PWM vs Analog Dimming: How a Constant-Voltage Driver Actually Reduces the Light

When a constant-voltage driver is told to dim, it can reduce the output in two fundamentally different ways. One switches the rail on and off at a high frequency; the other lowers the steady DC level. The choice between them determines how the light behaves, whether it flickers on camera, and how quiet the driver is in a silent room.

Pulse-width modulation (PWM) keeps the output rail at its full nominal voltage and switches the current to the LEDs on and off at a fixed frequency, varying only the ratio of on-time to off-time. The average current, and therefore the perceived brightness, is proportional to that duty cycle. At the top of the range the LED is on almost all the time; at 5 percent it is on for only a fraction of each cycle. Because the peak current never drops, the LED's colour temperature and forward-voltage behaviour stay constant across the whole dimming range.

The catch with PWM is where the switching frequency sits. If it falls between roughly 100 Hz and 20 kHz, two problems appear. Below a few kilohertz the modulation is slow enough that cameras pick it up as rolling bands, and moving objects under the light show a stroboscopic effect. In the audio band, the coils and capacitors of the driver can vibrate at the switching frequency, producing an audible whine that is especially noticeable in a quiet bedroom. Raising the PWM frequency above 20 kHz moves the modulation beyond human hearing, and moving it above about 3 kHz already solves most of the camera and strobe concerns, which is why high-frequency PWM is the accepted design.

Analog (or linear) dimming takes the other path. It lowers the DC level of the output rail itself, so the LED is always on but receives less current. There is no high-frequency switching of the light, no stroboscopic risk, and no audible whine from the dimming stage. The trade-off is that the LED's colour temperature drifts slightly as the current changes, and the controller has to be able to regulate a very low current accurately, which gets harder near the bottom of the range.

A well-built triac-dimmable constant-voltage driver usually combines both. The chopped mains waveform from the triac is filtered and reconstructed into a smooth command, and the output stage uses either high-frequency PWM above the audio band or a linear reduction, depending on which yields better colour and flicker performance at that point in the range. At low dimming levels, where a tiny PWM duty cycle becomes hard to regulate precisely, many drivers switch to analog control to keep the modulation clean.

The number to look for on the datasheet is the PWM dimming frequency. A figure above 20 kHz means the modulation is inaudible; a figure above 3 kHz means the camera and strobe effects are largely gone; a figure in the hundreds of hertz is a red flag on any product that calls itself flicker-free. For a driver that is also sold as 100-to-1 percent flicker-free, the internal dimming frequency should sit in the unrestricted band of the IEEE 1789 recommendation, and the datasheet should state it rather than leaving it to the specifier to guess.

There is also a resolution question that buyers rarely ask about. A PWM dimming command is a digital duty cycle, and its resolution is set by how finely the controller can divide one period. A coarse resolution shows up as visible steps near the low end, where each incremental change in duty cycle represents a large jump in perceived brightness. A driver that dims to 1 percent with smooth, stepless behaviour has a high-resolution dimming path, typically from an internal register with many steps or an analog level converted accurately. The smoothness of the transition from bright to dark is part of what makes a premium driver feel different from a cheap one, and it is separate from both flicker and frequency.