LED vs. regulator efficiency

Has anyone seen any analysis comparing the contribution of the LED die type & bin with that of the regulator & electronics to overall flashlight performance?

In other words how important is the regulator and electronics to getting a bright and efficient flashlight? On this forum people have tended to harp on emitter and bin chase (though both less so now than years ago when it was central) maybe because those are spec'd with data available from the manufacturer.

The electronics must be important — otherwise lights using the same led & bin would perform the same — except we know that's definitely not true.

Forgive any ignorance about flashlight drive circuitry, as only just started looking at that yesterday...

Some (many? most?) drive circuits use switching regulator ICs and those I've looked at do have slightly different efficiency curves with peak efficiencies between approximately 80-95%.

Another area that would come into play are the FETs which have varying drain to source pin resistance specified by Rdson. The FETs I've looked at for my robotics stuff vary from milliohms to an ohm or two. Rdson comes up among RC car nerds talking about their ESCs (electronic speed controllers)

The power lost to the FET varies linearly with Rdson and by the square of current. P=(I^2)Rdson.

OP, I think your thread would get more attention from the electronics experts here on CPF, if it were in the "Flashlight Electronics" subforum.

If you would like it moved, just let staff know.

Thanks, sure pls move if better placed there.

Illumination said:

.............The electronics must be important — otherwise lights using the same led & bin would perform the same — except we know that's definitely not true.

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Ya, I've done a lot of run time graphs comparing different flashlights with the same emitter. The difference is definitely the driver when all else is equal. Being an electronics guy, part of the hobby for me was in finding very efficient flashlights with well regulated output.

hiuintahs said:

Ya, I've done a lot of run time graphs comparing different flashlights with the same emitter. The difference is definitely the driver when all else is equal. Being an electronics guy, part of the hobby for me was in finding very efficient flashlights with well regulated output.

Click to expand...

This may be an aside to the original question, but I'm curious to know what the most efficient flashlights/drivers you found are through your testing.

Illumination said:

Has anyone seen any analysis comparing the contribution of the LED die type & bin with that of the regulator & electronics to overall flashlight performance?

Click to expand...

Such an analysis, if possible, is not necessary. If necessary, it's not possible (or at least not easy).

I don't say that facetiously. Take 2 lights, identical except for the LED. In order to analyze the effect of the LED, everything else about the light, i.e. driver, battery, etc, must be well understood and predictable over the range of conditions caused by the differing LED specs. If you have this, you simply look up the performance in the LED data sheets. If you don't, good luck making a comparison.

That said, there are some rules of thumb that might be useful to you. They are different for each type of driver, and whether you are in 'regulated' mode or not.

DIRECT DRIVE, FET DRIVERS, and RESISTOR LIMITERS

These are all basically the same thing, except that FET drivers can use PWM to change the apparent brightness. The circuit consists of a battery, an LED, and a bunch of resistors. The battery voltage minus the LED voltage, divided by the sum of resistors equals current. Unfortunately, it's hard to know the resistance of a battery or an LED, especially because they vary with state of charge, age, temperature, and current. The output of these lights drops continuously (though not linearly) from the moment you turn them on, until you turn them off. The LED's Vf and resistance have a huge impact on the initial brightness.

Because the efficiency of such a system is rather difficult to define in a way that everyone would agree on, much less find useful, it's rarely discussed with these lights. Maximum output and maximum current are the most commonly discussed characteristics here.

With the latest generations of LiIon cells and LEDs, it's now fairly easy to build a light that will fry the LED within seconds or minutes of turning it on.

LINEAR DRIVERS

These include the common AMC7135 based drivers that are found everywhere. A linear driver has a current sensor, a variable resistor, and a control circuit. The '7135 integrates this all into a single chip. The control circuit adjusts the resistor until the current is at the desired level. As the battery voltage drops, the resistance is reduced to keep the current the same. Of course the resistor has a minimum value, and if the battery voltage falls too low to supply the set current value, the current will drop below the setpoint, and the driver now looks like a resistor limiter. This is the point where the driver 'drops out' of regulation.

Of course the 'variable resistor' is actually an FET. The difference between linear drivers and FET drivers is that in a linear driver the FET is operated in its linear region where the resistance changes with gate voltage. In a FET driver, the FET is operated in the saturation region, where the gate voltage has minimal impact on the resistance. The point where a linear driver drops out of regulation is the point where its FET enters saturation.

When regulating, these types have an efficiency that's equal to the output voltage divided by the input voltage. Interestingly an LED with higher Vf causes the regulator to operate with higher efficiency. Unfortunately this generally corresponds to lower efficacy in the LED, so light output is not increased. Conversely, a lower Vf means lower efficiency and higher power dissipation in the driver, but because of the higher efficacy of the LED, you may get a modest increase in output. Here, it's lumens/amp or lumens/milliamp that matter. While this can usually be calculated from the datasheet, it's rarely specified directly.

