LED lighting design full revelation of the drive selection and design skills

LED Driver General requirements
The arrangement of LED street light  determines the basic drive requirements . LED driver is the main function of the operating conditions in a certain range of the current flowing through the LED restrictions , regardless of the input and output voltage changes . The basic work led driver circuit shown in Figure 1 , where the so-called "quarantine " means the AC line voltage and LED ( ie input and output ) between no physical electrical connection , the most common is the use of electrical isolation transformer , the " non-isolated " is not used to electrically isolate high-frequency transformer .


It is worth mentioning that, in the LED lighting design , AC-DC power conversion circuit with constant current drive two parts can adopt different configurations:
( 1 ) integral (integral) configuration, that the two together, are located within the lighting , the advantages of this configuration include optimizing energy efficiency and simplify the installation ;
( 2 ) distributed (distributed) configuration , that is, they exist alone, this configuration simplifies security considerations, and increase flexibility.
How to choose the LED drive
Today in the market typical LED driver includes two categories, namely linear drives and switch drivers ; probably scope shown in Figure 2 as current greater than 500mA high-current applications use switching regulators, since the linear drive is limited to its own structural reasons, can not to provide such a large current ; while current is less than 200mA low-current applications, usually separated using a linear regulator or regulator ; while in the middle of 200 to 500mA current applications , either using a linear regulator, also switching regulator can be used .


At low current LED applications, the resistance-type drive despite its low cost and simple structure, but the driver in low voltage conditions, the forward current is low , it will cause the LED brightness is insufficient , and in load dump transient conditions , etc. , LED may be damaged ; and resistance is energy-consuming components, energy efficiency is low throughout the program , shown in Figure 3 .


For example, in using the DC-DC power supply led lighting applications, you can use the LED drive a resistive, linear regulator and switching regulator , etc., the basic application is shown in Figure 4 .



Resistance-type drive mode , adjust the LED current sense resistor in series to control LED forward current, this drive easy to design , low cost , and no electromagnetic compatibility (EMC) problems , weaknesses is dependent on the voltage , need to be screened (binning) LED, and can lower efficiency .


Linear regulator is equally easy to design and no EMC problems , but also supports the current steady flow and overcurrent protection (fold back), and provides external current set point , less power dissipation problem lies , and is always higher than the input voltage to the forward voltage , and can efficiency is not high. Switching regulator control via PWM control module constantly switch (FET) on and off , and then control the flow of current .
Switching regulator with higher energy efficiency , voltage-independent , and can control the brightness , less is the relatively high cost , complexity is higher, and the presence of electromagnetic interference (EMI) problems . LED DC-DC switching regulators common topologies including buck (Buck), Step-Up (Boost), Buck - Boost (Buck-Boost) or single- ended primary inductance converter (SEPic) and other different types.
Among them, under all operating conditions the minimum input voltage is greater than the maximum LED string voltage step-down structure , such as using 24 Vdc drive 6 series LED; contrast, under all operating conditions the maximum input voltage is less than the minimum output voltage using boost structures, such as use of 12 Vdc drive 6 series LED; while the input voltage and output voltage range can be used when there is overlap buck - boost or SEPIC structure, such as use of 12 Vdc or 12 Vac drive four series LED However, the cost of this structure and the energy efficiency of the least desirable .
LED Driver Standard
LED street light driver itself is also evolving , with emphasis on further improving energy efficiency , increasing functionality and power density. U.S. " Energy Star" solid-state lighting norms that are energy-efficient lighting -class constraints, including power factor involved , including the specific product requirements. The EU IEC 61347-2-13 (5/2006) Standard for DC or AC powered LED module requirements include:

In addition , ANSI C82.xxx LED drive specification are still being developed . In terms of security , the need to comply with UL, CSA and other standards, such as UL1310 (Class 2), UL 60950, UL1012. Additionally , LED lighting design also involves the product life cycle and reliability issues.

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