THORLABS DC2100, Руководство по эксплуатации

Страница: 37 / 49

35 DC2100
5 Maintenance and Service
6
Application Note
This chapter contains the background knowledge about the LED driver and pulse
width modulation.
6.1
LED Driver
A simple definition of a LED driver:
A LED driver is a circuit that can produce enough current through a LED to get light.
There are many ways to realize such a circuit. The most simple solution would be a
voltage source, a series resistor and the LED. This is of course not economic and
will not meet most requirements especially for high power LEDs.
There are a wide range of solutions for 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 of the shunt is proportional to the
current and is measured and provides 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. Therefore it is applicable for
fluorescence microscopy illumination. Due to the adjustment of the LED current with
a linear driver a high dissipation loss can be generated. This results in heat and
depends on LED type and current. The efficiency can be quite low.
Switching Drivers
A switching driver with a constant current output is an efficient LED driver especially
for high power LEDs.
The working principle is based on an inductor in series with a LED load or a
capacitor parallel to a LED load. The inductor or capacitor, respectively,
accumulates the energy during the switch on state. This energy is used to supply a
current through the LED. There exists different concepts like the Buck driver which
converts a higher supply voltage into a lower LED forward voltage and the Boost
driver which converts a lower supply voltage into a higher LED forward voltage. It is
also possible to combine both methods to a Buck- Boost- solution.
The main advantage is the high efficiency. Furthermore, a high supply voltage range
can be realized without a high dissipation loss. Compared to linear drivers a
switching driver shows a not only a quite large ripple on the output current which can
be as high as 20%. They also need careful considerations regarding EMC. Because
of these disadvantages this method is not suitable for microscopy LED illumination.