Application information

L6599

18/36

7.2

Operation at no load or very light load

When the resonant half-bridge is lightly loaded or unloaded at all, its switching frequency will

be at its maximum value. To keep the output voltage under control in these conditions and to

avoid losing soft-switching, there must be some significant residual current flowing through

the transformer's magnetizing inductance. This current, however, produces some

associated losses that prevent converter's no-load consumption from achieving very low

values.

To overcome this issue, the L6599 enables the designer to make the converter operate

intermittently (burst-mode operation), with a series of a few switching cycles spaced out by

long idle periods where both MOSFETs are in OFF-state, so that the average switching

frequency can be substantially reduced. As a result, the average value of the residual

magnetizing current and the associated losses will be considerably cut down, thus

facilitating the converter to comply with energy saving recommendations.

The device can be operated in burst-mode by using pin 5 (STBY): if the voltage applied to

this pin falls below 1.25V the IC will enter an idle state where both gate-drive outputs are

discharge. The IC will resume normal operation as the voltage on the pin exceeds 1.25V by

50mV.

To implement burst-mode operation the voltage applied to the STBY pin needs to be related

to the feedback loop. Figure 23 shows the simplest implementation, suitable with a narrow

input voltage range (e.g. when there is a PFC front-end).

Figure 23. Burst-mode implementation: narrow input voltage range.

Figure 24. Burst-mode implementation: wide input voltage range.

L6599

STBY

4

5

RFmin

L6599

L6599

STBY

4

5

RFmin

7

LINE

B+

RC

RD