STMicro’s GaN Flyback Converters Simplify Design and Silence Noise

STMicroelectronics has introduced the VIPerGaN50W, a high-voltage converter meant to improve acoustic and electrical performance. Integrating a 700 V enhancement-mode GaN HEMT, a quasi-resonant (QR) flyback controller, and a gate driver in a 5 x 6 mm QFN package, the device targets USB-PD chargers and fast battery charging applications.

The VIPerGaN50W. Image used courtesy of STMicroelectronics

VIPerGaN50W Architecture and Features

A major benefit of the VIPerGaN50W is its efficient design. ST designed the VIPerGaN50W around a QR flyback topology with zero voltage switching (ZVS) at turn-on. The unique approach could minimize switching losses and provide efficient operation up to 50 W output power from 85-265 VAC input.

ST also integrated dynamic blanking time and valley synchronization delay functions that adapt the timing of switching events to align with the resonant valleys of the drain voltage waveform for maximum efficiency regardless of line and load conditions.

The converter's advanced power management includes a burst mode that reduces standby power to below 30 mW. In this mode, the device disables switching at very light or no-load conditions and reactivates when necessary to achieve low power consumption without generating audible noise. A patent-pending valley-lock feature further enhances this feature, as it stabilizes the number of skipped valleys during valley-skipping operation to avoid subharmonic oscillations that might cause magnetostrictive acoustic emissions.

Typical application diagram of the VIPerGaN50W. Image used courtesy of STMicroelectronics

To regulate output power across input voltages, the VIPerGaN50W employs line-voltage feedforward compensation. By adjusting the current limit based on sensed input voltage, the function ensures consistent output and protects against overpower conditions. Complementing this is the dynamic blanking time control, which scales switching frequency with input voltage to reduce losses at high-line operation.

Integrated safety mechanisms include output overvoltage, input overvoltage, overload, and brown-in/brown-out protections. All protection modes operate in auto-restart, and a thermal shutdown mechanism prevents overheating by disabling the device and enforcing a controlled restart.

Valley Switching in Flyback Converters

Valley switching is a powerful technique in QR flyback converters used to minimize power losses and electromagnetic interference. It refers to the timing strategy in which the power switch turns on at the valley point of the drain voltage waveform after transformer demagnetization. At this moment, voltage across the switch is at a minimum, allowing turn-on with reduced switching losses and voltage stress.

A flyback converter’s valley switching waveforms. Image used courtesy of Texas Instruments

To implement valley switching, the system must first precisely detect the transformer's zero-current point. In ST's device, this is achieved through a zero current detector (ZCD) that monitors auxiliary winding signals to trigger the gate driver at the appropriate instant. The VIPerGaN50W improves this with dynamic blanking time control, which adjusts the minimum off-time based on feedback voltage and input conditions to prevent false triggering and unlock flexible control across operating ranges.

In addition, valley-skipping allows the device to reduce switching frequency by intentionally delaying turn-on beyond the first valley, which reduces frequency at light to medium loads without compromising ZVS. However, uncontrolled valley-skipping can introduce frequency modulation in the audible range. The VIPerGaN50W's valley-lock mechanism counters this by fixing the number of skipped valleys under stable conditions, eliminating potential sources of audible noise.

Design Efficiency Without Compromise

The VIPerGaN50W delivers a compelling combination of power density, control flexibility, and EMI-silent performance in a single monolithic solution. By tightly integrating GaN switching and intelligent power management, STMicroelectronics provides a toolset that simplifies high-performance flyback converter design.

The VIPerGaN50W is available now in a 5 x 6 mm PQFN package, with the EVLVIPGAN50WF evaluation board as support.