Light-emitting diode (LED) Following in the small size of the screen backlighting applications such as portable products get a lot of use , along with its luminescent properties further upgrading and cost optimization in recent years, began to enter the general lighting applications such as architectural lighting , marking cards, landscape lighting , retail , lights , street lighting and residential lighting , its application can be described as the ascendant.On the other hand , LED street lighting design engineers to include the Chinese engineering community , including a challenge , not only because the LED lighting applications very wide range of applications, power level , you can use the drive power supply topologies such as types and , also varies . Engineers have an urgent need to systematically study and learn more about the basics of LED lighting design . For this reason, ON Semiconductor's director of product applications , Mr. Bernie Weir recent relevant training materials specifically written for engineers to impart basic knowledge related to the design , which relates to the general requirements for LED driver , power supply topologies , power factor correction, power conversion efficiency and driver standards, as well as other issues such as reliability and service life , better design to facilitate their entry and improve to better serve the LED lighting market. Due to space limitations , this article is a summary description of the training materials .One , LED drivers General requirementsLED drive faces many challenges, such as the forward voltage with temperature , the current changes, the different components , different batches , different suppliers of the LED forward voltage will vary ; addition, LED 's "color point " will be with the current, and temperature changes drift.In addition, applications often use multiple pieces of LED, which relates to the arrangement of multiple LED problem . Various arrangements , the preferred drive series single-string LED, because in this way regardless of how the changes in forward voltage , the output voltage (Vout) how to " drift " are providing excellent current matching performance. Of course, users can also be used in parallel , series - parallel combination and cross-connect other arrangement for the need for " mutual matching " LED forward voltage applications, and get other benefits . If the cross-connect , if one LED open circuit due to a fault , the circuit has only one LED drive current will be doubled, so as to minimize the impact on the entire circuit .Common LED arrangement
The arrangement of LED and LED light source specification 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 . LED driver circuit schematic of the basic work shown in Figure 2 , where the so-called "quarantine" means the AC line voltage and LED ( i.e. input and output) without 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 .LED driver circuit schematic of the basic work
It is worth mentioning that, in the LED lighting design, AC - DC (AC-DC) power conversion circuit with constant current drive two parts can adopt different configurations: 1 ) integral (integral) configuration, that both together , are located within the lighting , the advantages of this configuration include optimizing energy efficiency and simplify the installation, etc. ; 2 ) distributed (distributed) configuration , that is, they exist alone, this configuration simplifies security considerations, and increase flexibility.LED driver according to different application requirements , you can use a constant voltage (CV) output work , the output current range under a certain voltage clamp ; also using a constant current (CC) output , the output is designed to close a current-limited ; also may adopt constant current constant voltage (CCCV) output , namely to provide a constant output power , so as the load of the LED forward voltage to determine its current.
Using AC-DC power LED lighting applications, the power conversion building blocks include diodes , switches (FET), inductors and capacitors and resistors and other discrete components used to perform their respective functions, and pulse-width modulation (PWM) voltage regulator is used to control power conversion. Circuit is usually added to the transformer -isolated AC-DC power converter includes a flyback , forward and half-bridge topology, etc. , see Figure 3 , in which the flyback topology is less than 30 W of power in the standard choice for low-power applications , the half-bridge structure is best suited to provide higher efficiency / power density. The isolation structure of the transformer , its size and the size of switching frequency, and most isolated LED driver basically "e " transformers.
Using DC-DC power LED lighting applications, you can use the LED drive a resistive, linear regulator and switching regulator , etc., the basic application diagram 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 with no EMC problems , also supports the current regulation and over-current protection (fold back), and provides external current set point , drawback is that the power dissipation problem , and should always be higher than the input voltage is positive 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 , the cost of this structure and the energy efficiency is not satisfactory.AC power LED street lighting direct drive way in recent years also has been developed , and its application diagram shown in Figure 5. In this configuration , LED strings are arranged in the opposite direction , operates in half- cycle , and the LED forward voltage greater than the line voltage when turned on. This structure has its advantages, such as avoiding AC-DC converter power loss brought about . However , this structure in the low frequency switch the LED , the enemy eyes flicker may be observed . In addition, this design also need to protect the LED, so that the line from surges or transients .
U.S. Department of Energy (DOE) " Energy Star " (ENERGYSTAR ™) Solid State Lighting (SSL) specification provides that any power level are mandatory to provide power factor correction (PFC). This standard applies to a range of specific products , such as down lights, cabinet lights and lamps, which , residential applications, power factor LED driver must be greater than 0.7 , and commercial applications shall be greater than 0.9 ; But that standards are voluntary standards . EU IEC61000-3-2 harmonic content of the standard provides that a power greater than 25 W of lighting applications total harmonic distortion performance , the maximum limit of the total harmonic distortion (THD) <35%, and the power factor (PF ) > 0.94 . Although not all countries are absolutely mandatory for lighting applications to improve the power factor, but some applications may have this requirement , such as public utilities vigorously promote products with high power factor in the utility of commercial applications, in addition , utilities institutional ownership / maintenance lights , they can decide whether they wish to have a high power factor requirements ( typically> 0.95 + ) .Active PFC application circuit schematic
PFC technology including passive and active PFC PFC two kinds. Passive PFC larger programs , the need for additional components to better changing the current waveform , can reach about 0.8 or a higher power factor . Which, in less than 5 W to 40 W for lower power applications, is almost standard choice flyback topology requires only passive circuit components and minor modifications , you can achieve more than 0.7 power factor. Active PFC ( see Figure 6 ) is generally used as a dedicated power conversion section to the circuit to change the input current waveform . Active PFC typically provide boost, AC 100至277 Vac wide input range , PFC output voltage range of DC 450至480 Vdc. PFC stage if appropriately designed , can provide a 91% to 95 % of the energy efficiency . But increased active PFC, still require specialized DC-DC converter to provide a steady flow of current .Fourth, energy efficiencyThe energy efficiency of LED lighting applications require a combination of power output to consider. U.S. " Energy Star" solid-state lighting level specification of the lighting energy efficiency , but does not involve a separate LED driver efficiency requirements . As mentioned earlier , the use of AC-DC power LED application can be used two-stage distributed topology, it may use an external AC-DC adapter. The " Energy Star" does contain information about single output external power supply specification , the 2.0 version of the external power supply specifications in November 2008 came into force , requiring standard operating mode, minimum efficiency of 87%, while the low-voltage operation mode minimum energy efficiency up to 86 % ; in this specification , the power requirements of up to 100 W when PFC.DOE proposed in autumn 2008 LED lighting energy efficiency research and development goals
In using the AC-DC power LED applications , to provide greater energy efficiency AC-DC converter , it relates to cost, size , performance specifications and energy efficiency trade-offs between such factors . For example , if you use higher quality components , lower on-resistance (RDSon), can reduce losses and improve energy efficiency ; reduce the switching frequency will generally improve energy efficiency , but it will increase the system size. Resonance such as the new topology provides higher efficiency , but also increase the complexity of the design and components . If we design is limited to a narrow power and voltage range, can help optimize energy.
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