First we will take about some characteristics of the LED’s ,and put forward some require for LED power supply according to characteristics of LED. then description of work way with low power.
LED feature and drive power Requirements.
LED characteristics are as follows.LED is a PN junction device, with unidirectional conductivity, so it’s necessary to use a DC power supply or single pulse voltage. The forward voltage up to the start threshold after a current flowing through the LED.
To emit light, the LED light that are relative to this Current intensity.big current, current intensity; small, weak light, so LED must be supply power with constant current source. And voltage should be exceed in LED start voltage. And the LED characteristic is non-linear. As long as the forward voltage has little change. which Forward current changes will be large. The forward voltage and emit light color relative to the material of LED.RED GREEN and YELLOW led forward voltage drop is 1 .4～2．6V， white led forward voltage drop is large, for 3～4 .5v。when the forward current has been charged, the color of the emitted light will be offset, i.e., the color shift occurs. White led forward voltage Relationship with and white led forward current.Forward current
1、LED is non-line components for temperature sensitive. Temperature increases, the forward voltage drop is decreases. since the LED V-A characteristics is non-line, forward voltage drop has a little changed, then forward current will be changed is large. So must be using the constant current to supply power. or limiting current. Otherwise If using constant voltage to supply power. along with the temperature is increases. The forward voltage is decreases. The current is big and big, until it burned. Even the Devices in the same batch of production. High transparency plastic LED light strip includeing its forward voltage drop there will be some spectral rays. along with the temperature changed ,times flow ,equipment aging. So the driver circuit has feedback circuit is best. It can adjust of LED luminous color and intensity for real-time.
2、in order to highlight the effect of energy saving in LED,power conversion efficiency must be high, the demand of LED driver power can be attributed to the following.
3、Power supply must be DC voltage. further voltage is big than LED forward voltage.
4、requirement of power supply provide a current of constant and matching, to make a stable light and Brightness output.with power chang rate high,in order highlight the advantage of save-energy in LED.
5、in general, requirement of led has dimming function, so the driver power must be can adjust current. In order to carry out about the LED brightness, color and tone can be adjusted within a certain range.
6、has some protect function, such as open circuit protect,overvoltage protective and undervoltage lockout, and other functions.The original power supply LED The original power supply LED a lot supply by battery or AC power. such as batter supply power,in general the device of LED using rechargeable battery. Battery rang as 5～24v. rechargeable Battery has Rechargeable batteries are nickel batteries, nickel hydrogen batteries, lithium ion batteries.
1、nickel batteries
The first nickel battery was applied to mobile phone, its energy density is low, the volume is too big, capacity is too small,with more seriousis The memory effect of Ni MH battery, now has been replaced by the nickel hydrogen battery.
2、Nickel chloride (Ni – MH) battery Nickel battery are an environmentally friendly battery, rated voltage as 1 2v，energy density than nickel-chromium,battery has greatly improved.charge and discharge times of up to thousand times.price is very moderate.but the capacity is small than lithium-ion battery capacity。The advantages of lithiumion batteries is the higher the voltage of single cell, rated voltage of 3.6 ~ 3.7V. fully charged When the voltage is 4.2 ~ 4.4V, high energy density,small size:long cycle life, charge and discharge time Up to 500 ~ 1100 times: no memory effect;low self discharge rate,the monthly self discharge rate is 3% ~ 9% (nickel cadmiumElectricity is 25% ~ 30 ‰ Ni MH battery is 30% ~ 35%).Powered by batteries, in addition to bttery supply, also needs to use the DC, AC converter provides a constant current for LED, LED can also be composed of AC power supply, the need for a Ac to Dc converter,the first rectifier, filter,Then the cascade Dc, Dc buck converter, buck for LED power supply. Dc to Dc buck converter for switchType, it provides current to match the DC voltage and the constant and LED load.