AN805 VISHAY [Vishay Siliconix], AN805 Datasheet - Page 3

• Manufacturer Part Number: AN805
• Description: PWM Optimized Power MOSFETs for Low-Voltage DC/DC Conversion
• Manufacturer: VISHAY [Vishay Siliconix]

The following complementary n- and p-channel MOSFETs, all
LITTLE FOOT TSSOP-8 devices, represent the three
technologies under test:
Figure 4 shows test circuit used.
PWM Optimized MOSFET Performance
Reducing gate charge is one way in which PWM optimized
MOSFETs cut power losses. In a real application, another
component of power loss is crossover losses. These are also
minimized by the PWM optimized power MOSFET design, and
are discussed in detail in Appendix A.
Figure 5 shows oscillograms of the boost converter switching
waveform using the three different types of power MOSFETs.
Document Number: 70649
January 1997
PWM optimized MOSFET
Conventional MOSFET
Low-threshold MOSFET
. . . . . . . . . .
. . . . . . . . . .
Si6801DQ
Si6542DQ
Si6552DQ
The switching speeds are 4 ns for the Si6801DQ PWM optimized
MOSFET and 11 ns for the conventional MOSFET. The Si6801DQ
provides a nearly threefold improvement and thus lower losses. In
addition to the increase in basic switching speed, notice that the
PWM optimized MOSFET does not exhibit a large characteristic
step in the voltage waveform. This step is due to the feedback
capacitance from drain to gate of the MOSFET or “Miller”
capacitance (C
voltage is lower than the gate voltage during a switching transition
from an OFF state to an ON state. Effectively the gate voltage is
“stalled” while the Miller capacitance is charged, and this is
reflected in the voltage waveform from drain to source. This is
obviously an unwanted characteristic and has largely been
eliminated with PWM optimized MOSFET technology.
A final component that affects the switching speed of a MOSFET is
the effective gate resistance (R
resistance defines how fast the MOSFET capacitance can be
charged. It is therefore one of the dominant factors in determining
how fast a MOSFET will switch. Vishay Siliconix’s PWM optimized
MOSFETs provide a minimum effective gate resistance.
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in Figure 1) being charged when the drain
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G
in Figure 1). The effective gate
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