Datasheet

AN3303 Sensing optimization by waveform check
Doc ID 18164 Rev 2 15/27
performed by adding a capacitor on each DVS pin, as indicated before in Section 1.2. In
Figure 15 and 16, IC behavior corresponding to different values of RC sensing circuit is
shown.
It is important to point out that this fine tuning must be done at the maximum operating
temperature because the R
DS(on)
increases significantly with temperature, moving the
corresponding turn-off forward. An over-delayed turn-off must be avoided because, if the
MOSFET is turned off after the drain current goes to zero, the resulting current reversal
produces a series of possible adverse effects, ranging from an efficiency drop to the
converter's catastrophic failure. A good rule of thumb is to keep at least a 50/100 nS margin
before the zero current point, because it has negligible impact on efficiency and it is a safe
margin in the case of abrupt load changes or other disturbances.
Figure 17 shows the case of operation above resonance. Note how the current flowing
through the MOSFET exhibits a very steep edge while decreasing down to zero. In this
case, no external capacitors are implemented because a further delay could cause a current
reversal.
Figure 17. Operation above resonance frequency
Figure 15. SR MOSFET turn-off Figure 16. SR MOSFET turn-off - detail
CH3: SR FET current
CH2: SR FET gate
CH4: primary current
CH3: SR FET current
CH2: SR FET gate
CH4: primary current
CH3: SR FET current
CH2: SR FET gate
CH4: primary current