How to Test LED Power Supplies: Complete Guide for Display Engineers

6.1 Purpose

The temperature rise and derating test aims to evaluate the thermal stability of a power supply when operating in high ambient temperatures and to determine whether load derating is necessary under such conditions to ensure safe and reliable operation.

In real-world applications, LED power supplies are often deployed in high-temperature environments such as outdoor control boxes, sealed enclosures, or areas without active ventilation. In these settings, limited airflow can prevent heat dissipation, causing internal components—such as MOSFETs, electrolytic capacitors, and main transformers—to rise in temperature.

Continuous operation at elevated temperatures can accelerate component aging and reduce their service life. For example:

• The lifespan of electrolytic capacitors typically halves with every 10°C increase in temperature (based on the Arrhenius thermal acceleration model);

• Prolonged high temperature in MOSFETs increases leakage current, resulting in reduced efficiency;

• Overheating may trigger protection circuits or, in extreme cases, damage components permanently.

This test provides:

• The maximum safe operating temperature;

• The relationship between ambient temperature and output capacity;

• A technical basis to determine if load derating is needed.

6.2 Required Equipment

To ensure accurate and credible test results, the following equipment and materials are required:

• Temperature Chamber

  • Capable of maintaining a stable ambient temperature in the range of 0°C to 85°C;

  • Temperature control accuracy of ±1°C;

  • Interior space must accommodate the DUT (Device Under Test) and all wiring.

• Thermocouple Thermometer or Infrared Thermometer

  • K-type thermocouples are commonly used and should be directly affixed to component surfaces;

  • IR thermometers should have adjustable emissivity settings—using matte black tape on the component can improve accuracy;

  • Measurement accuracy should be better than ±1.0°C.

• Programmable Electronic Load

  • Should support constant current mode and handle the power supply’s rated load;

  • Models with thermal protection and monitoring are preferred.

• Device Under Test (DUT)

  • A fully functional sample with all heat dissipation components and circuitry in place;

  • If the DUT includes a fan, note whether it is thermally controlled or fixed-speed.

• Data Acquisition System (Optional)

  • A multi-channel temperature logger can automatically record temperature curves and load current data;

  • Useful for generating a visual correlation between ambient temperature and power output.

6.3 Test Procedure

Step 1: Pre-Test Setup

• Mount the DUT at the center of the temperature chamber, ensuring a minimum of 5 cm of clearance on all sides for airflow.

• Connect the electronic load and set the initial output power to 100% rated load.

• Attach thermocouples to at least three key components:

  • Main switching transistor (MOSFET or GaN);

  • Output electrolytic capacitor;

  • Main transformer core;

  • Driver IC or PWM controller.

• For passively cooled designs, turn off any internal fans in the chamber to simulate no-airflow conditions.

Step 2: Temperature Rise Test

• Set the ambient temperature to typical high-operating points, such as 40°C / 50°C / 60°C / 70°C.

• Begin full-load operation for 30–60 minutes or until temperatures stabilize.

• Record the stabilized temperatures of key components and the ambient temperature, then calculate the temperature rise.

• If any component exceeds its temperature limit (e.g., capacitor >85°C, MOSFET >100°C), proceed to derating tests.

Step 3: Derating Curve Test (if needed)

• At high temperature points, gradually reduce load current in steps (e.g., decrease by 10% per step).

• Observe whether component temperatures fall back into the safe range.

• Repeat the test at multiple ambient temperatures (e.g., 60°C / 70°C / 80°C) to assess stable load capacity.

• Use the data to plot a “Temperature vs. Maximum Output Power” derating curve, which serves as a reference for system design and deployment.

6.4 Precautions

• Thermocouple Placement: Sensors must be tightly fixed to the surface of the components using thermal paste or high-temperature tape to prevent measurement errors caused by air gaps.

• Avoid Air Interference: Forced air cooling inside the chamber should be disabled to avoid underestimating temperature rise.

• Multi-Point Monitoring: Record temperature at a minimum of three critical component locations to avoid relying on a single reference point.

• Understand Thermal Limits: All components must operate within their rated junction temperature specified by the manufacturer (e.g., typical MOSFET max junction temp is 150°C; recommended operating temp should stay below 100–110°C).

• Derating Recovery Strategy: If derating is required, clearly define the recovery point in the product documentation (e.g., “full power operation can resume when ambient temperature is below 60°C”).

• Recording Frequency: Log temperature readings every minute. Once temperature fluctuation remains within ±1°C for 5 consecutive minutes, the system is considered stable.

Summary

Temperature rise and derating tests are essential for validating thermal design effectiveness and ensuring high-temperature reliability of power supplies. These results help:

• Define proper usage guidelines for high-temperature environments;

• Determine the need for additional heatsinks or ventilation components;

• Evaluate suitability for deployment in various climates (e.g., tropical, Middle Eastern regions);

• Provide derating recommendations to customers, preventing failures due to overloading.

In the LED display industry, thermal management directly affects not only power supply lifespan but also system stability and long-term performance. Establishing a clear derating model and thermal boundary is a critical step in designing robust and reliable LED control systems.