Re: Constant current for 16 white LEDs?
- From: wrongaddress@xxxxxxx
- Date: 20 Sep 2006 21:04:21 -0700
Paul E. Schoen wrote:
<wrongaddress@xxxxxxx> wrote in message
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Paul E. Schoen wrote:
<wrongaddress@xxxxxxx> wrote in message
news:1158719221.448328.310740@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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
The boost converter I simulated (and also breadboarded) uses a 555 timer to
drive the gate of a MOSFET. I used a 5 uH inductor at about 100 kHz, and
also a 50 uH inductor at about 20 kHz. These inductors are about 1" long
and 1/2" diameter. I plan to use a toroidal inductor about 3/4" dia and
1/2" thick. These inductors have DCR in the uOhm range, and can handle
current up to several amperes. The overall volumetric efficiency increases
with frequency up to a point, where switching losses and other effects
become important. My circuit essentially pulls one side of the inductor to
ground, allowing the current to build up to several amperes (but below
saturation), and then opens the connection to allow the current to charge a
fairly large filter capacitor. The output voltage ratchets up over a period
of time until it reaches a maximum, which depends on the load and other
factors. I added a sampling resistor in series with the LED load (simulated
with a zener and a resistor), and drive an NPN transistor with its
collector to the RESET line of the 555. When the voltage is high enough to
make the zener conduct (or the LEDs light), current will increase until the
transistor shuts down the 555 until more voltage is needed. Remember that
you will need to store a lot more current in the inductor than what you
want to get out. For your 4.5 VDC to 48 VDC at 20 mA, assuming a 50% duty
cycle, the peak inductor current may need to be as high as 40 times the
output current, or close to 1 ampere. For my circuit, I was looking at peak
currents up to 50 amps, so I needed a MOSFET with a very low Rds-On. I will
be breadboarding a new version soon. A rough schematic is as follows:
L1 D2 R1
+12VDC----+----+-UUU--+--->|---+--/\/\/---> To LEDs
| | | |
| D1 - --D-- |
Q1-C <--4-8-- ^ |MOS| C1 -
|555|3-|---G|FET| -
--1-- | | | |
| | --S-- | R2
GND-------+----+------+--------+-/\/\/-+--> LEDs return
|
To Q1-B <--+
I have omitted the frequency determining components for the 555, pullup for
reset, and various additional components such as capacitors and diodes.
Basically, R1 allows C1 to charge up to more than 50 volts to current limit
the LEDs, and R2 is chosen so that it drops about 0.7 volts at the desired
maximum output current to shut down the 555.
There are certainly more elegant ways to do this. I was going to use a PIC,
and my other option was a PWM IC such as UC3526, but it is designed for
push-pull operation with two MOSFETs and a center tapped transformer.
It is interesting to see this circuit operate using a transient analysis
with Tina. The output voltage slowly rises and then oscillates with ripple
frequency as well as a lower frequency as the 555 turns on and off.
Paul
Yes, I have a similar circuit but the problem is getting enough
inductor current at low voltage. If I raise the supply voltage to 9
volts, I can get 60 volts out into 2700 ohms, or 1.3 watts which is
enough for this application. Input current is about 190mA, so the
efficiency is about 1.3/1.7 = 76%
But it won't work at low voltage of 4 volts, and that's the problem.
Why does Vce Sat increase as supply voltage decreases?
-Bill
.
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