Design buck converter to meet output ripple voltage specs

Light Units There are a number of  ways of measuring the light output from a lamp, LED, etc. Is a design buck converter to meet output ripple voltage specs of the luminance over some area.

For example a sheet of paper is made translucent by soaking it with oil. On one side is a candle made to a specification and burning at the specified rate. On the other side of the paper is a light source. By moving the light source toward or away from the paper a location will be found where the two sources are equal in brightness as seen by your eye. If the light source was another identical candle then it would be the same distance from the paper. But if a reflector was  on the light source candle it could be moved further away.

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The light output from the T series LEDs is specified in milli candelas. Lumens Are a measure of the total visible light in all directions. For example a 100 Watt light bulb in the U. For the 100 Watt bulb that’s about 17 Lumens per watt. Since power in equals power out the total power radiated from a lamp is the same as the power put in.

Design buck converter to meet output ripple voltage specs

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Most of the power out of electrical lamps is at non visible wavelengths, like heat. City governments or businesses are very concerned with Lumens per Watt since they pay the electrical bill. Note 1 – All fluorescent lights, both conventional and compact use Mercury. Note 3 – The most popular carbon filament lamp was the 16 candle power size that consumed about 60 watts and provided about 200 lumens. The smallest line powered lamp was the 8 CP that used about 25 watts and provided about 100 lumens. So a LED based lamp for general illumination needs to put out 100 to 200 lumens.

5 to 10 LEDs to make a useable lamp. W goes up as the current is turned down on modern LEDs, but then the cost of final lamp goes way up since many LEDs are needed. Many of the flashlight sellers make inflated claims about the brightness of their product. It’s probably best to ignore any claim made by the seller and instead look for a review that has some type of comparison testing.

The main problem I’ve found is the high amount of wasted power. For example the Made in China “Luxeon K2 Power 120 Lumens” flashlight that runs on two LIR123A cells. This is a cheap flashlight that uses a 4. 2 Ohm resistor to set the LED current.

The K2 is rated for up to 1. LIR123A cell is rated to deliver a maximum current of 800 ma where the K2  puts out 93 Lumens, not 120. There is no specification for  the 2C discharge capacity. A step up from a resistor is a regulated current source. For example the 7135 regulator is used in many flashlights.

This is a 300 ma linear regulator that has a constant current output. A flashlight might have three of these so by pressing the “Clickey” switch you get 0, 300, 600 or 900 ma current into the LED. The most efficient way is to use a buck type Switching Mode Power Supply. The reason I say with an input higher than the LED voltage is because the capacity of most batteries decreases with increasing current. It’s possible to use a boost type SMPS with a single cell battery to drive a high brightness LED.

This has the effect of greatly lowering the capacity of the cell in exchange for a much smaller light. It costs more to use a Switching Mode Power Supply and so the lights are more expensive, but the battery life is much much better. Unfortunately most of the advertisements don’t tell you if they use a resistor, linear regulator or SMPS type supply. High Brightness LED flashlights on the market have been designed more for advertising value than for benefit to the user.

Also See the Flashlight Patents page for tube sizes and other construction info. High Brightness LED Philips Luxeon aka. 4 type LED that runs at 20 ma. A white LED may have a forward voltage of 3. 2 v and with a current of .

1 Watts most of which is heat that needs to be dissipated. Oct 2007 – Phillips now has the Rebel which is a small Surface Mount Technology LED on a small heat sink. It’s designed to be used on a normal printed circuit board with a pattern of over a dozen plated through holes nearby to act as the heat sink. These appear to be high brightness LEDs packaged in the clear plastic T10 package. These packages have a built-in lens so the candela rating is very high. They have special mounting requirements and heat sink needs, which so far it’s not clear how to meet.

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The first generation emitters have a Tjunction max of 120 deg C and so need more heatsinking than the newer K2 emitters which have Tjmax of 180 deg C. 12 Feb 2007 – This is a first generation 1 watt emitter, the LXHL-BW03. It’s a warm white color that is very pleasing, not like the white that has the excess blue. I have it rubber banded to an aluminum bracket and the bracket does not get even warm.

