Grow LED Array Rev.2 Results AKA Why Rev.3

Why Rev.3?
- Topside ground plane connection underneath connector. Inaccessible, non functional without thru-hole plating without wire cludge.
- White rail dissipation in PNP pass transistors very far beyond spec. Red rail to spec only below 25C ambient.
- Transition to ethernet input.

 

 

Short term testing of hardware observations:
- Normal room temp.
- 25V unregulated psu.
- All circuits function as intended.
- Red and blue series transistors warm to touch.
- White series transistor hot set at 1/2 normal current.

Conclusions:
- Multiple rail PSU, <0.8V overhead at max string Vf. Blue rail is an example. Benefit is less dissipation per output.
- SOT223 instead of SOT23 3906 series transistors. Same dissipation but within spec. Benefit is single-rail lower-cost psu and a sexier transistor package.
- Dropping resistors for white and red LED rails. electrically same as SOT223 dissipation, retain cheaper SOT23 package.
- Higher dissipation SOT23 PNP for red rail viability without resistor. Must test at higher ambient.(MMBT4403?)
- Three channels current feedback SMPS. Much less dissipation, psu same level complication as tri rail. Cost benefit unknown.

Paper Test:

LED Vf(max)
Red =2.4
Blue =3.6
White =3.6

LED Vf(typ)
Red =1.7
Blue =2.8
White =2.8

String Vf(max)
8x Red =19.2
6x Blue =21.6
2x White =7.2

String Vf(typ)
8x Red =13.6
6x Blue =16.8
2x White =5.6

String W(max) @ 20mA
8x Red =0.384
6x Blue =0.432
2x White =0.114

String W(typ) @ 20mA
8x Red =0.272
6x Blue =0.336
2x White =0.112

System Output W(max) @ 20mA
4x Red Strings =1.536
3x Blue Strings =1.296
1x White Strings =0.228
RB Total =2.832
RBW Total =3.060

System Output W(typ) @ 20mA
4x Red Strings =1.088
3x Blue Strings =1.008
1x White Strings =0.288
RB Total =2.096
RBW Total =2.384

MMBT3904 Vce(max) @ 23V V[supply]-Vf[string_max]-Vbe[0.6V]
Red =3.2
Blue =0.8
White =15.2

MMBT3904 Vce(typ) @ 23V V[supply]-Vf[string_typ]-Vbe[0.6V]
Red =8.8
Blue =5.6
White =16.8

MMBT3904 mW(max) @ 23V/20mA
Red =64
Blue =16
White =304

MMBT3904 mW(typ) @ 23V/20mA
Red =176
Blue =112
White =336

MMBT2904 Temp(max) @ 23V/20mA 357C/W
Red =22.848
Blue =5.712
White =108.528

MMBT3904 Temp(typ) @ 23V/20mA 357C/W
Red =62.832
Blue =39.984
White =119.952

MMBT3904 mW_derate(max) 23V/20mA Ambient 25C 2.8mW/C
Red =63.9744
Blue =15.9936
White =303.8784

MMBT3904 mW_derate(typ) 23V/20mA Ambient 25C 2.8mW/C
Red =175.9296
Blue =111.9552
White =335.8656

MMBT3906 mW_overhead(max) 23V/20mA/25C 350mW[SOT23]-mW_derating-mW_used
Red =223
Blue =319
White =-257(!)

MMBT3906 mW_overhead(typ) 23V/20mA/25C 350mW SOT23
Red =-1
Blue =127
White =-321(!)

MMBT3906 Source, Vf(max)/If=20mA/23V/25C/77F
Red = OK
Blue = OK
White = FAIL

MMBT3906 Source, Vf(typ)/If=20mA/23V/25C/77F
Red = ALMOST, BUT FAILED
Blue = OK
White = FAIL

Posted by rmrubin April 2008

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