The led drive IC has wide applied space for design of led electricity circuit. in present led market ,it can divided into three kind:
first class, for consumer electricity production. these kinds of application features is that will drive the electricity circuit by using the battery .in generally , the scope of drive voltage is form 4.2V~8.4V, so this drive IC aims to promote the low voltage of LED usage .
second class, for vehicle lighting. because the input voltage is vehicle battery ,so the voltage scope is 12V~48V,buck led ic many opportunities to use it.
three class, for Architectural decorative lighting or home lighting, AC transform to DC is necessary for this kind drive IC. the main function of drive ic circuits is to convert AC currently to constant DC currently.
LED lighting device divide into low power（IF=15-20mA）and high power （IF>200mA）base on led power. the low power led generally use in led Fluorescent lamp, decorative lamp, grille lamp; and the high power led that can be use for Household lighting lamp, spotlight, underwater lamp, wall washing lamp, street lamp, tunnel lamp, automobile work lamp.
The led lighting device if device of low voltage and big current . so the currently of led was decide that led luminous intensity. if this is the case , the input currently are to strong will effect the led luminous attenuation. the input currently are to small that would interfere with led luminous intensity.
In summary , the electricity designer chose that led constant currently source to drive LED which we considered right.
There have two way to drive that led, may divide into resister currently limiter and constant current.
The resister limit current is one way of drive technology. the led has advantage of design simply and low cost . but it has a lot shortcoming:
*the current will change along with differ input voltage or forward voltage. for this result ，the indeterminateness of the led light produced by differ input voltage.
*the low efficiency: the drive efficiency are generally very low on high battery voltage drop. the reason for this is that most of power consumed in resister. the acquisition of heat is certainly result. so the dispel heat is big problem for resister currently limiter.
Other way is constant current source . is has advantage of current constant. but the price of constant current source is high than resister currently limiter.
The led application of vehicle system has many advantage. in car led system, most vehicles employed that 6000K color temperature led. because the band of color temperature close to natural light, so it is comfortable for human’eyes. other words: in night , the capability of distinctions color of person are the best in natural light.
The life time of led is long that traditional light device(just like Tungsten lamp) ,and more stable & reliable.
The starting speed of led is fast that traditional lamp. example the break led light starting time was shot in 50ms, and the tungsten lamp starting time about 300ms.
The led is solid state light source, led would come under greater shock.
The new current limiter —- limiting capacitor
A few days ago, We are talking about LED indicator light, some one mentioned that capacitor limiting .i think this is good idea for reducing energy loses. it is low power that resister limiting .
In general , the indicator led of patch board adopt 1/4w 220k resister to limiting current . the electricity consumption of resister as 0.2W, it looks small. if the indicator led work long hour, for example: which also consumes 1.75 kilowatt hours a year.
in the moment, we think that implementation of capacitor to limiting current for led indicator. because the capacitor is reactive energy device in alternating current .
First we need to clear out of two concept— led reverse voltage and reverse breakdown voltage 。
*led reverse voltage: we will call it “reverse voltage” when just make reverse voltage for led, in general there are not have numerical definition for reverse voltage ,and no matter the input voltage how big.,only condition is opposite directed . this is led reverse voltage.
*led reverse breakdown voltage, the most character of led reverse breakdown voltage is that the led reverse breakdown voltage has numerical definition . the PN junction have been zener or avalanche breakdown emerged from led reverse breakdown voltage(when the reverse current meet specification value).this situation we will call it” led reverse breakdown voltage”.
Many engineer ask me a question that use PWM to drive LED whether LED will be affected .
First LED is diode that it can achieve rapid switch(The application of quick switch technology in LED). which is incomparable to other component .
Hence, just only converted power supply to constant current source, then with adjust pulse width way to change the bright of LED. the way call PWM(pulse width modulation ). suppose the cycle of pulse as tpwm and width of pulse as ton, that duct cycle as ton/tpwm. adjust the duct cycle of constant current pulse can change the LED bright . so You trust to use the PWM.
BOOST and BUCK IC
We generally adopted led constant current ic to drive LED. the constant current ic is divided into three kind: BOOST , BUCK and BOOST & buck.
the engineer rarely use the BOOST & buck IC , because it’s much more expensive than others. now we are get a question that how to choose BUCK and BOOST for our project. base on the follow way to chose the appropriate DC-DC for your project.
Input voltage is big than load voltage, please chose the BUCK IC
contrary, the load voltage is big input voltage, please chose the BOOST IC.
Chromatogram offsets
The line of chromatogram curve offsets distinctly when adjust led currently. what the reason for that?
We are know that the white light come form blue led excite by yellow phosphor . when forward current is reduce , the blue led light actually increased and the yellow phosphor wasn’t given portion of reduce , and it will cause main wave length of spectrum is increased. so blue led light directly affect spectrum and color temperature . for example:
when the forward current at350mA, the color temperature as 5734K.
when the forward current current increase to 350mA, the color temperature will offset to 5636K. the color temperature will change to warm white when the current further reduce,