FOCUS
by
Geoff Archenhold
96 TECHNOLOGY
LED
average current through the
Figure 3: The inside PCB of a
inductor is no more than the cur-
typical LED driver high
rent through the LED.
voltage section.
In the boost configuration, all the
energy passes through the induc-
tor, as it is charged to ground
Driver efficiency can affect
and then discharged through the
reliability
LED, averaged by a capacitor.
The previous section discussed
In addition, the current through
different driver topologies and
the inductor is higher than the
their relative efficiencies however
current through the LED; losses
the overall driver efficiency is
increase with the square of the
important for system reliability as
current. In practice, in a circuit
shown by Table 4. Here a matrix
with comparable power output, a
of the overall driver efficiency
typical boost regulator will deliver
is created by looking at the ef-
around 85% efficiency versus
ficiency of the AC to DC and DC
around 95% for a typical buck
to DC stages of an LED driver.
converter.
As shown the overall efficiency
Unfortunately there is not one
of the LED driver significantly
best way for driving LEDs.
falls as both stages become less
However the majority of LED
efficient with anything less than
drivers use current mode switch-
90% efficiencies in both stages
ing techniques due to the high
resulting in significant losses in
efficiencies, good accuracy of
total driver efficiency eg <81%.
LED current regulation and wide
For LED driver PSUs with low
system flexibility.
wattages then the overall power
efficiency of a driver may be
New LED driver standard
managed however for larger
IEC 61347-2-13
wattages the inefficiencies are
From the 1st July 2009 all LED
Figure 4: The type of LED driver topologies used dependant
generated as heat within the
drivers need to adhere to the
upon input and output voltages of the system
driver and may become signifi-
IEC 61347-2-13 safety standard
cant as shown in table 5 for a
which specifies particular safety
40W LED driver.
requirements for electronic con-
trol gear for use on d.c. supplies
As shown in table 5, even if the
up to 250 V and a.c. supplies
AC to DC stage of an LED driver
up to 1 000 V at 50 Hz or 60 Hz
is at 90% efficiency and the DC
and at an output frequency which
to DC stage of the 40W driver
can deviate from the supply
is at 90% efficiency then more
frequency, associated with LED
than 7.6 Watts of heat is gener-
modules.
ated meaning that the system has
There are many improved
to be able to dissipate this heat
requirements that this new
to remain reliable in operation.
standards brings to LED drivers
The issue becomes significantly
however the most noticeable one
worse as the power of the LEDs
will be the change required in the
to be driven increases. For high
output connectors that connect
efficiency low wattage LED driv-
the driver to the LED fixture. The
ers (<20W) plastic cases can be
majority of LED drivers use the
used however for LED drivers
RJ45 connector originally de-
>20W it is recommended that
signed as network card connector
the driver case is constructed
in PC’s and these will need to be
from aluminium to enable
changed to double insulated type
improved heat dissipation and
connectors if the maximum out-
significantly improve reliability.
put voltage for a constant current
Obviously the design of an LED
driver is greater than 25Vdc.
driver becomes critical and the
It will be essential that all new
LED drivers meet this standard
so ask your suppliers to provide Table 3: The types of LED
you with the test reports to ensure driver topologies that are used
your installations are safe. in LED drivers.
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