When designing a custom LED module, total power alone does not determine how the module will behave thermally.
The same light-output target may be achieved with fewer LEDs operating at higher current or with more LEDs operating at lower current. Both approaches may work electrically, but they do not necessarily produce the same luminous efficacy, heat distribution, optical uniformity, or PCB requirements.
This becomes especially important in OEM projects where the PCB dimensions are already limited by the customer's product.
A practical design question is therefore:
Should a fixed PCB use fewer LEDs at higher current, or more LEDs at lower current?
The answer depends on the LED characteristics, available PCB area, target light output, operating current, component cost, circuit configuration, and the thermal path provided by the final product.
For a custom LED module, these factors should be evaluated together rather than independently.
1. Why PCB Size Changes the Design Options
In many OEM projects, the PCB dimensions are determined before the LED layout is developed.
The board may need to fit an existing enclosure, mounting pattern, optical cavity, or mechanical structure. In other projects, the customer may allow some adjustment to the PCB dimensions.
The available PCB area affects how many LEDs can reasonably be used and how widely they can be distributed.
If two modules operate at similar total power but one distributes its LEDs over a larger area, the larger board generally provides more opportunity to spread the heat sources rather than concentrating them into a smaller region.
It may also provide enough space to increase the LED quantity and reduce the operating current required from each LED.
However, this does not mean that a larger PCB automatically guarantees a lower final operating temperature.
The final thermal condition also depends on factors outside the LED PCB, including the mounting structure, thermal interface, housing or heatsink, airflow, ambient temperature, and operating environment.
This is why PCB dimensions are an important part of thermal design, but they cannot be used alone to predict the final product temperature.
For a broader discussion of these factors, see our guide to LED module overheating → Why Is My LED Module Overheating? Common Causes and Design Solutions.
2. LED Density Is Not Simply “More LEDs per PCB”
LED density is sometimes treated as a simple number: how many LEDs can fit onto a certain PCB area.
In an actual OEM module, it is more useful to consider three related questions:
- How many LEDs are required?
- How much current will each LED need?
- How can those LEDs be distributed across the available PCB area?
NKT normally tries to distribute LEDs as evenly as the mechanical design allows.
In practice, a PCB may also contain mounting holes, connectors, screws, board edges, or other components that interfere with the ideal LED pattern. A perfectly uniform layout is therefore not always possible.
The LED quantity also affects the required operating current.
If a customer wants a specific luminous output from a fixed PCB, using fewer LEDs generally requires each LED to contribute more light. That usually means increasing the current through each LED.
If PCB space allows additional LEDs, the required total output can instead be distributed across more LEDs operating at lower individual currents.
That trade-off is one of the reasons LED quantity and PCB layout should be determined together with the electrical and mechanical design.
3. Fewer LEDs at Higher Current Are Not Always the Better Design
Reducing LED quantity may appear attractive because fewer components are required.
But the fact that an LED can operate at a certain current does not automatically mean that this current represents the best operating point for the complete module.
A real Bridgelux SMD 2835 used in one of our OEM projects provides a useful example.
For the Bridgelux BXFN-XXG-21L-3C4 series, the datasheet specifies a nominal drive current of 65 mA.
The manufacturer also provides performance information at several commonly used drive currents.
For the 4000K version:
| Drive Current | Typical Pulsed Flux at 25°C | Typical Efficacy at 25°C |
|---|---|---|
| 40 mA | 22.3 lm | 208 lm/W |
| 60 mA | 33.4 lm | 205 lm/W |
| 100 mA | 55.0 lm | 198 lm/W |
| 120 mA | 65.5 lm | 194 lm/W |
| 150 mA | 81.1 lm | 189 lm/W |
These are manufacturer reference values under the stated test conditions rather than guaranteed finished-module performance.
The pattern is useful:
Higher current produces more light from each LED, but luminous efficacy decreases as the current rises across these listed operating points.
This is important when determining LED quantity.
The objective is not simply to ask:
“How much current can this LED handle?”
A more useful design question is:
“What LED quantity and operating current provide an appropriate balance for this module?”
For more background on the relationship between output and luminous efficacy. See our related LED performance article.
4. A Real OEM Example: 65 mA Nominal Current vs. Approximately 180 mA
We encountered this trade-off in a project for a European OEM customer.
The customer's proposed module used a relatively small number of Bridgelux 2835 LEDs. To achieve the required output with that LED quantity, the calculation resulted in an operating current of approximately 180 mA per LED.
