SRK2000 main characteristics AN3303
8/27 DocID18164 Rev 4
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
capacitance, but it may be necessary to add an external capacitance; in particular when
using power MOSFET packages with a high associated stray inductance, such as the
TO-220.
In several cases it can be advantageous to over-compensate with the external capacitor,
therefore introducing an additional delay to the power MOSFET turn-off. This solution,
shown on the right-hand side of Figure 5, reduces the current flowing through the power
MOSFET body diode increasing the efficiency. Power MOSFET turn-off fine tuning must be
handled carefully because, if the power MOSFET is turned off after the drain-source voltage
becomes positive, the current reverses and begins flowing from drain-to-source with
consequent converter malfunctioning.
Figure 5. Effect of parasitic elements on power MOSFET turn-off
A second effect associated to the parasitic elements is related to the power MOSFET turn-
on. Before turn-on, at the half-bridge inversion, the corresponding drain voltage starts
decreasing; the DVS voltage also drops but the RC formed by the parasitic capacitance of
the DVS pin (about 10 pF) and the sensing resistors introduces a time constant that slows
down the sensed signal. During this phase the power MOSFET stray inductance does not
contribute because there is no current flowing through it. As illustrated in Figure 6, this
results in a late power MOSFET turn-on which adversely affects efficiency.
This turn-on delay, which is negligible if the sensing resistor value is indicatively below 1 k,
becomes significant in a case where the resistor value is high and, obviously, further
increases if an external capacitor is mounted between the pin and ground as previously
indicated. To avoid this effect, a bypass diode can be mounted in parallel to the sensing
resistor. In this way, the parasitic capacitance is discharged through the diode dynamic
resistance instead of the sense resistor. A 100-200 resistor in series to the bypass diode
is recommended to limit the current sourced from the DVS pins in case SR power MOSFET
drain voltage goes excessively below ground.
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