GB2511641A

GB2511641A - Dimmer switches suitable for LED lamps

Info

Publication number: GB2511641A
Application number: GB1403673.5A
Authority: GB
Inventors: Terence Giles
Original assignee: ZANO CONTROLS Ltd
Current assignee: ZANO CONTROLS Ltd
Priority date: 2013-03-08
Filing date: 2014-03-03
Publication date: 2014-09-10
Also published as: GB201403673D0; WO2014135875A1; GB201304237D0; GB2511571A

Abstract

A two wire dimmer switch for an LED lamp 8 provides a bridge rectifier 10 for connection to an AC supply and a semiconductor switch 20 that can be turned on at a zero crossing of a half cycle of the supply and switched off part way through that half cycle under a brightness control 12. The duty cycle of the semiconductor switch therefore sets the brightness of the lamp. A programmable chip 40 acts as a control circuit and uses power stored during the period when the semiconductor switch was off in a preceding half cycle.

Description

Dimmer Switches suitable for LED lamps

Technical Field

[0001] The present invention relates to dimmer switches suitable for LED lamps.

While tungsten type lamps behave as a linear resistance, this is not the case for LED lamps due to the nature of their internal power supplies. An LED power supply is needed to reduce the mains voltage to a few volts and its current flow is very complex. Typically, the LED will take a high current for a short period at the start of the mains half cycle and virtually no current for the remainder of the cycle. Therefore, prior art designs such as disclosed in GB 2276286 A (NUTZ KARL-DIETHER) 21/09/1 994 are

not suitable.

Background Art

[0002] Dimmer switches used to control tungsten light bulbs have been available for more than 30 years. They have been designed as 2 wire devices so that they can replace simple on off switches and normally employ TRIACS or SCR5 as the switching element.

[0003] The main problem with any 2 wire dimmer design is that when it is switched ON the two terminals are connected together, so there is no voltage available to power any control electronics. Fortunately, a characteristic of a TRIAC or 8CR is that once it has been turned on it stays on until the current through it drops to zero. This leads to the prior art simple dimmer design that delays the turn on of the TRIAC, from the start of a mains cycle, the later the turn on, the less light output from the lamp.

The electronics to provide the delay will have power available, because when they are operating, the TRIAC switch is off. As soon as the switch is triggered the supply voltage will disappear but the TRIAC will latch on until the end of the cycle.

[0004] However the lamp is being switched on when the voltage of the AC waveform is well above zero leading to a high current transient causing electromagnetic interference and other radio interference.

[0005] This leads to the following disadvantages:-Expensive inductors are needed to filter the transients.

* TRIAC dimmers make a buzzing noise due to movement or magnetostrictive effects in the filter cores.

* Even with filtering it is difficult to meet the modern electromagnetic compatibility (EMC) regulations.

* The dimmer will not work with all LED or other type of electronic lamps because their power supplies are synchronised to work off the mains voltage crossings, a TRIAC dimmer suppresses the zero-crossings, as it only comes on pad way through the cycle.

[0006] In the NUTZ design, which is an example of a trailing edge or reverse phase dimmer circuit, the power for the control logic is provided at the start of the cycle. This is unsuitable for an LED lamp because that is when the LED needs most power.

[0007] The present invention seeks to solve the technical problems of noise, interference and requiring a separate power supply for a dimmer switch suitable for an LED lamp as well as other types of lamp.

Disclosure of Invention

[0008] The present invention recognises that, in order for a dimmer to work with LED lamps, it needs to switch on at the start of a mains cycle, i.e. just after the zero voltage crossing. The brightness of the lamp can then be determined by how far into a half cycle the switching element is turned off.

An AC mains power supply can be rectified using a standard bridge rectifier to provide two zero crossing in each full mains cycle. Therefore in this dimmer the brightness is determined by the duty cycle of the supply. A TRIAC or SCR is unsuitable for use as a switching element, because it cannot be turned off part way through a voltage half cycle, only going off once the current has stopped flowing at the end of a half cycle.

[0009] The dimmer of the present invention preferably uses a single high voltage MOSFET as the switching element preceded by a bridge rectifier, so that it will work on both the positive and negative halves of the mains cycle. An IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Tum Off SCR) could also be used as the semiconductor switch. A high voltage bipolar transistor could be used, but is inconvenient because of the high base drive current needed to hold the transistor ON, which is needed when there is no supply voltage available.

[0010] The requirement to turn the switch ON at the zero voltage point of the mains cycle, means that some power must be stored from the end of a previous half cycle, when the switch was open, to power a control circuit.

[0011] In order to have power available for the control electronics there must always be a period when the switch is off, this means in practice that the maximum duty cycle is likely to be in the region of 80-90%. Fortunately, at maximum duty cycle, the lamp off period is at the point on the sine wave when the voltage is lowest and so has negligible effect on the maximum lamp brightness.

[0012] The present invention accordingly provides atwo wire dimmer switch for an LED lamp, comprising a bridge rectifier for connection to an AC power supply, and a semiconductor switch connected to a DC side of the bridge rectifier to control power supply to the LED lamp, such that the semiconductor switch is turned on at a zero crossing of a half cycle and switched off part way through that half cycle, and a control circuit containing a control loop to monitor an input derived from the power line to output an accurate zero crossing point, the control circuit using power stored during a period when the switch is off in a preceding half cycle, the brightness of the lamp being varied by the control circuit adjusting a duty cycle of the supply to the lamp.

[0013] Other preferred features of the invention are set out in the appended claims.

