Current mirror for paralleled led strings?

[grizedale]

Hello,

The following shows how parallel led strings can be driven from a single current source and still have matched current............

**broken link removed**

However, over the typical +/-20% difference in led V(f) value, this would mean (in worst case) potentially 2W of dissipation in one of the BJT's of the current mirror. (considering 350mA led current.)

So how can this method be put forward realistically......its far too dissipative.?

One of the worst thing is that the reference BJT has to be the one in series with the highest voltage led string.....bu since you wouldnt know which string that may be you have to add a series resistor in the reference string so that it is definetly the one with highest voltage.....such a resistor adds even more to the dissipation.

Am i right to consider v(f) with +/-20% tolerance, and can you also see that this method is far too dissipative to be realistic, let alone the cost of matched bjt's

 

[dselec]

"BJT's of the current mirror"
what bjt ? where ?
anyway there is a problem . what will happen if 4 leds go +20% and the fifth is -20%
and in the next array 4 leds go -20% and 1 go +20% ???

 

[grizedale]

the bjts can be seen by clicking the pictures.

Its worse if all of one string are +20% and all of the other string are -20%.

I believe this parallel led driving solution is utterly bogus, and would appreciate your thoughts.

 

[dselec]

`1) bd139 can take 12.5w 1.5a and more for 0.5 amps it has vce sat 0.5 volt resulting about 0.35v * 0.37a= 1.2 watt
for perfect led lighting u will need to chose good led's and check each led luminance for given current.

any way read some more about the matching problem and solution so this post helped me understand more about high power leds
"Even if the LEDs are all from the same production batch and sequentially manufactured, the Vf of individual LEDs still has a ±20% tolerance. The tolerances mean that the total forward voltage for each string can be very different and therefore the current mismatch significant "
thanks for post

 

[grizedale]

Hello,

I now provide more information, as i ask you to concur with me (please) that driving LEDs in parallel is a hoax unless they are really low current like 30mA.................

So, Is the following current mirror circuit for matching led currents in parallel strings acceptable?
Led current is 350mA (white LEDs)

Schematic
https://i48.tinypic.com/16c0tit.jpg

(.....i simply used normal diodes instead of leds because all i was interested in was the overall forward voltage of the string.)

In the worst case, when the two leds in one series string are each 3.3V minus 20%, and the two leds in the other string are each 3.3V plus 20%, then the dissipation in the BJT with the minus 20% leds is 2W. (that seems an awful lot)

Also, the series resistor had to be added to make sure that the reference BJT always sits in the highest voltage led string.

Again, in the worst case, this resistor dissipates 0.9W


Surely this means that this current mirror method of matching LED currents is totally unsuitable?

View attachment _pllel leds plusminus20 RESBIGSTRING.txt

LTspice simulation file
......simply convert to .asc and run...

Version 4
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