This requires an important distinction.
For this Bridgelux series, the datasheet specifies:
Nominal drive current: 65 mA
Maximum drive current: 240 mA
The approximately 180 mA operating point was therefore still below the manufacturer's specified maximum drive current.
So the concern was not that the customer's proposed current exceeded the LED's maximum rating.
Instead, the engineering question was whether operating a relatively small number of LEDs at approximately 180 mA was the best overall module design when the PCB still had space for additional LEDs.
NKT explained the trade-offs of the higher-current approach and recommended evaluating an increased LED quantity with a lower operating current per LED.
After the potential issues were explained, the customer accepted the adjustment.
This illustrates an important distinction between a component rating and a module-design decision:
A drive current can remain within the LED manufacturer's specified maximum rating without necessarily being the preferred operating point for a particular OEM module.
5. What Changes When More LEDs Are Used at Lower Current?
Increasing the LED quantity while reducing the operating current per LED can affect several parts of the module design.
Luminous Efficacy
The Bridgelux data provides a useful example.
For the 4000K version, the manufacturer's table shows approximately:
205 lm/W at 60 mA
versus
189 lm/W at 150 mA
under the specified test conditions.
That is a difference of roughly 8% between those two listed operating points.
This does not mean that lowering LED current will always improve efficacy by 8%, nor should this percentage be applied to other LEDs.
The amount depends on the particular LED and operating conditions.
It does demonstrate why NKT reviews the actual LED datasheet rather than assuming that fewer LEDs at higher current will provide equivalent efficiency.
Thermal Distribution
More LEDs can distribute the electrical and thermal load across a greater number of heat sources.
If those LEDs can also be spread across the available PCB area, heat generation is less concentrated in a small number of locations.
This does not eliminate the need for an adequate thermal path into the customer's final product, but it can provide a more distributed thermal condition at the PCB level.
Optical Uniformity
Additional LEDs may also make it easier to distribute light more evenly across the required area.
However, LED quantity alone does not determine uniformity. LED spacing, viewing angle, distance to the illuminated surface, lenses or diffusers, and the final optical structure also matter.
Where these factors are important, the LED layout and optical requirements should be considered together during module development.
Component Cost
More LEDs also mean additional component cost.
Therefore, the solution is not simply:
“Use as many LEDs as possible.”
The objective is to find a practical balance between LED quantity, operating current, efficacy, thermal distribution, optical performance, PCB space, and cost.
6. Energy-Efficiency Requirements Can Also Affect the Decision
For some OEM products, luminous efficacy is not only an engineering consideration. It can also affect the efficiency margin available to the customer's finished product.
This was relevant when discussing the higher-current approach with our European customer.
Driving fewer LEDs harder may reduce component count, but if LED efficacy decreases as operating current increases, the finished product may have less efficiency margin after driver losses, optical losses, temperature effects, and other system losses are considered.
For applicable light sources placed on the EU market, Ecodesign requirements under Commission Regulation (EU) 2019/2020 include maximum on-mode power requirements calculated using useful luminous flux and applicable correction factors. The regulation does not simply prescribe one universal minimum lm/W value for every LED product.
For the LED module designer, the practical point is simpler:
Operating current can influence LED efficacy, and LED efficacy can influence the efficiency margin available to the finished product.
This does not mean that a particular LED drive current automatically determines whether a finished product complies with EU requirements.
NKT does not determine regulatory compliance for the customer's finished product. The customer evaluates the requirements applicable to its target market, while NKT considers the requested electrical and optical performance when developing the LED module.
7. What If the PCB Dimensions Are Fixed?
A fixed PCB size is common in OEM development.
The customer may already have defined external dimensions, mounting-hole locations, an existing housing, connector positions, optical components, or other mechanical restrictions.
In this situation, NKT normally begins with the available PCB dimensions together with the customer's voltage range and target luminous output.
Based on the selected LED datasheet, we estimate:
- LED quantity,
- suitable operating current,
- expected voltage range,
- expected luminous output, and
- circuit configuration.
If the initial LED quantity requires relatively high current to achieve the target output, we can evaluate whether additional LEDs can be accommodated within the available PCB area.
If space remains available, increasing the LED quantity may allow each LED to operate at a lower current.
If the PCB is already physically constrained, other options may need to be considered, such as reviewing LED selection or the customer's product-level thermal design.
