Why Modern LED Luminaires Exhibit Wattage Reduction After Activation？

Observed power reduction (typically 1%-5%) during sustained LED operation is a normal phenomenon. This results from the combined effect of driver temperature compensation and LED semiconductor properties—distinct from lumen depreciation as it represents actual input power reduction.

🔧 Core Mechanism: Driver-Temperature Interaction & LED Characteristics

1. Temperature Impact on LED Forward Voltage

• Semiconductor Nature: LEDs feature a negative-temperature-coefficient PN junction.

• Behavior: Rising junction temperature reduces forward voltage (Vf) at constant current.

  
  

  
  

• Governing equation: Vf = Vf0 + (ΔT × K)

• Vf0: Forward voltage at 25°C

• ΔT: Junction temperature rise (°C)

• K: Negative temperature coefficient (typically -1.8mV/°C for white LEDs)

2. Driver Operational Principle

• Dominant Type: >95% LEDs use constant-current drivers (prioritize current stability over voltage/power).

• Power Relationships:

• Output power: Pout = Iout × Vout

• Input power: Pin ≈ Pout / η (η: driver efficiency, minimal change post-thermal equilibrium)

• η: Driver efficiency (remains stable post-thermal equilibrium)

📉 Power Reduction Logic Chain

⚠️ Critical Parameters

📌 Special Consideration: Line-Loss Compensation

Drivers for high-power/long-cable applications may exhibit amplified initial power:

1. Cold State: Elevates voltage to overcome cable resistance

2. Thermal Equilibrium: LED Vf ↓ + cable resistance ↑
   → Result: Slightly larger power drop versus standard drivers

3. Increased cable resistance (copper TCR ≈ 0.4%/°C)
   → Net Result: Power reduction magnitude exceeds standard drivers by 0.5-1.2%

📐 Conclusion: Validating Normal Operation

Authentic power reduction stems from:

1. Physics: LED Vf ↓ with temperature

   Vf reduction correlates with measured Tj rise (ΔVf/ΔT ≈ -1.8mV/°C)

2. Engineering: Constant-current driver adjusting Vout

   Constant Iout maintained (variation < ±3% per ANSI C82.16)

3. Acceptability criteria:

• ≤5% power reduction

• Stable light output

• Driver temperature < rated limit

Abnormal indicators: Sudden power plunge or significant brightness loss requires immediate heatsink/driver inspection.

Industry Standards Compliance (Implicit Requirements)

1. UL 8750: Mandates junction temperature monitoring for Class 2+ drivers

2. IES LM-80-19: Requires thermal/cold power ratio recording at 1,000hr intervals

3. EN IEC 62384:2020: Specifies ≤500ms voltage response time for load changes