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AC/DC CONVERTER FOR LED (5 WATT)

Other Parts Discussed in Thread: LM3444, LM3445

I  want to drive 5 LED's of 1 watt each using AC supply of 230 v 50 HZ.

specification requirement:

(1) output 5 watt , 300 mamp of constant current

(2) Total harmonic distortion < 10%

While searching for the compatible IC in ti.com, i come across LM3444 evaluation module. Application note AN2082


I was going through this application note and found this warning message :

"Warning: The LM3444 evaluation board should not be powered with an open load. For proper operation, ensure that the desired number of LEDs are connected at the output before applying power to the evaluation board."

what if i power ON this evaluation board with an open load and why it is recommended NOT to switch ON with open load condition. Please help.

  • Hi Amit,

    I suspect that the LM3444 board may have a difficulty regulating without a load present. let find the right person to help you with this.

    Regards

    Peter
  • The main reason for this comment is to avoid destroying the EVM. If a flyback like this is run with the output open it will go to a very high output voltage. This would quickly destroy the 35V output caps, which is why we added the 30V zener across the output. If you power it up with no load that zener will short to protect the capacitors and it will need to be removed or replaced. If left on long enough, or possibly even if powered up again with no load, the zener will eventually open and the caps will fail.
    If allowed to go to a high voltage it will also reflect back across the transformer and you could damage the switching FET as well.
  • Thank you Clinton Jensen for your reply.... I appreciate your help.. Can i use circuit used in this evaluation board (APPLICATION NOTE: AN2097) to drive 5 watt, 7 watt and 10 watt LED after making necessary changes in "R21" resister value . Actually i want to use this circuit for residential lightning so ..will this circuit help or it still require some safety features for final use.
  • Thanks a lot for your quick reply.
  • Yes. The transformer on the board is sized to handle more power than the EVM is set up for as a default. There are actually some mods in the app note for higher power (refer to fig 11), so you can easily get your 3 different power levels just by adjusting R21. If your stack voltage will be above 30V you will also need to change/remove the zener and use caps rated for a higher voltage (they do not need to be a larger case size, you can get away with less uF at higher voltage due to the way caps store energy).
  • Thanks a lot... very informative.. I have some doubt in the circuit diagram.. Application note AN2097 , PAGE NO. 2 , in the given circuit diagram, there are two marker : first one is named as "FILTER" and second as " VCC". Is there any separate circuit diagram used whose output need to be connected at these marked position or it is used just to explain the circuit in the app note. I got this doubt because marker named as "V+" is the output of EMI FILTER that need to be connected to main circuit.
  • Those two markers are just informative. There isn't anything off to the side connected to them.

  • There is no information regarding "THD" in the application note. Do u have any graph or table which can indicate THD rating or can i get THD value of less then 10% with the same circuit at 230V AC.

  • If you look at section 14.2 it specifies that the THD was measured to be less than 1.2%.

  • I wanted to know whether input EMI filter used in the application note AN2097 can be used for other IC also like "LM3445" or "bq" series IC used for mobile charging application. whether this EMI filter is universal or its design parameter need to be changed even though i am using same input spec like 230v, 50hz and output current of 300 to 500 mamp. I don't have much knowledge of EMI Filter design. Please help.
  • The EMI filter was designed for the power levels for that EVM and the switching frequency of the circuit. Any circuit with similar power levels and a similar switching frequency could likely use the same EMI filter. However, note that if it is used with something like the LM3445 and triac dimming is required you will need additional snubbers to prevent triac mis-fire. You can check AN2069 for an example. It is the same design as in AN2097 except that it uses the LM3445 and is triac dimmable, so you can see what extra components are required. Unfortunately there is no universal filter design, it tends to be application specific. But if, like you mention above, you are just going to use the same circuit for different power levels you can just use what works for the highest power level and that should work for all of them.

  • Ok.. thanks for this information, its very much helpful to me..so at what max. Power level this EMI filter design used in application note AN2097 can be used keeping switching frequency as same as i am going to use same filter design for 5 , 7 and 9 watt.

  • That filter should be good up to about 12W or so, you should be fine using it.
  • With little finding i came to know that the EMI filter is designed to eliminate "differential mode noise" and "common mode noise" from the circuit which arises due to switching devices. To eliminate differential mode noise "X" rated capacitor are used and to eliminate common mode noise "Y" rated capacitor are used. Both of this X rated and Y rated cap. can be of ceramic or metallic paper/film type. ceramic type capacitor tends towards short circuit on failure which can cause fire if it is X rated cap. i.e. connected across the line and can cause shock hazard if it Y rated cap. connected between line and ground but metallic paper/film type tends to open circuit on failure which will not cause any type of hazard of fire. Even after this advantage of metallic paper/film type cap. the capacitor C4 and C1  used in this application note which i think is a "X" rated and "Y" rated cap respectively are of ceramic type. can u please explain the reason behind the use of ceramic cap. correct me if i am wrong please.

  • I didn't do the actual design for that EVM so I can't be 100% sure on why ceramics were chosen, but I'm pretty sure it was just due to size (trying to make the overall circuit smaller). The film type caps are the best option if they can be used, but there are ceramics that will still pass UL and other safety standards.