What Developers Should Keep in Mind During Design

Constant Power in an LED Driver What Developers Should Keep in Mind During Design

2026-09-11 A guest post by Frank Stocker* | Translated by AI 8 min Reading Time

Power supplies with a constant output can support different LED configurations using a single power supply model. However, to do so, developers must take into account forward voltage, temperature, manufacturing tolerances, and dimming conditions.

LED drivers: LED circuit boards connect the light fixture to the electronics. During design, the operating current, forward voltage, and temperature must be taken into account across the entire operating range.(Image: © Maria - stock.adobe.com) — LED drivers: LED circuit boards connect the light fixture to the electronics. During design, the operating current, forward voltage, and temperature must be taken into account across the entire operating range.
(Image: © Maria - stock.adobe.com)

When developing LED lights, it is often necessary to support different LED types, string lengths, and operating currents using as few power supply variants as possible. In this regard, constant-power power supplies offer more flexibility than traditional constant-current power supplies. However, this requires that the LED string’s forward voltage remain within the permissible operating window across all relevant operating conditions.

Mean Well’s XLC and XLN series demonstrate how such a concept can be implemented in practice. Depending on the model, these LED power supplies provide either constant voltage or constant power and can be configured via NFC. This makes them a good example for taking a closer look at the selection and design of constant-power power supplies.

Why LEDs Need Current Control

Figure 1: A 2.5 percent change in voltage can result in a 16 percent change in current for the LED shown. The specific values depend on the LED used and its operating point, but the basic behavior remains the same.(Image: Schukat) — Figure 1: A 2.5 percent change in voltage can result in a 16 percent change in current for the LED shown. The specific values depend on the LED used and its operating point, but the basic behavior remains the same.
(Image: Schukat)

LEDs are typically operated with a regulated constant current. The reason for this lies in their steep U-I characteristic curve: even a small change in forward voltage can cause a comparatively large change in current. Since the brightness of an LED is approximately proportional to the current within the normal operating range, both the brightness and the power dissipation change with the current.

A pure constant-voltage power supply is therefore not suitable for individual LEDs or LED strings without additional current limiting. In addition to supply voltage tolerances, temperature and component variation also affect the forward voltage of the LED. With a constant supply voltage, this could cause the LED current to vary significantly.

For constant-voltage light sources, such as 12-V or 24-V LED strips, the necessary current limiting is already built into the light fixture. This is typically achieved using series resistors or constant-current drivers on the LED strip. In this case, the external power supply merely provides the constant voltage.

Why a Constant-Power Power Supply Is Different

Figure 2: Illustration of the constant-power operating window for the Mean Well XLC-25-H, with a constant operating current ranging from 460 to 1,050 mA.(Image: Mean Well) — Figure 2: Illustration of the constant-power operating window for the Mean Well XLC-25-H, with a constant operating current ranging from 460 to 1,050 mA.
(Image: Mean Well)

A constant-current power supply regulates the output current to a specified value. The output voltage adjusts itself according to the connected LED load. With a constant-power power supply, on the other hand, an operating range is defined within which the product of the output voltage and output current remains approximately constant.

If the output current is changed, the power supply adjusts the output voltage accordingly within its specified limits. This allows different LED strings with varying forward voltages and operating currents to be powered by the same power supply model.

However, the CP topology does not make a power supply independent of the load. The operating window—defined by the minimum and maximum output voltages and the permissible output current—remains critical. Outside this range, the power supply can no longer deliver the intended power or switches to a different operating mode.

Figure 2 shows the operating window of the Mean Well XLC-25-H 25-W LED power supply. According to the manufacturer, the output current can be adjusted between 300 and 1,050 mA. In the range from 460 to 1,050 mA, the power supply can deliver 25 W of output power within the permissible output voltage range.

In terms of design, this means that for a given output current, the required voltage of the LED string must fall within the corresponding voltage range. If the required voltage is too low or too high, the constant-power range is exceeded.

Such a design range can reduce the number of variants. For example, a lighting designer can use it to design various 25-W luminaires with different LED configurations without having to provide a separate constant-current power supply for each variant. Whether multiple applications can actually be covered by the same power supply variant must be verified based on the respective LED data and the CP characteristic curve.

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Design an LED String Based on the Actual forward Voltage

An initial estimate of the required LED voltage is made by multiplying the number of LEDs in a string by the typical forward voltage of the LEDs used. However, this value is not sufficient for the design. The actual voltage of the string can vary significantly depending on the LED type, current, temperature, and manufacturing variations.

Therefore, the minimum, typical, and maximum forward voltages listed in the datasheet for the LED being used must be taken into account during the design process. In addition, the intended operating current, the temperature coefficient of the forward voltage, the cold-start behavior, and possible changes in the operating point during dimming must be considered. There should be a sufficient margin between the maximum required LED voltage and the upper output voltage limit of the power supply.

In a long LED string, positive and negative deviations in individual LEDs may partially cancel each other out. Nevertheless, the worst-case scenario should be considered during design. The sum of the forward voltages must remain within the power supply’s output voltage range across the entire intended temperature and operating range.

Temperature as a Design Factor

LEDs generally have a negative temperature coefficient for forward voltage. As the temperature of the LED chip rises, the required forward voltage decreases. Conversely, the forward voltage during a cold start may be higher than when the LED has reached thermal equilibrium.

A cold start can therefore represent the most unfavorable operating condition. The light must start up reliably even at low ambient temperatures and when the LEDs have not yet warmed up. This requires a sufficient voltage margin relative to the upper output voltage limit of the power supply.

