This is a large compact 84 freeform array built specifically
for the common 5630 and similar packages that produces an
asymmetric beam pattern for the use of an LPS (Low Pressure
Sodium) replacement system.
THE 84UP ARRAY OPTIC
developer forum | DIGITAL DIMMING CONTROL
which drives up the cost of the end product.
An active bleeder, as shown in Fig. 3, goes
some way toward mitigating efficiency loss
and temperature increase. Unlike the pas-
sive bleeder, the active circuit draws current
only when the LED current is other wise too
low to keep the triac turned on. This avoids
the burden of continuous power dissipa-
tion, which is imposed by the passive circuit,
thereby improving efficiency and reducing
thermal dissipation. On the other hand,
active bleeder circuits are difficult to con-
trol and require a relatively high number of
external components that increase overall
cost. Moreover, although an active bleeder
dissipates less power than a passive circuit,
the principle is the same: Deliberately dis-
sipating energy detracts from the efficiency
advantages that are a major reason for the
change to LED lighting.
Advantages of digital control
A more satisfactory approach is to recirculate bleeder current rather than dissipating
the energy as heat. This is not easy to implement in the analog domain. However, digital
technology permits complex dynamic control schemes that enable more intelligent use
of the energy required to keep legacy dimmers operating correctly.
In addition to eliminating flicker or
dropout when used with a conventional
triac dimmer, a suitable LED-driver circuit
must also manage AC-cycle inrush current
to avoid momentary overloads, minimize
audible noise resulting from interactions
bet ween the line cycle and internal magnetic
components, and meet regulatory standards
for power factor and electrical noise (EMI).
They must also be compatible with the widest possible range of dimmer types.
Fig. 4 shows a representative application
circuit for a non-isolated LED-driver circuit using the Dialog Semiconductor i W3688
controller. This device features a digital core
FIG. 3. An active bleeder circuit draws current only when needed but requires extra
components and is difficult to control.