AN3303 Datasheet, PDF (16/27 Pages) STMicroelectronics – Secondary-side rectification for an LLC resonant converter

Sensing optimization by waveform check
AN3303
3.2
MOSFET turn-on delay compensation
As discussed in Section 1.2, the RC circuit added to fine tune the turn-off timing has the side
effect of also delaying the turn-on. It was stated that to avoid this effect, a bypass diode plus
a series resistor can be mounted in parallel to the sensing resistor. Figure 18 and 19 below
show the turn-on delay improvement using this solution.
Figure 18. SR MOSFET turn-on
Figure 19. SR MOSFET turn-on - detail
CH3: SR FET current ^{CH2: SR FET gate}
CH4: primary current
CH3: SR FET current
CH2: SR FET gate
CH4: primary current
3.3
Sub-harmonic oscillation
Figure 20 shows the behavior described in Section 1.3. At light load the turn-off threshold (in
terms of current) is quite close to the peak current value. This causes a duty cycle oscillation
around the current peak. This duty oscillation causes the secondary current to fluctuate,
therefore amplifying the phenomenon. The 50 % blanking time implemented in the
SRK2000 limits the oscillation width, which would otherwise further increase causing output
instability.
Figure 20. Duty cycle oscillation
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Doc ID 18164 Rev 2