A common concern in LED lighting has been keeping THD (Total Harmonic Distortion) below 10%. Power sources act as non-linear loads and draw a distorted waveform that contains harmonics. These harmonics can cause interference in the working of other electronic systems. Therefore it’s important to measure the total effect of such harmonics. Total Harmonic Distortion gives us the information about the harmonic content in a signal w.r.t. fundamental component. Higher THD means higher distortion present on the input mains or lower power quality.
This requirement led me to test a design approach using a 15 W down-lighter (isolated) design based on TPS92314 configured for seven LEDs in series with 3.1 V forward voltage and 0.7 A rated current running from 150 – 265-V AC input. I followed below points to achieve a THD of 8.7 % at 240-V ac.
Before we get into actual implementation, here are two important equations from this application note that are needed to complete this test.
In this case, k comes out to be 1.68 and we plot THD vs “m” below for k = 1.68 with the help of above equations.
We also notice from the below figure that with increase in k (at a particular “m”, with m<k), THD increases.
Thus, going back to definitions of “m” and “k”, we notice that by increasing turns ratio (n = Np/Ns) and increasing the delay time of the converter, we lower the THD. Besides these parameters, the EMI filter design also plays an important role in THD improvement. The three design considerations to lower the Total Harmonic Distortion:
With the help of all of the above changes, I was able to achieve THD of 8.5% and PF of 0.98 at 240 V input with 21.8 V at the output. Using six LEDs at the output (18.8V output) in the same design, we achieved THD of 9% at 240 V. EMI filter of 80mH was realized on EE1685 core with 180 turns. The main transformer has a primary inductance of 2mH and peak primary current of around 0.5A.
The LED driver used in this test is the TPS92314 - a primary-side controlled off-line LED driver targeted at cost sensitive lighting applications (low external component count). It has a Constant-ON time architecture that provides natural power-factor correction without the requirement for difficult compensation techniques. Resonant valley switching also reduces the EMI and increases system efficiency. Other excellent features include cycle-by-cycle primary side current limit, VCC over voltage protection and under-voltage lock-out, output LEDs over-voltage protection and controller shutdown.
The complete schematic based on Texas Instrument’s TPS92314 can be seen below.
i would rate this apps notes very useful..
By increasing the Turns Ratio, you are making the supply into a linear one instead of switching, thereby increasing the transformer size and the weight of the supply.
Thanks Hung Huynh Trung.
Yes, Pravardhan. There's a compromise between the transformer size and THD, since you require higher number of turns to achieve a near perfect correlation between input voltage and input current.
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