Design buck converter to meet output ripple voltage specs

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Note at cold turn on the Vf is 3. I have this LED hung off the arm supporting a halogen desk light. The LumiLED is considerably dimmer than the 50 watt bulb, but it’s adequate for seeing what I need to see and the color is pleasing. All the fuss about hermetic storage relates to wave soldering. If an emitter is left out in the open it can get moisture inside, then when it’s wave soldered it may explode. But this isn’t a problem for hand soldering. It’s not a problem for the “Star” parts that are already solderded to an aluminum heat sink.

Efficiency is measured in Lumens per Watt. Here are some numbers for comparison. 50 deg C rise in temp. There is a tradeoff related to what current is used to drive a K-2. At higher currents you get more light.

But heat sinking comes into play at the higher currents. 1 – these 3 are all the same warm White 1 Watt emitter in 3 different packages. The emitter is the raw LED. The star is an emitter mounted to a PCB with an aluminum layer. The Star-O has a small parabolic reflector.

2 – the power supply has a 1 Amp current limit so did not test at 1. 1 Amp at turn on, then 3. 3 – the voltage drops to 3. 77 after a minute or so. 9 deg C temp rise or 2. I used a new bracket and applied a good size gob of silicon grease to this one.

K2 is good up to 1. 350, but can put out more light at 1. So the LM3402 can drive both types. The current in the LED is regulated at 354 ma and the Vf is 3. 046 so the power is constant at 1.

Note1 – At low input voltages the regulator is not putting out the full voltage and so the LED is not getting the constant power it does for higher input voltages. Note that driving a single LED is the lowest efficiency because most of the loss is independent of the drive voltage because of the constant current. So adding more LEDs improves the efficiency. The current is the same as before 354 ma but the voltage is now 6. 48 for a power of 2. 1 – although the LEDs will light at voltages below 12 volts, they are not as bright as at higher voltages so it’s difficult to calculate the efficiency. Low Current Operation I don’t have a good way to measure brightness so I’ll describe in words what I’ve observed.

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The efficiency gets better as the current is lowered. So if you run a 1 Watt LumiLED at 20 ma, it will put out more light than a 5mm type high brightness white LED. AA Battery 20 Feb 2007 – Normally  you would not use AA batteries to power a LumiLED that draws 350 ma. This is because at that current the batteries would last between 90 minutes and a few hours. Note that this is the case independent of the number of series batteries when a linear voltage regulator, like the LM317, is used. BUT it is what done in most of the cheap LED flashlights. So if 10 AA cells are used to drive a single LumiLED at 350 ma the battery current is 100 ma.

The Constant Current Performance chart on the Energizer E91 data sheet shows a run time of 15 to 30 hours for a 100 ma load. 38 watts and divide by 10 cells or 0. The run time ranges from 12 to 20 hours. Now looking at the Energizer L91 data sheet. At 350 ma constant current the run time is 8 hours and at 100 ma constant current is 30 hours. 38 watts Constant Power the run time is more like 40 Hours.

The L91 was designed to supply SMPS supplies where the current increases as the battery discharges. On the E91 Constant Current curve note that the slope gets steeper above 100 ma. A change in slope represents a change in amp hour capacity. So you get more amp hours when running at or below 100 ma. So by using a SMPS the battery lasts more than 4 times longer. It’s very hard to take a photo of a 1 watt LED operating becasue it makes a lot of light. This photo gives an idea of the relative brightness but is not in focus.

Single Cell Battery There are applications where the 1 watt LED needs to be powered by a single cell. The cell voltage varies with the chemistry from 1. 2 v or Ni-MH to 3. There are SMPS that are designed to raise the voltage but they do it by drawing more  current from the battery than the load consumes. As the current drawn from a battery increases the amp hour capacity decreases. Observations When any of these, except the Star-O which had the emitter hidden, are used in a way where you can directly see the emitter it’s not comfortable because of the brightness.

