(2)

ROPP=NAUXNPRI∙ αFFG1∙VFBH∙ VMIN+VMAX+VMIN∙VMAX

VR

With a proper compensation, it is possible to optimize the power capability change, as shown in the diagram of

Figure 16.

Feed-forward compensation affects the power level to enter burst-mode, thus the current IFF is linearly reduced,

starting from 100 % (at VFB ≥ 1.2 V) to 20 % (at VFB < 0.8 V). The residual IFF in burst-mode ensures a certain

level of compensation against the propagation delay TD.

Figure 16. Typical power capability variation vs. input voltage in QR flyback

5.7

Dynamic blanking time

Since the capacitive losses in a power system increase with the frequency, the tendency of the QR converters to

increase the switching frequency with the input line is one of the major limitations to consider when designing high

efficiency converters.

To solve this issue, the VIPERGAN65 offers a special dynamic blanking time function, which reduces the

maximum permitted switching frequency as a function of the input line.

Connecting the pin TB to the auxiliary winding through the resistor

RTB,thecurrentsourcedbythepinduringthe

on-time, which is proportional to the input voltage, is sampled and used to adjust the blanking time during the

off-time.

The advantage of this technique is that the frequency reduction is higher where mostly required, i.e., at high line,

and minimum at low line.

The following equation can be used to dynamically adjust the dynamic blanking time

TBLANKdyn:

Equation 3

(3)

RTB=NAUXNPRI∙ KBLANK∙VIN

TBLANKdyn −TBLANK

In this formula,

TBLANKisthedefaultvalueatVFB≥VFRandKBLANKistheblankingtimegain.

VIPERGAN65

Dynamic blanking time

DS14005 - Rev 2

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