GB2595922A

GB2595922A - Dimmer switches

Info

Publication number: GB2595922A
Application number: GB2008940.5A
Authority: GB
Inventors: Christopher Lewin Mark; Christopher Lambert Thomas
Original assignee: ZANO CONTROLS Ltd
Current assignee: ZANO CONTROLS Ltd
Priority date: 2020-06-12
Filing date: 2020-06-12
Publication date: 2021-12-15
Also published as: GB202008940D0

Abstract

The maximum duty cycle of a semiconductor switch 2 in a trailing edge LED dimmer is set by a processor 4 which measures a supply line voltage after each zero crossing of the mains power cycle and reduces the maximum duty cycle by a fixed percentage if that voltage has dropped by a predetermined amount. It may also restore the maximum duty cycle by fixed increments while the voltage is maintained at the predetermined amount. The processor also monitors the semiconductor switch 2 for any excessive ON voltage and responds to any detected fault condition to shut down the switch or further adjust the duty cycle. A voltage divider 10 scales down the voltage on the supply line so that it may be measured by the processor 4.

Description

Dimmer switches

Technical Field

[1] The present invention relates to trailing edge dimmer switches suitable for LED lights.

Prior Art

[2] LED dimmers typically utilise a high voltage MOSFET, an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn Off SCR) as a switching element to allow for the dimmer to turn on at zero volts of the AC waveform These LED dimmers are typically referred to as Trailing Edge Dimmers as they vary the switch off point (trailing edge) whereas older TRIAC or SCR dimmers are known as Leading Edge Dimmers as they vary the switch on point (leading edge).

[3] Such dimmer switches are described in GB2276286A TELEFUNKEN MICROELECTRON 1994-09-21 and GB251157 lA ZANO 2014-09-12 Technical problems [4] The main problem with these LED trailing edge dimmer designs is that the dimmer requires power to maintain its switched-on state, whereas, leading edge dimmers do not due to the latching state of their TRIACS / SCRs. During the switched-on state of a dimmer there is no voltage available as the two terminals are connected together. Therefore, in order for an LED dimmer to function properly, it requires the ability to store power during the on time of the light [5] LED dimmers will therefore collect and store power during the off-time of the light, but they have to guarantee that the amount of power stored during the off-time is enough to offset the power used during the on-time.

[6] This is done by using a control circuit to limit the maximum on-time for the LED dimmer, meaning, that the maximum light output available whilst using an LED dimmer is less than 100% [7] Furthermore, due to the possible variations in load, they must account for the worst case scenario leading to a maximum on time of only around 60% [8] In addition, it is desirable that the control circuit in the dimmer is able to monitor the performance of the switching element to detect when it is overloaded, too hot or there is a fault with its driver.

Solution of the Present Invention [9] The present invention is defined in the appended claims [10] Embodiments of the present invention recognise that, in order for a LED dimmer to achieve the maximum light output possible, it needs to monitor the amount of power it has stored and vary its maximum on time (duty cycle) accordingly.

[11] The dimmer of the present invention does so by measuring the voltage reached at the end of the off-time, where it would be at its highest. As the duty cycle approaches 100%, the voltage would begin to drop, the dimmer can then decrease the duty cycle from that point and confirm it has retained its peak voltage [12] In doing so, the dimmer is able to vary its maximum duty cycle for any load connected to it and reach over 90% light output when compared to the light output of the same load utilising a switch [13] In a preferred embodiment the correct operation of the semiconductor switching element, is monitored by a circuit and excessive ON voltage is reported to the control processor.

Description of the Drawings

[14] In order that the invention can be well understood some embodiments thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: [15] Figurel shows a block diagram of one embodiment of a dimmer switch.

Description of an Embodiment

[16] A dimmer switch has a switching MOSFET or another semiconductor switching element 2. An associated processor 4 controls the maximum duty cycle available and therefore the maximum brightness of a connected LED lamp 5. A power supply 8 drops the mains voltage to a level of about 15 volts and charges a capacitor 6 which stores power during the off time of the duty cycle [17] A voltage divider 10 scales down the voltage on the supply line so that at the maximum stored voltage (typically 15 V) the divider shows 3V. This facilitates the measurement of the supply line voltage by the processor 4 [18] The control circuit or microprocessor 4 monitors the ON voltage of the switching MOSFET 2. A circuit 9 senses the instantaneous voltage across the switch 2, which under normal circumstances, is in the order of a few 10's of millivolts. The following fault conditions can cause this voltage to rise: a. The dimmer output is short circuited or overloaded.

b. The semiconductor switch is too hot.

c. The supply voltage inside the dimmer is too low and the switch is not being turned on correctly.

[19] Detection of this condition can be used to curtail the current cycle, adjust the maximum duty cycle as described in paragraph [11] above, or actually shut the dimmer down, until it is reset either manually or after a fixed period of time.

[20] The maximum duty cycle is set by the processor 4 in response to the measured supply line voltage and the condition of the MOSFET switch as described in paragraph [18].

[21] The maximum duty cycle of the trailing edge LED dimmer is set by the processor 4 in response to measurements of the supply line voltage after each zero crossing of the mains power cycle and reduces the maximum duty cycle by a fixed percentage if that voltage has dropped by a predetermined amount and restores the maximum duty cycle by fixed increments while the voltage is maintained at the predetermined amount [22] The automatic maximum duty cycle is calculated by: * When the dimmer is turned on it will begin storing power, obtained via its 2 wire connection, into the supply line's capacitor 6, through power supply 8.

* After a set delay, around 50ms to allow the supply capacitors to fully charge, the processor will take a measurement of the voltage on the supply line, typically 12-15v. To do so, it uses the voltage divider to scale the highest expected voltage to 3v so that it may accurately measure it.

* The dimmer will now go into its normal operation mode and begin to detect the zero cross points of the mains power cycle.

* On each detection of a zero cross the processor will again read the voltage on the supply line. If the voltage has dropped more than 100mv the maximum dim level will be lowered by 1%.

* The maximum level will continue to be reduced until the supply line returns to its original voltage level.

Claims

Claims 1 A trailing edge dimmer switch for an LED light comprising a semiconductor switching element (2) that may be switched on at a zero crossing of a half cycle of an AC power supply and switched off part way through that half cycle, and a processor (4) powered using energy stored in a capacitor (6) in a supply line for the processor, during a period when the switch is off in a proceeding half cycle; characterised in that the processor sets the maximum duty cycle in response to measurements of the supply line voltage after each zero crossing of the mains power cycle and reduces the maximum duty cycle by a fixed percentage if that voltage has dropped by a predetermined amount.
2. A switch as claimed in claim 1, wherein a voltage divider is used to scale the voltage on the supply line and the processor measures the reduced voltage.
3 A switch as claimed in claim I or 2, further comprising means for monitoring the condition of the switching element, so that under fault conditions the dimmer can be shutdown or have its duty cycle adjusted.