What Happens When an LED Is Driven at Higher Current?

A customer once asked us to test the same constant-current LED module at three different current levels:

300 mA, 500 mA and 700 mA.

The PCB dimensions and LED quantity had already been specified by the customer, and NKT built the sample accordingly.

At the 700 mA module test point, the current through each LED was approaching 200 mA because of the module's series-parallel circuit configuration.

The LED used in this module was a Bridgelux SMD 2835 0.2W 3V. Its datasheet lists a nominal drive current of 65 mA and a maximum drive current of 240 mA.

So the individual LED current at the highest test point was still below the manufacturer's maximum rating.

But that does not answer the more useful engineering question:

Does it make sense to operate the LED that hard if the application does not require it?

That is why LED drive current should not be selected from the maximum-current rating alone.

Nominal Current and Maximum Current Tell Us Different Things

A maximum drive current tells us an important limit for the component. It does not automatically tell us where the LED module should operate.

For this Bridgelux SMD 2835, the difference between nominal and maximum current is substantial:

Nominal drive current: 65 mA
Maximum drive current: 240 mA

The datasheet also provides performance information at several current levels between these points.

That information is more useful than looking only at the maximum rating because it shows what changes as the LED is driven harder.

For the 4000K version, the manufacturer provides the following typical data:

Source: Bridgelux SMD 2835 0.2W 3V Product Data Sheet, DS503 Rev. A (05/2021), 4000K typical performance data. These are manufacturer datasheet values, not NKT test results.

There is a clear tradeoff in these numbers.

At 40 mA, the listed typical pulsed flux is 22.3 lm with an efficacy of 208 lm/W. At 150 mA, flux increases to 81.1 lm, but efficacy decreases to 189 lm/W.

So increasing current clearly produces more light from each LED.

But it does not produce that additional light at exactly the same efficiency.

This is one reason we do not look at an LED's maximum drive current and simply treat that number as the target.

Why We Questioned the 700 mA Test Point

The 300 mA, 500 mA and 700 mA values in this project were requested by the customer.

They were module test currents, not three current levels selected by NKT during the original design.

That distinction is important because module current and individual LED current are not necessarily the same number.

The module used a series-parallel circuit. At the 700 mA module test point, the resulting current through each LED was approaching 200 mA.

NKT tested the sample with an integrating sphere at the requested current levels. For this white-light module, the measurements included voltage, current, luminous flux, luminous efficacy, CCT and CRI.

After the testing, we provided the results to the customer and also advised avoiding such a high per-LED current if the application did not require it.

This was not because the test point exceeded the manufacturer's maximum current—it did not.

It was because “below maximum” and “appropriate operating point” are not the same engineering decision.

The Bridgelux datasheet itself illustrates why. As current increases, light output rises, but efficacy changes as well. Forward voltage also changes with current.

There is another reason not to turn this into a simple rule that “high current is bad.” Bridgelux includes reliability testing at 240 mA under defined test conditions in the same product datasheet. A high current should therefore not automatically be described as unsafe simply because it is close to the maximum rating.

The real question is whether that operating point makes sense for the module and the final application.

In this project, 700 mA was a requested test condition, not a confirmed final operating current. NKT's scope was to provide the requested module-level performance measurements. Final power-supply configuration and operating conditions were determined at the customer's system level.

If We Were Choosing the Operating Point From the Beginning

The design process is different when NKT is asked to develop a module around a target lumen output and known electrical requirements.

Suppose the PCB dimensions are fixed and the module needs to reach a certain luminous flux.

Increasing current is one possible way to obtain more light from the existing LEDs.

But it is not the only option.

If PCB space permits, we may instead consider using more LEDs and operating each LED at a lower current.

In other cases, a higher-efficacy LED may be a better option.

The decision depends on what the product actually requires. We look at the target lumen output, LED quantity, available PCB area, operating current and electrical configuration together rather than optimizing one parameter in isolation.

There is no NKT rule saying an LED should always operate at a fixed percentage of its maximum current.

Sometimes PCB dimensions are fixed. Sometimes LED quantity is fixed. Sometimes both are already specified by the customer. The practical design options change with those constraints.

This is why the answer to “How much current should we use?” often depends on the rest of the module.

Current Also Changes the Electrical Design

The same Bridgelux data shows another effect that matters when developing a module.

Typical forward voltage changes from 2.68 V at 40 mA to 2.78 V at 100 mA and 2.86 V at 150 mA.

For one LED, that difference may appear small.

Across multiple LEDs connected in series, however, it becomes part of the LED module's total forward-voltage calculation.

Series-parallel configuration matters too.

The customer example in this article is a useful illustration: the module was tested at 700 mA total current, while the individual LED current was approaching 200 mA because of the way the circuit was configured.

So when reviewing current, we also need to understand the series-parallel circuit configuration.

For a constant-current module, the resulting series voltage must also work within the constant-current driver's output-voltage range.

Current, LED quantity, forward voltage and circuit architecture are connected decisions.

Changing one can affect the others.

Datasheet First, Then the Actual Module

LED datasheets are where we start.

They allow us to estimate light output, forward voltage and electrical conditions before the first PCB is assembled.

But an OEM customer ultimately uses a module, not an individual LED from a datasheet.

Once a prototype is available, testing the actual LED module lets us compare the initial calculations with measured module performance.

That was the purpose of the 300 mA, 500 mA and 700 mA tests in this project.

The customer received actual performance data from the assembled module at the requested current points rather than relying only on component-level calculations.

The final product adds another layer. Housing, heatsink and operating environment affect thermal performance, so those conditions need to be evaluated in the complete application rather than assumed from LED current alone.

For us, that is the more useful way to think about LED drive current.

The question is not how close we can get to the maximum rating.

It is whether the combination of current, LED quantity, light output, efficacy, forward voltage and circuit configuration makes sense for the product being developed.

If you are working with defined PCB dimensions, mounting requirements, target lumen output or electrical parameters, you can review NKT's LED module engineering capabilities and our approach to custom OEM LED module development.

For a specific project, send us your LED module requirements.

Technical Reference

Component performance values referenced in this article are based on the Bridgelux SMD 2835 0.2W 3V Product Data Sheet, DS503 Rev. A, May 2021.

The 40–150 mA performance values shown above are Bridgelux manufacturer datasheet values for the 4000K configuration and are not NKT test results.

The 300 mA / 500 mA / 700 mA module test is from a separate NKT customer project. These values represent customer-requested module test conditions and should not be interpreted as Bridgelux recommended operating currents or as confirmed final operating conditions for the customer's product.