Datasheet

AN3303 SRK2000 main characteristics
Doc ID 18164 Rev 2 7/27
Figure 3. Power MOSFET drain voltage sensing and typical waveforms
Power losses are certainly much higher during phases a) and c), when the secondary
current flows through the SR power MOSFET body diode, than during phase b), when the
current flows through the power MOSFET channel. Therefore, minimizing phase a) and c)
duration is a good way to optimize the efficiency. In the following section, the reason circuit
parasitic elements play a fundamental role in this is described.
1.2 Drain sensing optimization
Drain voltage sensing must be very accurate to avoid disturbances and minimize the
parasitic elements that can affect it. The most important of these are shown in Figure 4.
Figure 4. Parasitic elements model
The stray inductance of the power MOSFET leads, internal bonding (L
source
, L
drain
) and
PCB trace (L
trace
) connecting the power MOSFET to the sensing trace, introduces a
discrepancy between the sensed voltage and the actual voltage drop across R
DS(on)
in the
effective drain voltage sensing. The contribution of L
trace
can be minimized by connecting
the sensing trace as close as possible to the power MOSFET but L
drain
and L
source
are
power MOSFET parameters and cannot be modified externally. As shown on the left-hand
side of Figure 5, this error causes an early power MOSFET turn-off. This anticipation is
partially compensated by the RC formed by the sensing resistor R and the DVS pin
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