Brief Description of Drawings

[0014] In order that the invention may be well understood, an embodiment thereof will now be described by way of example only with reference to the accompanying diagrammatic drawings in which: [0015] Figure 1 shows an AC supply waveform indicating the parts of the cycle in which the semiconductor switch in the dimmer of the present invention is ON and OFF; and [0016] Figure 2 is a circuit diagram of an embodiment of the dimmer of the present invention.

Mode(s) for Carrying Out the Invention [0017] A two wire dimmer switch circuit 2 is shown in Figure 2. The two connections to the dimmer are shown as 4 and 6 on the right hand side of the Figure and these are connected through the lamp 8 to the Live and Neutral of an AC mains power supply. The power supply is connected to a bridge rectifier lOso that the lamp is supplied with a waveform as shown in Figure 1 which has a duty cycle depending on the brightness set by the user with a brightness control 12 in order to move the point 14 at which a semiconductor switch 20 turns off.

[0018] With a 50HZ mains supply the lamp is being switched on and off at 100Hz but this does not produce any visible flicker. The duty cycle reflects the period for which the lamp is on during each cycle and varying that is perceived as adjusting the brightness of the light emanating from the lamp 8.

[0019] The main switching element is a MOSFET 20. The bridge rectifier 10 ensures that lamp current will flow between 4 and 6 independently of the polarity of the mains voltage. A transient voltage suppressor 16 is provided to remove spikes in the bridge rectifier output [0020] When the switch 20 is OFF current flows through resistor 22 and diode 24 to charge the reservoir capacitor 26 to provide a power store. When capacitor 26 reaches a target voltage, typically 15V, it will be clamped by a zener diode 28. A lower supply voltage, typically 3.3V, is provided by a linear ultra-low power regulator 32 and this is used by a programmable control processor 40 and other circuitry. The control processor 40 is a customised low power CMOS integrated circuit that contains both analogue and digital circuitry.

[0021] A switching control output from the control processor 40 switches between 0 and 3.3V and this is converted toO and 15V by a MOSFET driver chip 42. The output current is limited by resistor 44 before it reaches a gate of the switching MOSFET or IGBT, 20. Diode 46 clamps the gate of the switching element 20 to stop it exceeding iSV or dropping below 0.7V negative. An 9CR 48 is triggered if the voltage across a current sensing resistor 50 exceeds about 0.7V which shorts the drive voltage to the MOSFET, turning it OFF so protecting the circuitry from a current overload.

[0022] A transistor 52 connected to the output of the bridge rectifier provides a synchronising pulse to ensure the start of the ON period occurs at the zero crossing of the mains. This is important to minimise electromagnetic and radio interference. The controller chip 40 has an internal algorithm that averages the fall and rise times of its ZX input from transistor 52 to provide an accurate zero crossing point and therefore control the switching element 20 to turn on at the zero crossing point. This provides a self-adjusting zero crossing detector.

[0023] As power for the circuit is only available when the switch is OFF, the duty cycle of the switching waveform is very critical, the lower the duty cycle the more power is available, but the less light output. The improved dimmer uses a control loop that monitors the supply voltage with resistors 54 and 56. If the voltage drops below a pre-set value, it automatically reduces the duty cycle. In this way the maximum light output is achieved whilst maintaining sufficient supply voltage to keep the switching MOSFET hard ON. If this voltage drops too low, the MOSFET will go into its linear region and overheat. The switch therefore automatically adjusts the dimmer duty cycle to achieve maximum lamp brightness when the user brightness control is set to maximum. The MOSFET transistor 20 acts as a nearly perfect switch and when it is ON it is saturated to a very low voltage. This ensures that there is very little power lost and so no heat sink is required, allowing a very compact switch replacement package.

[0024] The duty cycle and hence lamp brightness is set by potentiometer 12 which provides the user brightness control. The supply to the top of the potentiometer is switched ON and OFF by an SW output of the control chip 40 to reduce power consumption. The controller chip 40 can be programmed with the control potentiometer to set a minimum value of lamp brightness.

[0025] As an alternative to using a potentiometer to provide the user brightness control 12, it would also be possible to use encoder switches, up/down buttons, a 0-1OV control signal or provide for remote control over a radio link.

[0026] It will be appreciated that switching ON at the zero voltage crossing minimises electromagnetic and acoustic noise which have been significant technical problem with prior art switches. Therefore although the described dimmer switch is particularly suitable for an LED lamp which requires switching at the zero crossing, it is also possible to use such a dimmer switch with other types of lamp. The design will also work with low power LEDs and those employing simple capacitive dropper power supplies.

[0027] The design will work with MOSFET, GTO 5CR or IGBT transistors as the principal switching element 20.

Claims

Claims 1. Atwo wire dimmer switch (2) for an LED lamp, comprises a bridge rectifier (10) for connection to an AC power supply, and a semiconductor switch (20) connected to a DC side of the bridge rectifier to control power supply to the LED lamp, such that the semiconductor switch is turned on at a zero crossing of a half cycle and switched off part way through that half cycle, and a control circuit (12, 40) containing a control loop to monitor an input derived from the power line to output an accurate zero crossing point, the control circuit using power stored during a period when the switch is off in a preceding half cycle, the brightness of the lamp being varied by the control circuit adjusting a duty cycle of the supply to the lamp.
2. A dimmer switch as claimed in claim 1, wherein the semiconductor switch (20) is a high voltage MOSFET, an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn Off SCR) transistor.
3. A dimmer switch as claimed in claim 1 or 2, wherein the control circuit (40) has an internal algorithm that averages the fall and rise times of an input derived from the power line to provide an accurate zero crossing point and therefore control the switch (20) to turn on at the zero crossing point.
4. A two wire dimmer switch (2) for an LED lamp substantially as herein described with reference to the accompanying drawings.