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Current mirror for paralleled led strings?

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grizedale

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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
 

"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% ???
 
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.
 
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Reactions: dselec

    dselec

    dselec

    Points: 2
    Helpful Answer Positive Rating
`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
 
Last edited:

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...


Code: 
Version 4
SHEET 1 880 1392
WIRE -256 -1120 -880 -1120
WIRE -256 -1088 -256 -1120
WIRE -256 -960 -256 -1008
WIRE -256 -960 -448 -960
WIRE -96 -960 -256 -960
WIRE -96 -896 -96 -960
WIRE -96 -720 -96 -816
WIRE -96 -608 -96 -656
WIRE -448 -512 -448 -960
WIRE -96 -512 -96 -544
WIRE -96 -416 -96 -448
WIRE -448 -400 -448 -448
WIRE -448 -304 -448 -336
WIRE -96 -304 -96 -352
WIRE -96 -208 -96 -240
WIRE -448 -128 -448 -240
WIRE -96 -112 -96 -144
WIRE -448 -32 -448 -64
WIRE -96 -16 -96 -48
WIRE -880 32 -880 -1120
WIRE -448 64 -448 32
WIRE -96 144 -96 48
WIRE -448 160 -448 128
WIRE -96 240 -96 208
WIRE -448 272 -448 224
WIRE -96 352 -96 304
WIRE -448 368 -448 336
WIRE -96 448 -96 416
WIRE -448 480 -448 432
WIRE -96 560 -96 512
WIRE -448 592 -448 544
WIRE -96 656 -96 624
WIRE -96 848 -96 720
WIRE -96 848 -304 848
WIRE -448 944 -448 656
WIRE -96 944 -96 848
WIRE -304 992 -304 848
WIRE -304 992 -384 992
WIRE -160 992 -304 992
WIRE -448 1088 -448 1040
WIRE -96 1088 -96 1040
WIRE -448 1216 -448 1168
WIRE -288 1216 -448 1216
WIRE -96 1216 -96 1168
WIRE -96 1216 -288 1216
WIRE -880 1344 -880 112
WIRE -448 1344 -880 1344
WIRE -288 1344 -288 1216
WIRE -288 1344 -448 1344
WIRE -448 1360 -448 1344
FLAG -448 1360 0
SYMBOL diode -112 -512 R0
SYMATTR InstName D1
SYMATTR Value MURS120
SYMBOL diode -112 -608 R0
SYMATTR InstName D2
SYMATTR Value MURS120
SYMBOL diode -112 -720 R0
SYMATTR InstName D3
SYMATTR Value MURS120
SYMBOL schottky -112 240 R0
SYMATTR InstName D4
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -112 352 R0
SYMATTR InstName D5
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -112 448 R0
SYMATTR InstName D6
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -112 560 R0
SYMATTR InstName D7
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -112 656 R0
SYMATTR InstName D8
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -112 144 R0
SYMATTR InstName D9
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL diode -112 -416 R0
SYMATTR InstName D10
SYMATTR Value MURS120
SYMBOL diode -112 -304 R0
SYMATTR InstName D11
SYMATTR Value MURS120
SYMBOL diode -112 -208 R0
SYMATTR InstName D12
SYMATTR Value MURS120
SYMBOL diode -112 -112 R0
SYMATTR InstName D13
SYMATTR Value MURS120
SYMBOL diode -112 -16 R0
SYMATTR InstName D14
SYMATTR Value MURS120
SYMBOL diode -464 -304 R0
SYMATTR InstName D15
SYMATTR Value MURS120
SYMBOL diode -464 -400 R0
SYMATTR InstName D16
SYMATTR Value MURS120
SYMBOL diode -464 -512 R0
SYMATTR InstName D17
SYMATTR Value MURS120
SYMBOL schottky -464 160 R0
SYMATTR InstName D18
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -464 272 R0
SYMATTR InstName D19
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -464 368 R0
SYMATTR InstName D20
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -464 480 R0
SYMATTR InstName D21
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -464 592 R0
SYMATTR InstName D22
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -464 64 R0
SYMATTR InstName D23
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky -464 -32 R0
SYMATTR InstName D24
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL diode -464 -128 R0
SYMATTR InstName D25
SYMATTR Value MURS120
SYMBOL voltage -880 16 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR InstName V1
SYMATTR Value 15
SYMATTR SpiceLine Rser=0.001
SYMBOL current -256 -1088 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName I1
SYMATTR Value 0.7
SYMBOL npn -384 944 M0
SYMATTR InstName Q1
SYMATTR Value BC337-40
SYMBOL npn -160 944 R0
SYMATTR InstName Q2
SYMATTR Value BC337-40
SYMBOL res -464 1072 R0
SYMATTR InstName R2
SYMATTR Value 0.56
SYMBOL res -112 1072 R0
SYMATTR InstName R3
SYMATTR Value 0.56
SYMBOL res -112 -912 R0
SYMATTR InstName R4
SYMATTR Value 7.54
TEXT -946 1374 Left 2 !.tran 0 10m 0 startup
TEXT -312 -848 Left 2 ;3.43W

Next time use code tags [alexan_e]
Click to expand...
 
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