5.3

Zero current detection and triggering block

The zero current detection (ZCD) and triggering block switches on the power GaN if a negative-going edge

falling below 60 mV is applied to the ZCD pin. To do so, the triggering block must be previously armed by a

positive-going edge exceeding 110 mV.

This feature is used to detect transformer demagnetization for the QR operation, where the signal for the ZCD

input is obtained from the transformer auxiliary windings used also to supply the IC.

The triggering block is blanked after GaN turn-on to prevent any negative-going edge that follows leakage

inductance demagnetization from triggering the ZCD circuit erroneously.

The switching frequency is top limited below 240 kHz.

To prevent the tendency of the system to excessively increase the frequency at light load, a variable blanking

time, function of the FB pin voltage, is implemented. This blanking time is maximum for VFB = VFBB and

decreases linearly down to the minimum for VFB = VFBR.

In QR systems the switching frequency also increases with the input line. To solve this issue, the VIPERGAN65

offers a special dynamic blanking time function, which reduces the maximum permitted switching frequency as the

input line increases. This function is settable through the TB pin and is explained in the relevant section.

In this way, the switching frequency is progressively reduced with the input voltage and output load, resulting in

lower frequency-related losses.

If the demagnetization completes - hence a negative-going edge appears on the ZCD pin - after a time exceeding

time TBLANK from the previous turn-on, the GaN is turned on again, with some delay to ensure minimum voltage

at turn-on (“QR mode”).

If the negative-going edge appears before TBLANK has elapsed, it is ignored and only the first negative-going edge

after TBLANK turns on the GaN. In this way one or more drain ringing cycles is skipped (“valley-skipping mode”)

and the switching frequency is prevented from exceeding 1/TBLANK.

The blanking time limits and the mode of operation are reported in Figure 12.

In case the amplitude of residual oscillation on ZCD is not enough to trigger again the switching (it could happen

during low frequency operation), an internal function forces the GaN to turn on, TFORCE after the blanking time is

elapsed. A starter block is also used to start up the system when the signal on the ZCD pin is not high enough

to trigger the GaN. After the first few cycles initiated by the starter, as the voltage developed across the auxiliary

winding becomes large enough to arm the ZCD circuit, the GaN's turn-on starts to be locked to transformer

demagnetization, hence setting up the QR operation.

The ZCD pin voltage is both top and bottom limited by a double clamp. The upper clamp is typically located at

3 V, the lower clamp at -60 mV. The interface between the pin and the auxiliary winding is a resistor divider. Its

resistance ratio as well as the individual resistance values are properly chosen, see “Line voltage feed-forward

block” and “Output overvoltage protection”.

Figure 12. Frequency limits and modes of operations

VIPERGAN65

Zero current detection and triggering block

DS14005 - Rev 2

page 13/35