Also note that as the battery voltage drops, the efficiency of the regulator increases. I've seen efficiencies go over 99% as the regulator approaches and enters the dropout region. As I recall, the '7135 tops out around 94-95%. But with a battery initial voltage of 4.0V and an LED voltage of 3.0V, the initial efficiency is only 75%.

SWITCHING DRIVERS

Switching drivers, by using magnetic devices (inductors, usually, but transformers can be used too) to store energy and translate voltages, can overcome the efficiency limitations of the linear driver. It's possible to top 95% over a broad range of conditions. Unfortunately, doing so requires complex circuitry, and it's increasingly difficult to do as the voltages decrease. At single cell voltages, it's difficult to top 90% in a simple driver (though modern ICs are making it possible, only high-end drivers will use these). Low-end drivers will typically be in the 80-90% range, while super-cheap ones are likely to be less, especially at high currents.

Keep in mind that any given switcher will have a range of efficiencies. Low voltages and high currents push efficiencies down.

Here Vf has a different effect. It shouldn't change the output, as this is determined by current, which should be unchanged while the driver is in regulation. But it does change the power output of the driver, which in turn changes the power input, which changes the load on the battery. A 10% drop in Vf should result in about 10% increase in battery life.

I hope this helps. I know it's a lot to absorb; feel free to ask questions (even a lot of them) if need be.

D

FLfrk said:

This may be an aside to the original question, but I'm curious to know what the most efficient flashlights/drivers you found are through your testing.

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Typically what I have found is that efficiency follows price. The Fenix, Nitecore, Jetbeam, EagleTac, Sunwayman, Olight, etc tend to do better compared to those flashlights that cost way less. They are all fairly comparable if they have the same LED and the output is similar when comparing. I know Zebralights are very efficient, but I have steered away from them due to price. One area that I have tested a lot of lights is in the single AA. I haven't tested so much with the 18650 lights. I've tested a few CR123A/16340 lights. In the top end of the budget category, The latest arrivals of the Lumintop tool AA 2.0 and the Sofirn SF14 do just about as well, though the graph of light isn't as smooth.........meaning probably not as much output capacitance on the driver...............but doesn't matter since your eyes can't detect it and the substantial less price more than makes up for that. The Jetbeam Jet I-MK is another that is fairly inexpensive and performs very well. I've pretty much stayed away from PWM lights.

My flashlight hobby has been dropping off over the last year and so haven't kept up with some of the newer lights. I have really taken a liking to Sofirn lights lately. Overall I own 25 or so lights in my possession right now but I have to say that I've gone through probably 100 or more. I tend to sell the lights that I didn't think were as efficient, I didn't like the user interface, mode spacing wasn't to my liking or I didn't like the tint of the lights.

If you do a Google search for "hiuintahs candlepowerforums run time test" you will find some of the graphs that I have posted.

With a single lithium ion and a led with a vf near 3.3v (typical cree single die) - is buck or linear more efficient?

It would depend a lot on the design of the regulator, and what you expect to happen when the battery discharges to near 3.3V. If you expect to continue running and have gradually reducing output, then the overhead of the regulator will determine the efficiency.

Overhead is the minimum input-to-output differential.

Either type can have fairly high or nearly zero overhead voltage. With low overhead voltage the efficiency can be very high for either design (I've seen 99% in both types I've built). With higher overhead, you will see decreased output sooner in the discharge, and you will probably never see very high efficiencies (it's impossible in a linear, unlikely in a buck). Overhead voltages can range from a few millivolts to a full volt, even two volts.

Unfortunately, overhead voltage is rarely specified for inexpensive drivers. Occasionally, you will get this spec incorrectly (IMHO) listed as dropout voltage. The correct use (again, IMHO) of the term dropout voltage is the input voltage below which the driver cannot maintain regulation. At this point, the output voltage plus the overhead equals the dropout voltage.

Notice all these are voltages. LED drivers regulate current, not voltage. The LED determines the voltage. This throws a bit of a spanner in the works. If that's not bad enough, most of the specs depend on operating conditions.

It's not impossible to get useful information, it's just that most driver manufacturers don't give it to you. Primarily, it's high-end manufacturers that publish proper data sheets that will give you what you need. Think LumiLeds, TaskLED, and the like.

Lastly, if you think the typical Cree die is 3.3V, you are either pushing them fairly hard, or you are using data from some years ago. Most of the Cree datasheets I've looked at in the last few years show nominal voltages around 3.0V or less. That has significant impact on this discussion, but does not invalidate any of it.