The thermal design of the luminaire also affects the eventual operating temperature of the LEDs. Heat dissipation, the installation configuration, and the housing can therefore indirectly influence the voltage requirements of the LED string.

The dimming condition must also be taken into account during design. If the LED current is reduced, the operating point shifts along the U-I characteristic curve. With linear dimming, this can also cause the forward voltage of the string to change. Therefore, to ensure reliable operation, it is not sufficient to test only the rated operating conditions at full brightness. Rather, all operating conditions under which the luminaire is intended to be used are relevant.

Set the Output Current to Match the Application

Figure 3a: The NFC programming tool from the Mean Well app. The app can be used to read, write, and back up data.(Image: Mean Well) — Figure 3a: The NFC programming tool from the Mean Well app. The app can be used to read, write, and back up data.
(Image: Mean Well)

Figure 3b: The NFC programming tool from the Mean Well app. The app can be used to read, write, and back up data.(Image: Schukat) — Figure 3b: The NFC programming tool from the Mean Well app. The app can be used to read, write, and back up data.
(Image: Schukat)

A constant-power power supply offers an advantage only if the output current can be adjusted to suit the specific LED application. Various methods can be used to achieve this. Possible options include, for example, an integrated potentiometer, external fixed resistors, or DIP switches.

For Mean Well's XLN and XLC series, parameterization can also be performed via NFC, depending on the model. This can be particularly helpful when manufacturing multiple luminaire models with different LED currents.

NFC for Manufacturing and Service

NFC stands for Near Field Communication. This contactless transmission technology operates over short distances and is used, among other things, for contactless payments. Many current smartphones and tablets feature a corresponding interface. The Mean Well power supplies mentioned above have an integrated NFC interface for configuration. According to the manufacturer, the Mean Well app can be used to read, among other things, the set output current and device information such as the GTIN/EAN number. In addition, users can set a desired output current and transmit it to the power supply. Password protection is designed to prevent parameters from being overwritten unintentionally or without authorization.

To transfer the data, hold the mobile device over the marked area of the power supply unit. According to the manufacturer's instructions, the device does not need to be turned on for this. This allows parameterization to be performed, if necessary, even before electrical commissioning.

This can offer several advantages for manufacturing and service. Different luminaire variants can be configured using the same power supply unit and digitally parameterized. The settings can be reproduced and documented, eliminating the need for mechanical access to potentiometers or DIP switches. In addition, manual setting errors can be reduced, and device information can be retrieved.

NFC does not handle operational control in this context. For dimming or control during operation, other interfaces are available for the corresponding models, such as DALI-2, a 0- to 10-V signal, or a push-button interface.

What to Consider When Choosing an LED Driver

When selecting a constant-power power supply, you should first consider the LED string, its intended operating current, and its forward voltage. The typical values are provided only as a general guide. For the actual design, you must take into account the LEDs’ tolerances, temperature dependence, and possible operating conditions.

The required LED current must be appropriate for the thermal design and the desired brightness. At the same time, it must fall within the power supply’s adjustable range. The minimum and maximum string voltages must remain within the output voltage range under all relevant operating conditions. This includes cold start, thermal run-in, LED manufacturing variations, and dimming or partial-load operation.

In addition, the system requirements must be checked. These include cooling, the installation location, the required protection class, the available dimming interface, the necessary certifications, and the inrush current. The rated power alone is therefore not sufficient for selecting an LED driver. The key factor is whether the actual LED operating point falls within the power supply’s operating window.

According to the manufacturer, the XLC-25-H series offers an output voltage range of 9 to 54 V and an adjustable output current of 300 mA to 1,050 mA. Depending on the LED type and configuration, this allows for powering both short LED strings and longer strings or COB LEDs. However, whether a specific application is suitable depends on the combination of LED current, LED voltage, and the CP operating window.

Inrush Current in Many Power Supplies

Figure 4: The XLC-25H power supply offers a number of advantages. These include, among other things, built-in inrush current limiting.(Image: Mean Well) — Figure 4: The XLC-25H power supply offers a number of advantages. These include, among other things, built-in inrush current limiting.
(Image: Mean Well)

Another aspect of the system is the inrush current. If numerous LED power supplies are operated on a shared fuse circuit, the inrush currents of the individual devices can add up. If they are turned on simultaneously, this can cause a circuit breaker to trip, even though the subsequent continuous load is significantly lower.

According to the manufacturer, the XLN and XLC series feature built-in inrush current limiting. For the XLC-25, Mean Well specifies a maximum inrush current pulse of 10 A with a pulse duration of 100 µs and 50 percent I_peak. Under the conditions specified by the manufacturer, this should allow up to 71 power supplies to be operated from a single B16 circuit breaker.

However, this number is not a universally applicable planning guideline. The actual maximum number depends, among other things, on the circuit breaker used, the line impedance, the wiring, and the timing of the switching operations. For a specific installation, therefore, the manufacturer’s specifications and the characteristics of the electrical circuit must be considered together.

Additional Constraints

For these series, Mean Well specifies a maximum ambient temperature of 50 °C (122°F) without derating and a service life of up to 100,000 hours, depending on the model. The actual service life depends, among other factors, on ambient temperature, load, cooling, and installation conditions. Depending on the model, the devices are designed for input voltages ranging from 100 to 305 V_AC. International certifications listed by the manufacturer include ENEC, UL, CB, CCC, BIS, and EAC. The specific certifications and technical limits applicable to a particular model should be verified in the respective data sheet and certification documents.

*Frank Stocker is a Field Application Engineer and Product Manager for Power Supplies at Schukat