To my eye the brightest by far is the PW14 emitter. So far Philips has not offered the K2 in any of the “Star” packages which mount the emitter on a PCB that has an aluminum core that acts as a heat sink. One of the reasons for this may be that the max temperature on the K2 emitters is much higher than on the other types allowing it to be mounted on top of a normal FR4 PCB. Power Supply The high brightness LEDs are best driven by a current source.

There are a number of different ways to do that. Cheap and dirty is to use batteries that have enough internal resistance to limit the current to a safe level. The problem with this is that you are dissipating a lot of power in the 317 and if near the LED the heat combines. Use a Switching Mode Power Supply. This has a big advantage in terms of efficiency. If the source is a battery, like the BA-5590 family that has 12 or 24 volt capability then the SMPS is working in the buck mode where the input current is lower than the LED current by about the ratio of the voltages.

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White LEDs really are a blue LED that illuminates a phosphor. The light contains a lot of UV and can burn  your eye  just like looking into an arc welder. It’s been 4 days and I’m no longer suffering like I was when this was written. It may be that it’s just the point source visible light that’s causing the problem.

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I’m looking into what caused it. The company has many patents going way back in the area of ceramic circuits like used for microwave components. The key technology here is the ability to get a good heat path through an insulator. The disk has four LED chips mounted in a square pattern. At 1 amp forward the Vf is 7. LED, but I like the feel of the warm light.

The data sheets have quite a bit of data at 350, 700 and 1000 ma. The photo at left was taken with 7. LED was so bright that the flash did not fire and the image came out mostly black. I’ve tweaked it in photoshop to make it viewable. Thermal Joint Compound, not the 120 Lamina recommends. The way I read the specs this is twice as good when first installed and even better after the 120 has aged for 6 months. Since the disk with the LED needs to be attached to the heatsink using thermal grease and held in place with screws you can not attach the optic until that’s done.

So to make a task light some sort of can needs to be added to what you see in the photo. There are four LEDs inside the Lamina LED assembly. I’ve had it out in my office area and you can see that it’s covered with dust. Note the use of 2-56 Round Head screws since they have a smaller head diameter than pan heads. If you look at the back of the optic you can see the clear plastic part that has a hemispherical hole that fits over the LED silicon. The optic is actually seated on the silicon and can be rocked a little. Drilling holes in the heat sink would serve no purpose.

Design buck converter to meet output ripple voltage specs

A better solution is to put the star LED board and optic into a aluminum can from the front. The parts shown are available as an assembled board that has a combined buck Switching Mode Power Supply and a micro controller to allow programming the drive current and a number of handy user functions and the assembled LED head on a 6″ length of Loc-Line. The Loc-Line is stiff enough to hold the head where you point it. Much superior to the old flex metal goosenecks that had a lot of built-in spring.

The relative scale of the Task Light and the Lamina Atlas in the above photo is close to accurate. The Task Light head is about 1″ dia and the Lamina 10 deg optic is about 2″ dia. There are a number of settings that relate to batteries such as warning when the battery voltage goes below some value or an auto off mode that will turn the light off after some time period. LED will blink to acknowledge the commands.

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Not the most user friendly interface, but just the thing for a bicycle or other application where once setup very simple button presses suffice. I got this as a universal lamp for testing various flashights. Specified to be 1 watt with a 2. 8 to 12 Volt input range, 35 Lumen 120 degree beam.

E10 based lamps do not have pre focused filaments and many flashlights have a way to change the axial position of the lamp to find focus. That’s the next test scheduled for this lamp. But didn’t expect it to be so high tech. Note 1 for voltages between 2. 3 the brightness is increasing and constant for higher voltages. This type of load curve incicates a Switching Mode Power Supply, not a simple resistor or linear voltage regulator.