NCP4304 Datasheet, PDF (13/32 Pages) ON Semiconductor – Secondary Side Synchronous Rectification Driver for High Efficiency SMPS Topologies

NCP4304A, NCP4304B
Zero Current Detection and Parasitic Inductance
Compensation
Figure 39 shows the internal connection of the ZCD
circuitry on the current sense input. The synchronous
rectification MOSFET is depicted with it’s parasitic
inductances to demonstrate operation of the compensation
system.
Figure 39. ZCD Sensing Circuitry Functionality
When the voltage on the secondary winding of the SMPS
reverses, the body diode of M1 starts to conduct current and
the voltage of M1’s drain drops approximately to −1 V. The
CS pin sources current of 100 mA that creates a voltage drop
on the R_{shift_cs} resistor. Once the voltage on the CS pin is
lower than V_{th_cs_on} threshold, M1 is turned on. Because of
parasitic impedances, significant ringing can occur in the
application. To overcome sudden turn−off due to mentioned
ringing, the minimum conduction time of the SR MOSFET
is activated. Minimum conduction time can be adjusted
using R_{_Min_Ton} resistor.
The SR MOSFET is turned−off as soon as the voltage on
the CS pin is higher than V_{th_cs_off}. For the same ringing
reason, a minimum off time timer is asserted once the
turn-off is detected. The minimum off time can be externally
adjusted using R_{_Min_Toff} resistor. MOSFET M1 conducts
when the secondary current decreases, therefore the turn−off
time depends on its R_DS(on). The 0 mV threshold provides
an optimum switching period usage while keeping enough
time margin for the gate turn off. The R_{shift_cs} resistor
provides the designer with the possibility to modify
(increase) the actual turn-off current threshold.
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