The PWM Stage

(Last edited 5/14/2026)

You might hear this called the "main isolation stage," though I avoid that term since isolation truly occurs at the main transformer (more on that later). While pulse width modulation commonly controls FET switching, labeling this simply as the "PWM stage" might seem overly generic. However, I find this designation practical as in a very basic PSU with only three integrated circuits, you'll typically find a housekeeping IC, a PFC controller, and the PWM controller. Of the three, the PWM controller being the one controlling this part of the PSU.  Those familiar with power supply design often name this stage after its specific topology instead, using terms like "Double Forward," "LLC," or "Active Clamp". We'll explore those terms in the next chapter.

This final section of the primary side performs a crucial function: transistors convert the rectified high-voltage, low-frequency DC into high-frequency pulses that drive the main transformer. The pulses aren’t quite AC, but are a square waveform.

The frequency of the voltage prior to the PWM stage is the same as the mains frequency. The PWM stage increases this frequency anywhere from between 20 kHz to 500 kHz. With these higher frequencies, the transformer can use a much smaller ferrite core and fewer turns, which drastically reduces its size and weight. Coincidently, with higher frequency voltage comes higher frequency ripple, and this can be filtered with smaller capacitors than ripple at a lower frequency.

Double-Forward vs. LLC vs. Active Clamp

Converting high-voltage to lower-voltage requires specific topologies. The double-forward (or " two-switch forward") uses a pair of MOSFETs that transfer energy to the main transformer during their "on" cycle.

Today's designs favor LLC converters for their superior efficiency. The name derives from its resonant tank components: resonant inductor (Lr), resonant capacitor (Cr), and transformer magnetizing inductance (Lm). This tank configuration reshapes voltage and current waveforms, enabling zero voltage switching (ZVS) that virtually eliminates switching losses.

ZVS represents a significant improvement over conventional switching. Traditional MOSFET operation creates power loss when voltage and current simultaneously flow through the switch, generating heat. ZVS activates the MOSFET only when voltage is absent, preventing this overlap and nearly eliminating associated losses.

While double-forward converters regulate through duty cycle adjustments, LLC converters modulate switching frequency. The controller increases frequency above resonance when load decreases, and conversely, shifts below resonance when load increases.

The active clamp (or active clamp reset forward) addresses a fundamental challenge in double-forward designs. When the primary switch deactivates, residual magnetizing energy remains in the transformer core. Without proper reset, this energy could saturate the core during the next cycle, potentially causing damaging current spikes. Active clamp technology recycles this energy back to the input or redirects it to assist the subsequent switching cycle, improving efficiency—though still not matching LLC performance.

Why Does LLC Sometimes “Click” at low-loads?

A reason a PSU can make clicking noises during low-load is from the implementation of burst mode in the LLC stage. The LLC stage follows the PFC stage and converts rectified high‑voltage, low‑frequency DC into high‑frequency pulses that drive the main transformer.

Burst mode is a light‑load efficiency technique. It periodically enables (“bursts”) and disables (“sleeps”) the switching drive signals.

At very light load, conventional Pulse‑Frequency Modulation (PFM) struggles to maintain regulation without excessive switching losses. Burst mode allows the controller to completely turn off switching when the load is too light. During this “sleep” period, the output capacitor supplies the load, significantly reducing switching losses.

When the output voltage falls to a defined threshold, the controller briefly resumes switching (the “burst”) and then enters another idle period. This on/off cycling maintains regulation while minimizing losses.

Burst mode can cause audible noise because the burst envelope, which is the low‑frequency repetition rate of the bursts, falls within the audible range, even though the switching frequency itself is typically tens or hundreds of kilohertz.

Another noise source is the magnetic components. When bursts start and stop, magnetic flux changes abruptly. These sudden transitions can cause the transformer or output inductors to “snap” due to magnetostrictive stress, creating clicks, ticks, or chirping at the burst frequency.