LM3450 Datasheet, PDF (18/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
www.ti.com
PHASE DIMMER OPERATION
A simplified schematic of a phase dimmer is shown in Figure 25. An RC network consisting of R1, R2, and C1
delay the turn-on of the triac until the voltage on C1 reaches the trigger voltage of the diac. Increasing the
resistance of the potentiometer (wiper moving downward) increases the turn-on delay which decreases the on-
time or “conduction angle” of the triac (θ). This reduces the average power delivered to the load.
FORWARD PHASE DIMMER
R1_MAX 250 k:
BRIGHT
DIM
R2 3.3 k: ^TRIAC
DIAC
C1 100 nF
AC
MAINS
LOAD
Figure 25. Basic Forward Phase Dimmer
Phase dimmer voltage waveforms are shown in Figure 24.
Figure 24a shows the full sinusoid of the input voltage. Even when set to full brightness; few dimmers will provide
100% conduction angle.
Figure 24b shows a waveform from a forward phase dimmer. The off-time can be referred to as the firing angle
and is simply 180° – θ.
Figure 24c shows the waveform of a reverse phase dimmer (also called an electronic dimmer in the lighting
industry). These typically or more expensive, microcontroller based dimmers that use switching devices other
than triacs. Note that the conduction angle starts from the zero-crossing, and terminates some time later. This
method of control reduces the noise spike at the transition.
Any form of phase dimming modulates the incoming AC waveform by chopping part of the sinusoid, reducing the
average power to the load. These dimmers work very well with standard incandescent bulbs, but not with power
converters. A converter attempts to regulate the load in with presence of any input, effectively ignoring the phase
angle. To implement a dimmable converter, the angle must be sensed at the input, decoded and used to properly
control the LED current regulator.
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