Sensing optimization by waveform check

AN3303

16/27

Doc ID 18164 Rev 2

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.

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

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