VERSATILE IESNA LM-79
HIGH-SPEED MOVING MIRROR TYPE C
GONIOPHOTOMETER SERIES 6400T
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developer forum | DIGITAL DIMMING CONTROL
and allows a low-cost solution that is compatible with a wide range
of triac dimmers.
As the diagram shows, the circuit uses only one external MOSFET. This is possible because the driver IC uses the same switch
to keep the triac dimmer circuits working properly and to provide
power for the control circuit itself. This design eliminates the need
for a secondary winding on the main magnetic core, which is usually needed to power the controller. This approach gives designers
the option to use low-cost, off-the-shelf inductors for non-isolated
applications, or to use a standard flyback transformer if the application requires isolation. Such an architecture helps reduce component count, saves energy, and minimizes heat dissipation, thereby
simplifying thermal management.
The IC’s digital circuitry monitors the relevant voltages and currents and permits the controller to dynamically modulate the main
power MOSFET to achieve the required level of dimming and keep
the triac turned on even when the demand from the LED load is very
low. Any additional current needed to keep the triac operating that
is not already being drawn by the main power-conversion stage is
used internally rather than being converted into heat.
The IC also integrates intelligent features that are capable of making dynamic impedance adjustments depending on the characteristics of the dimmer. The design allows the device to work with almost
any standard triac dimmer and allows LED brightness to be dimmed
down to 1% of maximum. The low dimming level enables a much
stronger level of perceived dimming by humans compared to earlier driver circuits that were unable to support dimming below 5%
or even 10%.
When no dimming is to be applied, the main power converter that
delivers current to the LED load is operated in quasi-resonant mode
to provide high power efficiency and low EMI. Power factor is also
optimized to enhance efficiency and minimize current harmonic
distortion on the AC line as well as to meet regulatory requirements
around the world.
In dimming or no-dimming modes, the
driver in the near future will be required to
comply to emerging guidelines governing
the flicker produced by lighting and SSL in
particular. These include the IEEE 1789 recommendations, published earlier this year.
Other bodies such as Energy Star are considering introducing flicker criteria. Among
the proposals of IEEE 1789 is a limit on maximum acceptable flicker (see p. 35 for more
End users of energy-saving lighting expect
a completely seamless transition from existing lighting technologies to LED replacements. This requires replacement bulbs to
meet current industry-standard form factors, and to operate faultlessly with existing
triac-based dimmers down to low dimming
levels and within minimum flicker guidelines. At the same time, the new technology must be offered at a
very competitive price, while also maximizing energy efficiency and
reliability. Digital driver-control technology offers a superior solution compared to conventional analog circuitry and resistive bleeders, enabling manufacturers to meet such high expectations.
FIG. 4. Rectifier,
using the i W3688