LED Driver Noise and Audible Hum: Causes and Mitigation

LED lighting is often described as silent, and compared with the magnetic ballasts of fluorescent fixtures, that description is largely fair. But LED drivers are not acoustically inert. In quiet environments, an LED driver can produce a faint hum, buzz, or whine noticeable enough to generate complaints. The noise is not a sign of imminent failure in most cases, but it is a quality issue specifiers should understand and, in sensitive applications, prevent. The causes are rooted in the physics of magnetics and the mechanics of switching power conversion, and the mitigation strategies are well understood.

The primary source of audible noise in an LED driver is magnetostriction in the transformer and inductor cores. Magnetostriction is a property of ferromagnetic materials, such as the ferrite and manganese-zinc compounds used in driver transformers, in which the material changes shape slightly when subjected to a magnetic field. In a switching power supply, the transformer core is magnetized and demagnetized at the switching frequency, typically 50 to 150 kilohertz. The core expands and contracts by a few micro-strain at that frequency, producing mechanical vibration. At 100kHz, the vibration is far above the human hearing range, which extends to roughly 20kHz, so it is inaudible.

A second and more common noise source is the 100Hz or 120Hz ripple on the output. In a driver with a single-stage PFC and a relatively small output capacitor, the output voltage and current carry a ripple at twice the line frequency. This ripple current flows through the output inductor and the LED load. The output inductor's core experiences a magnetic field varying at 100 or 120Hz, and if the core is not sufficiently clamped or potted, it can vibrate at that frequency, producing a low-frequency hum.

A third source is mechanical resonance. The driver's enclosure, the PCB, the transformer mounting, and even the luminaire housing can have natural resonant frequencies within the audio band. If a vibration source, such as a magnetostrictive transformer, excites a resonant frequency, the amplitude of vibration is amplified by the quality factor of the resonant structure, and the noise becomes much louder. This is why a driver that is quiet on the bench can become noisy when mounted in a specific luminaire: the luminaire's sheet metal housing resonates at the driver's vibration frequency and acts as a sounding board.

PWM dimming can introduce noise even in a driver that is quiet at full output. If the driver uses PWM to dim LED current, and the PWM frequency is set within the audio band, typically below 20kHz, the LED current is switched on and off at an audible frequency. The output inductor and LED module vibrate at the PWM frequency, producing a tone that changes pitch as the dimming level changes. This is most noticeable at low dimming levels, where the PWM duty cycle is low and current pulses are short and sharp.