AB171 LUXEON SunPlus 35 Line for Horticulture Application Brief 20161110 ©2016 Lumileds Holding B.V. All rights reserved.

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7.2 Active Control LED Current Driver

There are two types of active control LED current drivers:

– Linear regulated power supply

– Switch mode power supply

The main difference is how the power supply is being regulated. In linear regulated power supply, the transistor connected

to a load is used to control the output voltage or current. In order to do this, the transistor must operate in the ohmic or

linear region as shown in Figure 6. Since there is current flowing through the transistor (operating in ohmic region), the

transistor behaves like a variable resistor and generates heat.

In a switched mode power supply, as the name implies, the transistor is used as a switch to either switch in or out electrical

storage elements such as inductor (current source) or capacitor (voltage source) to a load. In order to achieve this, the

transistor must be operating in the saturation region as shown in Figure 6. In the “ON” or close switch, there is hardly

any resistance through the transistor and hence does not generate any heat when a current flows through it. In “OFF” or

open switch, no current is flowing through the transistor. This type of power supply has high power efficiency than the

linear regulated power supply. However due to the switching of the circuit, it generates electromagnetic interference (EMI)

which must be taken into account and minimized during the design stage. There are several switch mode power supply

configurations such as buck, boost and buck-boost convertor.

Figure 6. Typical transistor operation regions. Ohmic or linear region is shaded in yellow

while the saturation or switching region is shaded green.

A summary of the major differences between these two active control LED current drivers is given in Table 3 below.

Table 3. Comparison of a typical linear regulated versus switch mode power supply.

LINEAR

SWITCH MODE

Cost

Cheap

Expensive

EMC

None

Potential

Circuitry

Simple

Complicated

Power Efficiency

Low (~50...~70%)

High (~75%...~95%)

Size and Weight

Small and light

Notes for Table 3:

1. If magnetics are used to step down the voltage before the regulator.