Comparing effects of using pulsed and steady state power to illuminate a high brightness LED

I was having a discussion here with a colleague about the merits of powering a high brightness LED (HBLED) using pulsed power versus using steady state DC power.

My opinion was: “Basically, amperes in proportionally equates to light flux out, so you will get about the same amount of illumination whether it is pulsed or DC.”

His argument was: “Because the pulses will be brighter, it’s possible the effective illumination that’s perceived will be brighter. Things appear to be continuous when discrete fixed images are updated at rates above thirty times a second, and that should apply to the pulsed illumination as well!”

I countered: “It will look the same and, if anything, will be less efficient when pulsed!”

So instead of continuing our debate we ran a quick experiment. I happened to have some HBLEDs so I hooked one up to an N6781A DC source measure module housed in an N6705B DC Power Analyzer sitting at my desk, shown in Figure 1. The N6781A has excellent current sourcing characteristics regardless whether it is DC or a dynamic waveform, making it a good choice for this experiment.

Figure 1: Powering up an HBLED

First we powered it up with a steady state DC current of 100 mA. At this level the HBLED had a forward voltage drop of 2.994 V and resulting power of 0.2994 W, as seen in Figure 2, captured using the companion 14585A control and analysis software.

Figure 2: Resulting HBLED voltage and power when powered with 100 mA steady state DC current

We then set the N6781A to deliver a pulsed current of 200 mA with a 50% duty cycle, so that its average current was 100 mA. The results were again captured using the 14585A software, as shown in Figure 3.

Figure 3: Resulting HBLED voltage and power when powered with 200 mA 50% DC pulsed current

Switching back and forth between steady state DC and pulsed currents, my colleague agreed, the brightness appeared to be comparable (just as I had expected!).  But something more interesting to note is the average current, voltage, and power. These values were obtained as shown in Figure 3 by placing the measurement markers over an integral number of waveform cycles. The average current was 100 mA, as expected. Note however that the average voltage is lower, at 2.7 V, while the average power is higher, at 0.3127 W! At first the lower average voltage together with higher average power would seem to be a contradiction. How can that be?

First, in case you did not notice, the product of the RMS voltage and RMS current are 0.3897 W which clearly does not match our average power value displayed. What, another contradiction? Why is that? Multiplying RMS voltage and RMS current will give you the average power for a linear resistive load but not for a non-linear load like a HBLED. The average power needs to be determined by taking an overall average of the power over time computed on a point-by-point basis, which is how it is done within the 14585A software as well as within our power products that digitize the voltage and current over time. Second, the average voltage is lower because it drops down towards zero during periods of zero current. However it is greater during the periods when 200 mA is being sourced through the HBLED and these are the times where power is being consumed.

So here, by using pulsed current, our losses ended up being 4.4% greater when powered by the comparable steady state current. These losses are mainly incurred as a result of greater resistive drop losses in the HBLED occurring at the higher current level.

There is supposed to be one benefit however of using pulsed power when powering HBLEDs. At different steady state DC current levels there is some shift in their output light spectrum. Using pulsed current provides dimming control while maintaining a constant light spectrum. This prevents minor color shifts at different illumination levels. Although I would probably never notice it!

Consider using an electronic load for generating fast, high-power current pulses

Often there is the need for generating high-power current pulses, typically of short duration, and having rise and fall times on the order of microseconds. This is a common need when testing many types of power semiconductors, for example.

When looking for a DC power supply capable of generating very fast, high-power current pulses, one will find there are not a lot of options readily available that are capable of addressing their needs. There are specialized products dedicated for specific applications like this; an example of this is Keysight’s B1505A purpose-built semiconductor test equipment. They are capable of generating extremely fast, high-power current pulses.

Apart from specialized products however, DC power supplies generally to not offer this kind of speed when operating in a constant current mode (or current priority mode). One exception that comes to mind that we provide is our N6782A and N6782A DC source measure modules. They can create fast current pulses having just a couple of microseconds of rise and fall time. However, they are limited to 20V, 3A, and 20W of output. Most of the higher power, more general-purpose DC sources are not able to generate these kinds of fast, high-power current pulses and most are really more optimized to operate as voltage sources.

One alternative to consider for generating fast, high-power current pulses when working with general-purpose test equipment is to use an electronic load. You may initially say to yourself “an electronic load is for drawing pulses of current, not sourcing them!” but when coupled to a standard DC power supply operating as a voltage source, the setup is able to source fast, high-power current pulses. Most electronic loads are designed to have very fast current response. To illustrate this, I helped one customer needing to test their high brightness LED (HBLED) arrays with fast pulses of current. This was accomplished with the setup shown in Figure 1.

