The VIPERGAN50 is the first member of the VIperPlus series to offer up to 50 W in a wide range of operating voltages (9 V to 23 V). It is also our first VIPer device with a gallium nitride (GaN) transistor. Thanks to the 650 V GaN device, the VIPERGAN50 needs less than 30 mW in standby with the adaptive burst mode on. Additionally, its protection features improve its robustness and help shrink the bill of materials. Its QFN 5 mm x 6 mm package also makes it one of the smallest devices in the industry for its power output. The VIPERGAN50 is available in volume, and a development board with a USB-C power delivery port is coming.
Why a QR ZVS Flyback Converter?
Applications Keep Requiring Greater Power Density
In our application note AN1326 from November 2002, we explained that engineers often used quasi-resonant (QR) zero voltage switching (ZVS), also called valley switching at turn-on, in the switched-mode power supply (SMPS) of TVs and other appliances. While still true today, the topology is now in a lot more products. The reason is that power envelopes have gotten significantly denser with each passing decade. For instance, TVs now push more pixels while having even more stringent power consumption requirements. Similarly, while 50 W chargers are not new, consumers demand supplies that don’t look and feel like giant hot bricks and fast charge laptops, tablets, phones, and more.
QR ZVS Flyback Converter Keep Needing Greater Efficiency
The industry often turns to QR converters because of their efficiency. In traditional PWM converters, the device turns on when the voltage is at its highest, which causes power losses that are compounded as switching frequencies increase. Engineers can mitigate this with a snubber circuit, but the best way to improve efficiency is to soft-switch, meaning to switch when the voltage, or current, is at zero. To do that, a resonant (inductor-capacitor or LC) transforms the square-shaped signal into a sinusoidal waveform. In ZVS, turn-on happens at the bottom, or valley, of the curve. Over the years, engineers attempted to improve the efficiency of QR ZVS flyback converters, and GaN just gave them a new answer.
Why VIPERGAN50 Today?
The GaN Transistor
The VIPERGAN50 uses the same 650 V GaN transistors as the MASTERGAN series and thus offers similar benefits. For instance, GaN’s high electron mobility means that the device tolerates high switching frequencies. Consequently, it can handle greater loads while suffering far fewer losses. GaN thus enables the creation of supplies that can output more power while shrinking their overall footprint. VIPERGAN50 is also highly symbolic because it is the first of many. As ST continues to make GaN a priority, we will use higher-rated transistors. The industry can thus expect future VIPERGAN models with greater output power.
The Multi-Mode Operations
The new device optimizes its performance thanks to multi-mode operations. In a nutshell, the VIPERGAN50 adapts its switching frequencies according to its load. During a heavy load, the quasi-resonant circuit synchronizes the GaN turn-on with the transformer demagnetization (ZCD pin) to minimize losses. Similarly, a heavy or medium load will trigger valley-skipping. In a nutshell, the transistors can skip one or more valleys when the load decreases. In this scenario, the switching frequency decreases to limit losses.
Similarly, the frequency foldback mode lowers the frequency during medium and light loads but ensures it remains above a certain threshold to prevent noise. Finally, burst-mode, at light or no load, can limit the switching frequency to a few hundred hertz while keeping a constant peak current to prevent noise. In this last mode, the VIPERGAN50 has a power consumption of less than 30 mW and a quiescent current of only 900 µA. The new device can thus help meet new environmental regulations that demand greater power efficiency to save global resources.
The Protection Features
Traditionally, engineers add external devices to provide safety features and protect their circuits. The fact that VIPERGAN50 vastly increases efficiency means ST had room to include more safety functionality. Designers thus need fewer components on their boards, reducing their BOM. For instance, the new device is the first in the VIPer Plus series to offer input overvoltage protection (iOVP) to protect against sudden voltage spikes. Similarly, the brown-in/brown-out feature monitors the supply voltage to shield the system against unreliable mains by setting the minimum input voltage that will start operations and the minimum one that will stop it. And these features are on top of more common over-temperature and overload / short-circuit protections.
