Figure 14. Burst-mode operation

5.6

Line voltage feed-forward block

The power that a QR flyback converter with a fixed overcurrent set point can deliver changes considerably with

the input voltage.

In wide-range mains applications, the deliverable power at high line can be more than twice than at minimum

voltage. This is a problem because it means that at high line the system could be operated at a power level much

higher than that which triggers overload at low lines, thus requiring an overrating of the power components.

The VIPERGAN65 implement a feed-forward function able to solve this issue: the voltage across the auxiliary

winding is monitored and the information is used to compensate the converter (see Figure 15).

Figure 15. Feed-forward compensation: simplified internal schematic and pin configuration.

During GaN’s on-time the current sourced from the ZCD pin through the ROPP resistor is mirrored inside the

“Feed-forward Logic” block to provide a feed-forward current IFF, proportional to the input voltage according to the

formula:

Equation 1

(1)

IFF=NAUXNPRI∙ VINROPP

The optimum value of the compensation, which minimizes the power capability variation over the input voltage

range, is the one that provides equal power capability at the extremes of the range, and is defined by the proper

selection of the resistor, according to the equation below:

Equation 2

VIPERGAN65

Line voltage feed-forward block

DS14005 - Rev 2

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