400W Class D Amplifier Circuit Using LM311 and IR2153 – Schematic & Working Explained

This Class D amplifier circuit is designed to deliver approximately 400W of output power and uses the LM311N comparator as the core of the PWM generator. The input audio signal is conditioned and compared with a reference waveform inside the LM311 to produce a PWM signal whose duty cycle varies with the input amplitude. This PWM signal represents the audio information in a high-frequency switching format suitable for digital amplification.

The PWM output from the LM311 is buffered and processed through a transistor stage (Q1 and Q2), which modulates the operation of the IR2153 half-bridge gate driver. Although the IR2153 typically contains an internal oscillator for SMPS applications, in this design it is used to drive high- and low-side MOSFETs (Q8 and Q3), controlled indirectly by the PWM. The circuit cleverly repurposes the IR2153 to function as a gate driver for the MOSFET bridge, allowing efficient switching of high currents.

The power stage includes two IRFP260N MOSFETs configured as a half-bridge, driven by the IR2153. The switching output passes through a low-pass LC filter made with a toroidal inductor (35 µH) and high-quality capacitors (C18–C20) to reconstruct the amplified analog audio signal from the PWM.

The circuit uses a split ±50V power supply, capable of supporting the high voltage and current demands of a 400W load, while capacitors (C14–C17) provide necessary decoupling and energy storage.

While this is a functional and clever implementation, there are a few caveats. The IR2153 is not traditionally designed to be controlled externally via PWM, which may limit timing precision and lacks built-in dead-time control, risking shoot-through if not carefully tuned.

There’s also no visible overload or thermal protection. Despite these limitations, this design offers a compact and cost-effective approach to Class D amplification using widely available components like the LM311 and IR2153, suitable for DIY and learning projects with proper testing and safety measures.