What are harmonics in led power supply/led driver?

In an ideal power system, voltage and current should be perfect, smooth sine waves (known as the fundamental wave, with a frequency of 50Hz or 60Hz). However, in reality, many electrical devices (like LED power supplies) "pollute" this perfect waveform, causing the current waveform to become distorted and no longer a smooth sine wave.

Mathematically, this distorted waveform can be broken down into a 50Hz/60Hz fundamental wave and a series of sine waves whose frequencies are integer multiples of the fundamental frequency (e.g., 100Hz, 150Hz, 200Hz...). These high-frequency sine waves are called harmonics.

2nd Harmonic: Frequency is 2 times the fundamental (100Hz/120Hz)

3rd Harmonic: Frequency is 3 times the fundamental (150Hz/180Hz)

5th Harmonic: Frequency is 5 times the fundamental (250Hz/300Hz)

...and so on.

Specific Cause in LED Power Supplies:

Modern LED power supplies typically use Switched-Mode Power Supply (SMPS) technology. A key part of converting Alternating Current (AC) to Direct Current (DC) for the LED chips is the rectification and filtering circuit.

Rectification: AC power passes through a diode bridge rectifier and becomes pulsating DC.

Capacitor Filtering: A large electrolytic capacitor is responsible for "smoothing" this pulsating DC into stable DC.

The problem lies here: This filter capacitor only draws current from the grid for a very short time near the peak of the AC voltage to charge itself quickly. For most of the rest of the voltage cycle, it draws no current.

This results in the LED power supply drawing current not as a continuous sine wave, but as sharp, narrow pulses when viewed from the grid side. This non-sinusoidal, pulsed current contains a significant amount of harmonic components, especially the 3rd, 5th, 7th, and other odd-order harmonics.

Visual Understanding:

Imagine the left side is the ideal sinusoidal current, and the right side is the distorted current waveform (pulse-like) produced by an LED power supply. The latter can be decomposed into the fundamental wave and various harmonics superimposed on it.

2. The "Role" of Harmonics (Actually, the Negative Effects)

In the field of power electronics, harmonics are almost always considered a negative phenomenon. Their "role" is to cause a series of problems and hazards.

1. Increased Losses and Heating in Lines and Equipment

When harmonic currents flow through lines and transformers, they cause additional heating due to the skin effect, which increases resistance. This leads to:

Overheating wires, accelerating insulation aging and even posing a fire risk.

Overheating transformers, which must be derated, reducing their load capacity.

2. Causes Excessive Neutral Line Current

In a three-phase four-wire system, the 3rd harmonic and its multiples (3rd, 9th, 15th...) are called "zero-sequence harmonics." They do not cancel each other out in the neutral line; instead, they add up. This can cause the neutral current to be even greater than the phase current, leading to neutral line overheating, which is very dangerous.

3. Impacts Power Grid Quality and Other Equipment

Voltage Distortion: Harmonic currents create harmonic voltages across the grid impedance, causing the grid voltage itself to become distorted. This affects the normal operation of other sensitive equipment (like precision instruments, communication devices) connected to the same grid.

Circuit Breaker Nuisance Tripping: Can cause circuit breakers or ground fault interrupters to trip without an actual fault.

Reduces Power Factor: While a low traditional "Displacement Power Factor" can be corrected, the presence of harmonics leads to a decrease in the True Power Factor.

4. Damage to Capacitors

Capacitors used for power factor correction in electrical systems are very sensitive to harmonics. Harmonics can cause them to become overloaded with current. In severe cases, this can even lead to resonance, causing capacitors to bulge, fail, or explode.

3. How to Deal with Harmonics? — Power Factor Correction (PFC)

To solve the harmonic problem, high-quality LED power supplies incorporate a circuit called Power Factor Correction (PFC).

One of the main goals of the PFC circuit is to manage harmonics. It controls the current waveform to make it closely follow the shape of the voltage sine wave, thereby:

Changing the current waveform from sharp pulses to a smooth sine wave.

Greatly suppressing the generation of harmonic currents.

Simultaneously improving the power factor (usually to above 0.9).

Depending on the implementation, PFC is divided into:

Passive PFC: Lower cost, average performance, typically only raises the power factor to 0.7-0.8, with limited harmonic suppression capability.

Active PFC: Uses specialized ICs and switching circuits, is very effective, can raise the power factor to above 0.95, and significantly reduces harmonic content. This is the mainstream configuration for mid-to-high-end LED power supplies.

Concept: Harmonics in LED power supplies are integer multiples of the fundamental frequency generated due to the non-linear operating characteristics (pulsed current draw) of the power supply, which distort the current waveform.

Role (Effects): Harmonics are primarily negative, causing increased system losses and heating, overloading the neutral line, polluting the power grid, and interfering with other equipment.

Countermeasure: By designing a Power Factor Correction (PFC) circuit (especially Active PFC) into the LED power supply, harmonics can be effectively suppressed, returning the current waveform to a sine wave. This allows the power supply to meet strict international harmonic standards (such as the EU's EN 61000-3-2 standard).