Figure 19. Blanking time variation vs. FB voltage with dynamical Blanking time active

5.8

Valley synchronization

The main feature of the QR converters is that the turn-on occurs at the valley of the resonance after the

secondary demagnetization, which is a function of primary inductance, LP and total capacitance of the drain node,

CD. With respect to the transformers’ demagnetization instant, the first valley occurs with a delay given by the

formula:

Equation 4

(4)

TVALLEY=π∙ LP∙CD

Since

TVALLEYcanchangealotbetweenonedesignandanother,itisnoteasytominimizetheturn-onlosses

using a fixed turn-on delay time after ZCD triggering. For this reason, the VIPERGAN65 integrates a special

function that can increase the default turn-on delay time after trigger,

TD−ONMIN

(197 ns typ.) to exactly synchronize the turn-on time at the valley of the resonance, minimizing the turn-on losses.

The pinout configuration is shown in Figure 1: the voltage divider made up by RTB and RDELAY samples the

auxiliary winding voltage which, during the GaN off-time, is representative of the output voltage.

Since in steady-state the output voltage is constant, and RTB value has already been selected to get the desired

dynamic blanking time, RDELAY can now be chosen to set the TB pin sampled voltage to the value corresponding

to the desired turn-on delay, according to Figure 20.

Figure 20. Typical Turn-on delay after triggering as function of the voltage on TB pin.

VIPERGAN65

Valley synchronization

DS14005 - Rev 2

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