Support

Preface

LED light-emitting diodes are semiconductor devices with full semiconductor characteristics. They are different from incandescent lamps and energy-saving fluorescent lamps. Their service life is directly related to junction temperature and operating current. The relationship between current and voltage is nonlinear. LEDs must be driven by a constant-current source to ensure the service life and light attenuation of the entire lamp. The operating current varies according to the characteristics of different LEDs. These are important considerations for LED engineers during design.

1. Why must an LED power supply be constant-current?

The characteristics of LED semiconductor materials make them highly sensitive to environmental changes. For example, when temperature rises, LED current increases; when voltage increases, LED current also increases. Long-term operation above the rated current will greatly shorten the service life of LED chips. Constant-current driving ensures that the operating current remains stable despite changes in temperature, voltage, and other environmental factors.

2. How to match the LED power supply with the LED module?

Some customers design the LED board first and then look for a power supply, only to find it difficult to get a suitable one: either the current is too high and the voltage too low (e.g., 7×1W with I＞350mA or V＜20V), or the current is too low and the voltage too high (e.g., I＜200mA or V＞25V). This results in severe heating, low efficiency, or insufficient input voltage range.

In fact, an optimal series-parallel configuration ensures equal voltage and current across all LEDs and maximizes power supply performance. The best approach is to communicate with power supply manufacturers for customized solutions, or produce the power supply in-house.

3. What is the optimal operating current for an LED power supply?

For an LED with a rated current of 350 mA, some factories use the full 350 mA, which causes severe heating. After comparative testing, 320 mA is generally ideal. This reduces heat generation and converts more electrical energy into visible light.

4. Operating voltage of the LED power supply

The recommended operating voltage for most LEDs is 3.0–3.5 V. Testing shows most LEDs work best at 3.2 V, so 3.2 V is a reasonable design value.

Total voltage of N series LEDs = 3.2 × N

5. Series-parallel connection and wide voltage range

To operate LEDs over a wide input range of AC 85–265 V, the series-parallel configuration of the LED board is critical. Wide-voltage designs should be avoided where possible; separate AC 220 V and AC 110 V versions are more reliable.

Most current power supplies are non-isolated buck constant-current types:

For AC 110 V input: output voltage ≤ 70 V, series count ≤ 23

For AC 220 V input: output voltage ≤ 156 V, series count ≤ 45

Excessive parallel connections lead to high current and severe heating.

Another wide-voltage solution uses APFC (active power factor correction): L6561/7527 boosts voltage to 400 V before buck conversion, equivalent to two switching power supplies. This is used only in specific applications.

6. LED series-parallel connection and PFC (Power Factor Correction)

For isolated power supplies:

AC 220 V: electrolytic capacitor ≈ 1 µF/W

AC 110 V: electrolytic capacitor ≈ 2 µF/W

Three types of PFC are commonly used:

No PFC: PF ≈ 0.65

Passive PFC (valley-fill): PF ≈ 0.92, most widely used and reliable

Active APFC (7527/6561): PF up to 0.99, but cost is double and reliability lower

For passive PFC, the DC voltage is about half the peak AC input voltage. For AC 220 V, peak voltage ≈ 312 V, half-peak ≈ 156 V. For non-isolated designs, output is positive half-wave only.

Thus, LED series count should generally be ≤ 45.

To achieve high power factor, the series count cannot be too low.

For isolated designs, series count relates to secondary winding turns and must satisfy output power.

Lower operating current within the rated range reduces heating and extends component life.

LEDs are sensitive to AC ripple: higher ripple degrades light quality. Use electrolytic capacitors to suppress ripple:

Non-isolated output: 1 µF ＜ 6 mA

Dimmable LED power: 1 µF ＜ 0.5 mA

Otherwise, flickering may occur.

7. Constant-current accuracy of LED power supplies

Many power supplies have poor constant-current accuracy, with errors up to ±8%. A reasonable standard is ±3%. Fine-tuning during production is required to achieve ±3% error.

8. Isolated / Non-isolated

Isolated power supplies above 15 W are bulky, expensive, and difficult to fit inside LED tubes. Non-isolated designs are mainstream, smaller (can be as thin as 8 mm), and cost-effective. Safety can be fully ensured with proper protection. Isolated types may be used if space permits.

9. Efficiency of LED power supplies

Efficiency = (Output Power / Input Power) × 100%

Higher efficiency means less power lost as heat. High internal temperature shortens the life of electronic components. Efficiency is a key factor determining power supply lifetime. Non-isolated designs typically achieve ＞80% efficiency, which is related to the LED board connection.

10. LED heat dissipation

Keeping LED chips below thermal overload greatly extends service life. Aluminum alloys and aluminum PCBs are commonly used for heat dissipation. Maximize external heat dissipation area.

Summary

LED chips and power supply must be well matched to ensure long service life of the entire lamp.