16-07-2014, 09:59 AM
[attachment=65874]White Paper
Introduction – LED Lighting
A luminaire using HB-LEDs is more complex than a typical incandescent or fluorescent
fixture, and contains several elements that are specific to the LEDs. Firstly, the LEDs
themselves cannot be used directly, but must be mounted onto a circuit board that provides
physical support, interconnection and cooling. The complete assembly of LEDs onto the
circuit board is commonly referred to as a light engine, which is cooled by a heatsink.
Second, the LEDs must be supplied with a DC current that is accurately controlled to
provide the required light level without exceeding the LED rating. The current control
function is provided by a driver. Finally, in the case of LEDs operating from AC power, there
must be a power supply (PSU) that converts the AC into DC for the driver, and provides
safety isolation. Figure 1 shows an outline block diagram.
Driver combined with power supply
This configuration offers the advantage of simplicity for the luminaire designer, because the
power supply provides all the functions needed and the light engine does not require any
components other than the LEDs themselves. It is commonly used in applications such as
street lights or high bay lights where the main focus is on achieving a high light output with
good efficiency and high reliability, rather than a wide range of customer controls.
Pulse-width modulation dimming
In pulse-width modulation (PWM) dimming, the LEDs are driven with full amplitude pulses of
current and (as the name implies) the width of the pulses is varied to control the apparent
brightness. This type of dimming relies on the capability of the human eye to integrate the
average amount of light in the pulses. Provided the pulse rate is high enough, the eye does
not perceive the pulsing at all but only the overall average. PWM dimming may be
implemented in the power supply or in the driver.
For example, at 50% brightness the LEDs are driven with pulses where the ON-time and
OFF-time are equal, as shown in Figure 8. As mentioned above, the pulse rate must be high
enough so that the eye perceives only the average light intensity. Typically a pulse rate of
approximately 200Hz is high enough to achieve this objective, without any visible flickering.
This corresponds to a period of 5ms per pulse, as indicated
Phase control dimming of LEDs.
As shown in Figure 1, all LED luminaires contain a power supply and driver whose primary
function is to maintain the LED current at a constant level regardless of the AC input
voltage. Figure 13 shows a simplified block diagram of a typical constant-voltage power
supply.
AC input voltage control
Another method of dimming that can be used with many lighting technologies including
incandescent, metal halide and high pressure sodium is simply to reduce the AC input
voltage. This system is particularly suitable for wide area applications such as street lighting,
where it can be very effective in reducing energy consumption during off-peak hours. The
normal AC voltage can be reduced by 10% in the late evening and by 20% after midnight,
and then brought back to normal in the early morning. To minimize cost, a single voltage
controller can be used at a central point where power is distributed to a number of lights in
an area
Logic control
A simplified form of dimming control can be implemented where there are only a small
number of fixed brightness levels (two, or possibly three), selected by input pins. As an
example, this type of control can be used with a motion sensor so that the light output is at a
low level until the motion sensor activates, when the light switches to full brightness.
Again, the dimming of the LEDs themselves can be achieved by either analog or PWM
control. Since this type of dimming is intended to be as simple as possible, the most likely
approach is to simply provide two (or three) output current set-points from the driver
selected by one or more logic inputs, as shown in Figure 20.
Summary
Lighting technology using HB-LEDs can fully support dimming control through the AC wiring,
through an analog or digital input or through a wireless mesh network. The virtually
instantaneous response of LEDs to changes in drive current allows great flexibility in the
end application and will support ongoing and future initiatives to reduce the amount of
energy used for lighting. As a leader in LED power supplies, AEG Power Solutions offers
products with or without integrated LED drivers that are compatible with the various forms of
dimming discussed in this White Paper
Introduction – LED Lighting
A luminaire using HB-LEDs is more complex than a typical incandescent or fluorescent
fixture, and contains several elements that are specific to the LEDs. Firstly, the LEDs
themselves cannot be used directly, but must be mounted onto a circuit board that provides
physical support, interconnection and cooling. The complete assembly of LEDs onto the
circuit board is commonly referred to as a light engine, which is cooled by a heatsink.
Second, the LEDs must be supplied with a DC current that is accurately controlled to
provide the required light level without exceeding the LED rating. The current control
function is provided by a driver. Finally, in the case of LEDs operating from AC power, there
must be a power supply (PSU) that converts the AC into DC for the driver, and provides
safety isolation. Figure 1 shows an outline block diagram.
Driver combined with power supply
This configuration offers the advantage of simplicity for the luminaire designer, because the
power supply provides all the functions needed and the light engine does not require any
components other than the LEDs themselves. It is commonly used in applications such as
street lights or high bay lights where the main focus is on achieving a high light output with
good efficiency and high reliability, rather than a wide range of customer controls.
Pulse-width modulation dimming
In pulse-width modulation (PWM) dimming, the LEDs are driven with full amplitude pulses of
current and (as the name implies) the width of the pulses is varied to control the apparent
brightness. This type of dimming relies on the capability of the human eye to integrate the
average amount of light in the pulses. Provided the pulse rate is high enough, the eye does
not perceive the pulsing at all but only the overall average. PWM dimming may be
implemented in the power supply or in the driver.
For example, at 50% brightness the LEDs are driven with pulses where the ON-time and
OFF-time are equal, as shown in Figure 8. As mentioned above, the pulse rate must be high
enough so that the eye perceives only the average light intensity. Typically a pulse rate of
approximately 200Hz is high enough to achieve this objective, without any visible flickering.
This corresponds to a period of 5ms per pulse, as indicated
Phase control dimming of LEDs.
As shown in Figure 1, all LED luminaires contain a power supply and driver whose primary
function is to maintain the LED current at a constant level regardless of the AC input
voltage. Figure 13 shows a simplified block diagram of a typical constant-voltage power
supply.
AC input voltage control
Another method of dimming that can be used with many lighting technologies including
incandescent, metal halide and high pressure sodium is simply to reduce the AC input
voltage. This system is particularly suitable for wide area applications such as street lighting,
where it can be very effective in reducing energy consumption during off-peak hours. The
normal AC voltage can be reduced by 10% in the late evening and by 20% after midnight,
and then brought back to normal in the early morning. To minimize cost, a single voltage
controller can be used at a central point where power is distributed to a number of lights in
an area
Logic control
A simplified form of dimming control can be implemented where there are only a small
number of fixed brightness levels (two, or possibly three), selected by input pins. As an
example, this type of control can be used with a motion sensor so that the light output is at a
low level until the motion sensor activates, when the light switches to full brightness.
Again, the dimming of the LEDs themselves can be achieved by either analog or PWM
control. Since this type of dimming is intended to be as simple as possible, the most likely
approach is to simply provide two (or three) output current set-points from the driver
selected by one or more logic inputs, as shown in Figure 20.
Summary
Lighting technology using HB-LEDs can fully support dimming control through the AC wiring,
through an analog or digital input or through a wireless mesh network. The virtually
instantaneous response of LEDs to changes in drive current allows great flexibility in the
end application and will support ongoing and future initiatives to reduce the amount of
energy used for lighting. As a leader in LED power supplies, AEG Power Solutions offers
products with or without integrated LED drivers that are compatible with the various forms of
dimming discussed in this White Paper