Built around a VIPERGAN50 high-voltage GaN converter, the 15 V / 50 W QR flyback converter evaluation board (EVLVIPGAN50FL) consists of a main board and a daughterboard.

The main board contains, on the primary side, a diode bridge for double wave rectification, an input pi filter for EMI, a flyback transformer, the VIPERGAN50 and all the related components needed for polarization and setting features; and on the secondary side, the output capacitors and the output rectifier, realized by a power MOSFET driven by a synchronous rectifier for more efficient optimization.

The series resistor (RHV) between the rectified mains and the Startup pin (HV) is used to activate the internal high-voltage current source, which charges the capacitor connected between VDD and GND at startup. To minimize the residual consumption from the mains, the RHV value is selected in the range of several tens of Mohms.

When the high-voltage current source is turned off, the Startup pin (HV) is internally connected to the Input OVP protection pin (IOVP); this allows the series resistor (RHV) between the rectified mains and the Startup pin to also act as the high-side resistor of a voltage divider sensing the rectified mains to ensure input overvoltage and brown-out protection.

Quasi-resonant switching is done via the voltage divider connected across the Zero-cross detect (ZCD) pin and the transformer’s auxiliary winding used also to supply the IC. When the transformer completes the energy transfer to the output, the auxiliary winding voltage decreases, and as the ZCD voltage falls below 60 mV, the GaN is enabled to switch, after some delay. The optimum value of this delay is the one for which turn-on occurs exactly at the valley of the resonance following the demagnetization, which minimizes the switching losses. It can be found experimentally, and easily set through the selection of the voltage divider across the Blanking time adjustment and valley synchronization pin (TB) and the auxiliary winding. During the VIPERGAN50’s off time, the ZCD voltage is proportional to the output voltage, thus the ZCD voltage divider is used also to set the output overvoltage protection threshold.

An optocoupler brings the voltage reference error signal to the primary side, where it adjusts the Feedback (FB) pin voltage level (and thus the drain peak current) to the value required by the control loop for output voltage regulation.

The voltage reference, voltage divider for the output voltage setting, compensation network and output connectors are contained in the daughterboard, which is soldered orthogonally to the main board.