The appropriate solution depends on the project rather than on a fixed rule.
8. What If the PCB Size Can Be Adjusted?
Some customers allow the PCB dimensions to change slightly during development.
This provides additional design flexibility.
A modest increase in PCB area may make it possible to use more LEDs, distribute them more evenly, reduce individual LED current, or improve the overall layout.
However, increasing PCB size can also affect the customer's enclosure, mechanical structure, material usage, and assembly.
NKT therefore does not treat “make the PCB larger” as an automatic solution.
The PCB needs to fit the complete OEM product rather than being optimized independently.
9. PCB Material Is Another Part of the Thermal Path
PCB area and LED quantity are not the only factors that influence thermal behavior.
PCB construction also matters.
For LED modules that do not require complex routing, NKT may use an aluminum PCB to provide a practical thermal path between the LEDs and the customer's housing or heatsink.
FR4 may be appropriate when the electrical design requires more complex routing or other PCB characteristics.
In NKT's actual supply environment, standard aluminum LED PCBs can also be economically practical. Therefore, we do not automatically assume that FR4 will be the lower-cost choice.
The appropriate PCB construction depends on the electrical design, layout requirements, power level, mechanical integration, and thermal path of the specific module.
10. Circuit Configuration Must Still Work With the New LED Quantity
Increasing the LED quantity affects more than PCB layout.
The additional LEDs still need to form a practical electrical configuration.
The designer needs to consider LED forward-voltage range, available operating voltage, LEDs per series string, number of parallel branches, total module current, PCB routing, and connector requirements.
For this reason, adding LEDs simply to reduce individual LED current is not always practical.
The resulting LED quantity must still work with a suitable series-parallel LED configuration.
This is another reason LED quantity, PCB dimensions, operating current, and circuit architecture should be developed together.
11. Why We Do Not Predict Final Product Temperature From the PCB Alone
Even after PCB size, LED quantity, and operating current have been optimized, the LED module is still only one part of the complete thermal system.
The same LED module can operate differently depending on how it is installed.
The customer's housing, heatsink, thermal interface, airflow, ambient temperature, and operating conditions can significantly affect the final temperature.
The Bridgelux datasheet itself notes that actual performance may vary depending on the thermal design of the luminaire and the exposed environment.
For this reason, NKT does not claim to predict the exact operating temperature of the customer's finished product from the PCB design alone.
During prototype validation, NKT evaluates the electrical and optical performance of the LED module. Final thermal behavior in the complete product remains dependent on the customer's actual housing, heatsink, and operating environment.
For more information about the broader thermal path, see LED module thermal management.
12. So, Should You Use Fewer LEDs or More LEDs?
There is no universal answer.
Fewer LEDs may reduce component cost and simplify the physical layout.
More LEDs may allow lower operating current per LED, improve luminous efficacy, distribute the thermal load more widely, and support more uniform light distribution.
But additional LEDs also consume PCB space, increase component cost, and may require a different circuit configuration.
The correct decision therefore depends on the complete product requirements.
Instead of asking:
“What is the minimum number of LEDs we can use?”
a more useful question is:
“What combination of LED quantity, operating current, PCB area, circuit configuration, efficiency, and cost makes the most sense for this OEM product?”
That is the type of question NKT evaluates during custom OEM LED module development.
Key Takeaway
PCB size and LED density should not be evaluated independently.
For a given light-output target, using fewer LEDs generally requires more output from each LED and therefore often requires higher operating current. Adding LEDs can allow the required output to be distributed across more devices at lower individual current, which may improve luminous efficacy and thermal distribution.
However, more LEDs also increase component count, consume PCB area, and affect the electrical configuration.
The best solution is therefore not automatically the fewest LEDs, the most LEDs, or the largest PCB. It is the configuration that provides an appropriate balance of light output, operating current, efficacy, thermal distribution, PCB constraints, electrical architecture, and cost for the final OEM product.
Developing a Custom LED Module for Your Product
If you already know your available PCB dimensions, mounting-hole positions, voltage range, and target luminous output, these parameters provide a useful starting point for custom LED module development.
NKT can evaluate LED selection, LED quantity, operating current, expected voltage range, PCB layout, and expected optical performance before prototype validation.
If an existing design requires relatively high LED current to reach the target output, we can also evaluate whether a different LED quantity or configuration may provide a more practical solution.
Request a Quote
Send us your PCB dimensions, mounting requirements, voltage range, and target light output to start the discussion.