The LED Driver RMS Value Measurement and Analysis
As we known the quality of LED lighting products depend on two points: One is LED chip(light source); Second is LED power driver. In the practice, there only have few company can provide qualified LED chip such as PHILIPS, OSRAM and CREE in the market. Generally, the LED chip have very small change to get quality problem, but we heard many news about low quality LED lighting products was found and caused big losses to customers because of un-qualified LED driver. As considering the importance of LED driver, this article will discuss some technical introduction and measurement technology.
There have some necessary test items of LED driver such as Crest Factor, Effective Power, Reactive Power, Apparent Power, Power Factor, Voltage/Current (RMS value, Average value) and THD harmonic parameters etc. In the practice measurement, lots of people found it have big difference between get from LED driver tester and multimeter, and this difference focus on Voltage/current rms, apparent power, power factor, RMS(rms) and average. Many users feedback the voltage/current(RMS date) from multimeter is lower or bigger than LED driver tester, it is necessary to make a explain here that the values from LED driver tester are RMS(true and effective value), the average curren we often talking is also RMS(average value), the average current only can get when it satrt test and it is RMS average value during start process; the software data is keep some with front panel of LED driver tester. In the meantime, according to international standards, the RMS(true and effective value) is needed only when test LED driver, it is meaningless to measure average value. The LED driver tester will show voltage, current and power, but the power is not equal of voltage multiply current; the voltage multiply current directly get inspecting power, the LED driver tester showed value is active power. If only use multimeter to measure average value, then there is no need use professional LED driver tester. According to IEC standard, the Voltage True RMS (Vrms), True RMS Current (rms), Active Power (P) and Power Factor (PF) calculate way as below:
In the above formula, “N” is stand the number of sampled points during the period (the period depends on the frequency of the signal to be measured) and the value at a certain sampling time. The LED driver tester provides two test methods to calculate the total harmonic distortion (THD), namely IEC and CSA, respectively:
In the above two formulas:
THD: Represents the relative value of voltage/current total harmonic distortion
C：Represents the K times harmonic effective value of the voltage or current
K：Number of harmonics
N：The maximum harmonic frequency (This LED driver tester is 50)
C：RMS value of fundamental voltage or current (ie, 1 harmonic)
In the practice measurement, when use RMS value or average value of LED constant output current to judge quality of LED driver, there have two ideas in the market: some use RMS value but some use average. Caused this two difference idea due to LED driver have big current ripple and each LED driver is different, then the RMS value and average value of output current have big difference, but according to the actual situation and standards, the only judge factor is focus on RMS value, we can refer average value but can’t see it as key judge factor.