5

Application information

5.1

Multi-mode operations

The VIPERGAN65 is an offline Quasi-Resonant ZVS (Valley Switching at switch turn-on) flyback converter.

Depending on converter’s load condition, the device is able to work in different modes (see Figure 10):

1.

QR mode at heavy load. Quasi-resonant operation is achieved by synchronizing the GaN turn-on to the

transformer demagnetization by detecting the resulting negative-going edge of the voltage across any

winding of the transformer. The system then works close to the boundary between discontinuous (DCM) and

continuous conduction (CCM) of the transformer. As a result, the switching frequency is different for different

line/load conditions (see the hyperbolic-like portion of the curves in Figure 10). The minimum turn-on losses,

low EMI emission and safe behavior in short-circuit are the main benefits of this kind of operation.

2.

Valley-skipping mode at heavy/medium load. The device defines the maximum operating frequency of the

converter. As the load is reduced the GaN turn-on no longer occurs on the first valley but on the second

one, the third one and so on. In this way the switching frequency no longer increases, limiting the switching

losses.

3.

Frequency foldback mode at medium/light load. The maximum switching frequency limit is progressively

reduced with the FB pin voltage, still maintaining ZVS operation. This maximizes efficiency at light load, still

ensuring noise-free operations, since the switching frequency is bottom limited above the audible frequency

range.

4.

Burst-mode with no or very light load. When the load is extremely light or disconnected, the converter

enters a controlled on/off operation with constant peak current. Decreasing the load then results in frequency

reduction, which can go down even to few hundred hertz, thus minimizing all frequency-related losses and

making it easier to comply with energy saving regulations or recommendations. As the peak current is very

low, no issue of audible noise arises.

Figure 10. Multi-mode operation of VIPERGAN65

5.2

High voltage start-up generator and supply structures

Based on a double FET structure, the HV current generator is supplied through the DRAIN pin and is used to start

up the device.

The internal schematic is shown in Figure 11. Before startup, the switch SW1 is closed, SW2 and SW3 are open,

whereas the low-side MOSFET (LS MOSFET) is open.

When the power is applied to the circuit, the gate of the high-side GaN (HS GaN) is pulled up through the resistor

RHV and capacitor CHV. If the voltage is high enough, the internal current generator draws the current ICHARGE

(3.55 mA typ.) to charge the capacitor connected between VDD and GND. To avoid excessive IC dissipation

in case VDD is accidentally shorted to GND, this current is initially reduced to 0.6 mA, until VDD is lower than

VDD_FOLD.

VIPERGAN65

Application information

DS14005 - Rev 2

page 11/35