Re: Constant current for 16 white LEDs?

Paul E. Schoen wrote:
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What is a good approach to drive 16 white LEDs (3 volts each at 20mA)
from three 'C' cell batteries (1.5 volts each) to maintain constant
current and brightness as the battery voltage falls?

I have tried a Boost converter (inductor, transistor, diode, and filter
cap) with poor results.

-Bill


A boost converter should work, but you must make sure the output voltage
does not fall below the total forward voltage, unless you want a PWM
brightness control. You can tie all the LEDs in series, which will require
48 VDC, and make a boost converter that puts out at least 50 VDC. The
tricky part is to regulate the current without burning up too much power in
a series dropping resistor. It is better to use inductance to maintain a
constant current.

You could use a 5 volt to 15 VDC DC-DC converter, and add a current
regulator, to drive 4 strings of four in series.

I am working on a design to drive up to 12 high power white LEDs* (3 watts
each, at 700 mA, 4.5 Volts), from a 12 VDC SLA battery. I worked out some
boost circuits using the Tina simulator, and it looks promising. I will be
building an actual prototype soon. The same basic design could be used for
your application. I can send you a schematic you may use, if you have a
copy of Tina. You can get a free demo, or purchase a copy for about $30,
from www.tina.com, or get a free limited version at www.ti.com.

(See the CREE website for Cree XLamp3_7090.pdf)

Paul

Well, the problem with the boost converter is the ramp current won't
increase past a few hundred milliamps before the transistor comes out
of saturation. I have tried several inductors and transistors and ended
up with the same problem.

I am experimenting with a large 25mH inductor with low resistance of
200 milliohms or less and a power MOSFET (IRF620). I am driving the
gate with a (0 to 8 volt) squarewave at about a 800Hz rate. It works
well at a 1/4 watt power level, but if I try and increase the ramp time
by lowering the frequency (from a generator) to obtain more inductor
current, the transistor comes out of saturation and starts dropping a
large portion of the supply voltage and the efficiency goes way down.
The supply voltage is 4 volts.

I must be missing something fundamental. As I understand it, inductor
current will increase with time and stored energy is 1/2 LI^2. So, to
increase output power, the time should be extended so that more current
flows into the inductor. But that idea only seems to work within a
small range.

-Bill

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