THORLABS DC4104, Operation Manual

Page: 43 / 56

© 2018 Thorlabs
6 Appendix
41
6.6 Tutorial
A simple definition of a LED driver:
A LED driver is a circuit that can produce a sufficient for light emission current through a LED.
There are many ways to make such a circuit. A simple solution would be a voltage source, a
series resistor and the LED. This leads of course to a waste of energy and won't meet most re-
quirements, particularly for high power LEDs.
LED drivers which can be divided in three categories:
1. Linear LED driver
2. Switching LED driver
3. Combination of linear and switching LED driver
Linear Driver
A linear voltage driver can be used to generate a constant current. A shunt is in series with the
load (LED). The voltage drop across the shunt is proportional to the current and used as a
feedback signal to adjust the output voltage and therefore the current.
The main advantages are the low output current ripple and its EMC compatibility. There are no
switching elements in the circuit. For this reason it is advantageous for fluorescence micro-
scopy illumination. Thorlabs DC3100 and DC4100 LED drivers use this principle of operation.
The disadvantage is the low efficiency of the linear current control: linear regulators waste en-
ergy as they operate by dissipating excess power as heat. This loss depends on LED type and
current.
Switching Drivers
A switching driver with a constant current output is an efficient way to drive especially high
power LEDs.
The operating principle is based on an inductor in series with a LED load or a capacitor parallel
to a LED load. These reactances accumulate energy during the switch-on state, which is used
to supply a current through the LED.
Further, two types of switching converters exist: The Buck driver is a step-down DC-to-DC con-
verter, which converts a relatively high supply voltage into the lower LED forward voltage, while
the Boost driver is a step-up power converter with an DC output voltage greater than it's input
DC voltage. It is also possible to combine both methods to a Buck-Boost solution.
The main advantage of switching power supplies is their high efficiency of up to 95%; so the
dissipated as heat power loss is much less. The disadvantage is resulting from the switching
mode operation - a switching driver shows a ripple on the output current of up to 20%. This
ripple amplitude in conjunction with the switching operation requires careful considerations un-
der point of electromagnetic emissions and interferences (EMC).
Particularly the ripple on the output current might be a disadvantage for a number of micro-
scopy LED illumination applications.
The LEDD1B is a switching driver.
Combination of Linear and Switching Drivers
This method combines the advantages of both principles. A switching driver is used to adjust
the supply voltage to a value close to the LED forward bias. It is followed by a linear driver, gen-
erating constant current with low ripple. In such a design the heat dissipation is reduced to a
minimum. Thorlabs DC2100 LED Driver is based on this approach.
The disadvantage of this method is the use of more, often expensive components and the re-
quirement for more space than conventional constant current linear or switching drivers.