LNK306 constant current Rfb value?
Hello,
When using the PIXl designer, it shows a value for Rfb and Rbias. It seems that Rbias is always 2K, and Rfb changes according to output voltage, which makes sense. However, when designing for constant current LED supply, are these values correct, or should I use the 2K rbias with the 300-ohm shown in your reference LED disign, such as that in AN-37? I am trying to get 350mA for a single string of 20 LEDs. It seems to work, but only puts out about 220mA. I have tried a 4.7mH inductor, as well as 8.2mH, but both put out approx. the same current. I am using an Rbias of 2k, with an Rfb of 78k, as determined by PIXl designer. What is the correct Rfb I should be using? Thanks.
Comments
Hello,
Thank you for the info. I now have it connected with the 2k and 300-ohm combination as in fig 6 of AN-37, with Rsense of 5.7-ohms, an 8.2mH inductor, and a .1uF for Csense. My input cap is two 22uf electrolytic caps in parallel. My output cap is a 10uF electrolytic. My input DC voltage is approx. 144 volts with about 4v of ripple. I used the equation given for Csense and it came up with 52uF, which doesn't seem right to me. I tried a 4.7uF for Csense, and it started flashing the leds at the .8S rate. With the .1uF for Csense, the LEDs stay on, but the current remains at 240mA. The output is only about 56 volts with less than 300mV of ripple. This is with 18 LEDs in series. I can't figure why the voltage would be low. Any hints? Thanks again!
Hi,
Just wondering if you read my second post on this thread. I'm not able to get any more than 255mA out of the circui as explained. Any help would be great! Thanks.
Hi,
This may be too much power for a LinkSwitch-TN device to handle. What temperature is the PI device running at? At 144 VDC input, can you send me the drain voltage switching waveform? Can you deliver more power when you increase the input voltage?
The LED's are not running at rated current so their terminal voltage is slightly lower.
Last thing - Try in reduce the Rsense resistor (AN 37, fig 6) and see if you can get more output current.
Hi again,
I tried to reduce the current sense resistor - no change in current except that at 4.3 ohms the leds start to flash at .8s. -Rick
Hi,
I should have done this to begin with, but...what device would you recommend for this? 20 Leds @ 350ma, single string, universal power input, also a 5v, 15mA winding or second voltage possibly required. Thanks.
Rick
The LinkSwitch-PH family of devices was just introduced a few days back and is targeted specifically for LED drivers. It will simultaneously give you constant current at the output and also achieve good PF (better than 0.9). These devices are used in Flyback topology and can be used upto 50 Watts output power.
Take a look at the following webpage to see some examples using LinkSwitch-PH
http://www.powerint.com/en/products/linkswitch-family/linkswitch-ph
http://www.powerint.com/sites/default/files/PDFFiles/rdr194.pdf
Hi,
If you are designing a CV power supply, then the value of Rbias would be about 2K and you set the output voltage using Rfb. If you are designing a CC power supply such as the LED reference design shown in the AN-37, you will need to sense current instead of output voltage using a series pass resistor. This resistor will have to be sized to generate a voltage drop proportional to the output current, so as to set a limit on the output current.
Often it is advisable and appropriate to have a feedback scheme that also limits the maximum output voltage in case the load gets disconnected. This is espceially required so as to not to exceed voltage ratings of the output capacitors. The optional DFB, VRFB network shown in Figure.6 of AN-37 is to limit the output voltage making the power supply a CV power supply once the load gets disconnected.
If you are using a CV feedback scheme , it is possible that the LED string current is being set by the impedance of the LED's. In order to achieve a higher current, you will need to use the scheme similar to the one shown in Fig.6 of AN-37.
Hope this helps.
Regards
PI-Sarek