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

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