A method as claimed in claim 1 or claim 2, in which the portion x of the period t for which the lighting load is on is reduced in response to the monitored current exceeding the first threshold value by decreasing a delay between switching the lighting load on and switching the lighting load off.
5. A method as claimed in any preceding claim, further comprising switching the lighting load off permanently if the load current exceeds a second threshold value.
6. A method as claimed in claim 5, in which the second threshold value is for example 150% of the first threshold value.
7. A method as claimed in claim 5 or claim 6, wherein, when the second threshold value is exceeded, the lighting load is switched off until a reset signal is generated within, or supplied to, the dimmer.
8. A method as claimed in any preceding claim, in which the lighting load comprises a plurality of lights.
9. A dimmer for connection to a lighting load, said load having a brightness and powerable by an AC power supply, said AC power supply having a cycle of period t, said dimmer comprising: a semiconductor switch operable to switch the lighting load on and off; wherein in normal operation the switch is arranged to switch the lighting load on and to switch the lighting load off, the switch being operable to alter the brightness of the lighting load by altering the * switching on or the switching off so that the lighting load is on for a longer or shorter portion x of the period t; the dimmer further comprising a monitor arranged to monitor a load current passing through the switch to the lighting load and, when the monitored current exceeds a threshold value,S-18 -to reduce the portion x of the period t for which the lighting load is on, so that power dissipated in the switch is also reduced.
10. A dimmer as claimed in claim 9, in which the semiconductor switch is a field-effect transistor, a triac, a BJT or IGBT.
11. A dimmer as claimed in claim 9 or claim 10, further comprising a.controller that monitors the load current and controls the switching of the switch.
12. A lighting installation including a dimmer as claimed in any of claims 9 to 11.
13. A method of protecting a lighting installation substantially as herein described with reference to the accompanying drawings.
14. A dimmer substantially as herein described with reference to the accompanying drawings.