Figure 1: Load setup generating fast, high power current pulses for LED array testing

In this setup the power supply operates as a fixed, static voltage source. The power supply’s output voltage is set to the combined total of the full voltage needed to drive the HBLED array at full current plus the minimum voltage needed for the electronic load. The minimum voltage required for the electronic load is when it conducting maximum current and most of the power supply voltage is then applied across the HBLED array. The electronic load’s required minimum voltage is that which supports its operation in its linear range and maintains full dynamic response characteristics. In the case of Keysight electronic loads this minimum voltage for linear dynamic operation is 3 volts.  Conversely the maximum voltage required for the electronic load is when it drops down to minimum current level, where the power supply’s voltage is instead now being dropped across the electronic load instead of the HBLED array. Note that the electronic load may need to maintain a very small amount of bleed current to maintain linear operation in order to provide truly fast rise and fall times. In this way the electronic load is able to regulate the current across the full range with excellent dynamic response. This can be seen in Figure 2 where we were able to achieve approximately 15 microsecond rise time right from the start.

Figure 2: Pulsed current rise time in HBLED array

One advantage of this setup is the wide range of voltage and power that can be furnished to the DUT using a relatively low power electronic load. A common characteristic of electronic loads is that they can dissipate a given amount of power over an extended range of current and voltage. When the electronic load is at maximum current it is at minimum voltage. Conversely when it is near or at zero current it is then at its maximum voltage. In both cases there is only a small amount of power that the electronic load needs to dissipate. For an HBLED array it does not conduct a lot of current until it reaches about 75% of its full operating voltage. As a result the electronic load does not see a lot of power even on a transient basis. For this particular situation we chose to use the Keysight N3303A 240V, 10A, 250W electronic load. This gave a wide range of voltage, current, and power for testing a comparably wide range of different HBLED array assemblies.

So next time you need to source fast, high-power current pulses, you may want to think “load” instead of “source”!

Quickly Measure a High Brightness LED’s (HBLED) Forward Electrical Characteristics

It’s not hard to notice (or extremely hard not to notice!) how high brightness LEDs, or HBLEDs, are quickly becoming commonplace all around us in our daily lives. LEDs are no longer relegated to being an indicator light on a display panel. HBLEDs have drastically ratcheted up their output to become sources for illumination.  More and more autos use them for their tail and brake lights. It’s easy to see the “instant on” they have when the auto in front of you hits its brakes, not to mention the deep purity of color they have in comparison to the incandescent predecessors.  They are also turning up in the headlights, the traffic lights, even high power street and parking lot illumination lights, and in countless other places. A lot of testing, characterization, and development work has, and continues to take place, to achieve this level of performance from HBLEDs. This includes making careful measurements of electrical power being provided and the corresponding luminous efficacy outputted, in order to assess its performance.

In my title above I am using the term “quickly” for two reasons in my posting today. First, it is important when trying to capture the forward characteristics of an HBLED that it is performed in a minimum amount of time in order to minimize temperature change due to self-heating.  The temperature an HBLED is running at has in impact on its performance. Minimizing the amount of temperature change improves accuracy of test results in determining the performance of the HBLED, for a given operating temperature. My second reason for using quickly is providing a means to make these HBLED measurements with just a little time and effort.

It turned out using the N6784A four-quadrant SMU module in an N6705B DC power analyzer mainframe worked out really well on both counts of quickly. This set up is depicted in Figure 1.

Figure 1: HBLED test characterization set up

While the N6784A is an extremely fast voltage source it is even a faster current source. With current rise and fall times of just a few microseconds was a simple matter to generate sub-millisecond-long high amplitude pulses of current with fast settling edges to provide the necessary stimulus for performing the forward electrical characterization of the HBLED. This allowed testing to take place in minimum time and avoid significant heating of the HBLED die.

One of the outcomes of the testing is shown in Figure 2, displayed graphically by the 14585A software.  Here a ramped current pulse was used instead of a flat top pulse. The HBLED’s voltage and current were simultaneously digitized as the current was ramped up. This gave a way of characterizing the HBLED’s forward voltage drop for all levels of drive current, from zero to maximum.

Figure 2: HBLED forward characterization results

The N6705B DC Power Analyzer mainframe and 14585A companion software made quick work of the setup, testing, and display of results.  A ramp waveform from the library of pre-defined ARBs was selected and used to generate the current ramp. In this instance it was set to ramp up to 1.2 amps in 1 millisecond. The oscilloscope mode was used to set up the simultaneous capture of voltage and current, synchronized to the current ramp stimulus. As voltage and current were captured it is also a simple matter to display the power, being the point-by-point product of the voltage and current. The electrical power in can then be correlated with a light output measurement on the HBLED for evaluating its performance.

Not only is this setup able to measure the HBLED’s forward characteristics, as the N6784A can source negative voltage and measure down to nanoamp levels it can quickly test the HBLED’s reverse leakage characteristics as well.