Page 65 - Energize June 2022

TECHNICAL

temperature. The ratio between currents in
the channels gives the desired result.

Voltage control
Voltage control is used to control strip
lighting, where the LEDs are in parallel
across the supply lines. Varying the voltage
varies the current flowing in the LED. Typical
systems use a 10 V DC variable supply.

Current control
Figure 7: Inclusion of a green diode balances the chromaticity (LEDdynamics) Current control is generally applied to single
chips, and is activated by a remote signal.
HCL controls
Controls are a critical element in white-tuning systems. The most successful products have PWM control
clever algorithms for tuning from one CCT to another without changing light levels in the middle Changing the forward current flowing in a
ranges. These algorithms are built into the driver or an interface between the controller and the LED also changes the chromaticity or colour
driver. It is common to see a dimmer for each control signal: one for power and intensity level, of the LED slightly. PWM allows a change
and a second for colour. The two separate controls can also be combined into a user interface in intensity while maintaining colour. PWM
which has separate dials, sliders, or buttons for controlling colour and intensity independently. 4 turns the LEDs on and off at a fixed duty
There is a wide variety of control systems available, as well as chipsets, varying in quality cycle and frequency. Assuming the switching
and complexity. The simplest solutions use manual control, in the form of a wall-mounted or multiplexing is sufficiently fast, typically
panel or a remote control which allows adjustment of the colour of the light as well as its 200 Hz or greater, the human eye perceives
brightness independently. These do not mimic daylight but rather allow for personal preference. the LEDs to be on continuously. The PWM-
Tuneable systems are able to interface with other standard lighting controls such as DALI (Digital dimmed LED current is determined by the
Addressable Lighting Interface). following equation
Control over colour is implemented by varying the brightness of the different LED
components of the system. Changing the brightness of one component relative to the others I Dim = D * I LED
can change colour temperature, while lowering or raising brightness of all components changes
the overall brightness without changing colour. Achieving the right lighting condition involves Where I LED is the dimmed LED current, D
changing both the overall brightness intensity and relative brightness. is the on-duty cycle of the PWM dimming
Basically, there are two ways to achieve LED dimming: analog or linear voltage or current signal, and I is the constant current supplied
control and pulse-width modulation (PWM). Both dimming methods change the LEDs’ brightness to the switched LED string. Many LED driver
by controlling the average current through the LED string. Both can be implemented in either a ICs have a PWM dimming input pin that
switch-mode or linear LED driver. Figure 7 shows a two-string LED driver. Both LED strings can accepts a PWM dimming signal from a
be dimmed with either analog or PWM techniques. There are advantages and disadvantages microcontroller.
to each. In most applications, the dimming method is chosen based on the color mixing More complex control strategies are
performance requirement. Every channel supplies the dedicated LED string with a certain colour designed to manipulate the circadian rhythm
of room occupants. This basically means the
settings are programmed into the control
architecture of the building itself. Shifts in
colour temperature and light level can be pre-
set on a schedule or instigated by a manual
over-ride. This method is what is needed in
order to take full advantage of the benefits
of HCL.

Dim to warm
Dim to warm is the capability of reducing
the colour temperature of a light source in
proportion to its intensity. This mimics the
colour shift of incandescent lamps as they are
dimmed to a lower intensity (warmer colour
temperature at lower light levels, cooler
Figure 8: Tuneable white LED driver (Infineon) colour temperature at higher light levels).

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