9/17

L6565

the L6565 puts a limit on the minimum OFF-time of the switch. This is done by blanking the triggering block of

the ZCD circuit as mentioned before. The duration of the blanking time (3.5µs min.) is a function of the error

amplifier output VCOMP, as shown in the diagram of figure 6.

If the load current and the input voltage are such that the switch OFF-time falls below the minimum blanking

time of 3.5µs, the system will enter the "Frequency Foldback" mode, a sort of "ringing cycle skipping" illustrated

schematically in figure 14.

Figure 14. Frequency foldback: ringing cycle skipping as the load is progressively reduced

In this mode, uneven switching cycles may be observed under some line/load conditions, due to the fact that

the OFF-time of the MOSFET is allowed to change with discrete steps (2·Tv), while the OFF-time needed for

cycle-by-cycle energy balance may fall in between. Thus one or more longer switching cycles will be compen-

sated by one or more shorter ones and vice versa. However, this mechanism is absolutely normal and there is

no appreciable effect on the performance of the converter or on its output voltage.

t

t

(limit condition)

t

Figure 15. Frequency Foldback: qualitative

frequency dependence on power

throughput

Further load reductions involve lower values for

VCOMP, which increases the blanking time. There-

fore, more and more ringing cycles will be skipped.

When the load is low enough, so many ringing cycles

need to be skipped that their amplitude becomes

very small and they can no longer trigger the ZCD cir-

cuit. In that case the internal starter of the IC will be

activated, resulting in burst-mode operation: a series

of few switching cycles spaced out by long periods

where the MOSFET is in OFF state.

Voltage Feedforward block (see fig. 17b):

The power that QR flyback converters with a fixed

overcurrent setpoint (like fixed-frequency systems)

are able to deliver changes with the input voltage

considerably. With wide-range mains, at maximum

line it can be more than twice the value at minimum

line, as shown by the upper curve in the diagram of

figure 16. The L6565 has the Line Feedforward func-

tion available to solve this issue.

Figure 16. Typical power capability change vs.

input voltage in ZVS QR flyback

converters

Pin

without frequency foldback

with frequency foldback

Vin fixed

11.5

22.533.5

4

0.5

1

1.5

2

2.5

system optimally

compensated

system not

compensated