MTW24N40E

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4

POWER MOSFET SWITCHING

Switching behavior is most easily modeled and predicted

by recognizing that the power MOSFET is charge

controlled. The lengths of various switching intervals (

∆t)

are determined by how fast the FET input capacitance can

be charged by current from the generator.

The published capacitance data is difficult to use for

calculating rise and fall because drain−gate capacitance

varies greatly with applied voltage. Accordingly, gate

charge data is used. In most cases, a satisfactory estimate of

rudimentary analysis of the drive circuit so that

During the rise and fall time interval when switching a

times may be approximated by the following:

where

During the turn−on and turn−off delay times, gate current is

not constant. The simplest calculation uses appropriate

values from the capacitance curves in a standard equation for

voltage change in an RC network. The equations are:

a voltage corresponding to the off−state condition when

At high switching speeds, parasitic circuit elements

complicate the analysis. The inductance of the MOSFET

source lead, inside the package and in the circuit wiring

which is common to both the drain and gate current paths,

produces a voltage at the source which reduces the gate drive

current. The voltage is determined by Ldi/dt, but since di/dt

is a function of drain current, the mathematical solution is

complex.

The

MOSFET

output

capacitance

also

complicates the mathematics. And finally, MOSFETs have

finite internal gate resistance which effectively adds to the

resistance of the driving source, but the internal resistance

is difficult to measure and, consequently, is not specified.

The resistive switching time variation versus gate

resistance (Figure 9) shows how typical switching

performance is affected by the parasitic circuit elements. If

the parasitics were not present, the slope of the curves would

maintain a value of unity regardless of the switching speed.

The circuit used to obtain the data is constructed to minimize

common inductance in the drain and gate circuit loops and

is believed readily achievable with board mounted

components. Most power electronic loads are inductive; the

data in the figure is taken with a resistive load, which

approximates an optimally snubbed inductive load. Power

MOSFETs may be safely operated into an inductive load;

however, snubbing reduces switching losses.

10

5

5

20

25

GATE−TO−SOURCE OR DRAIN−TO−SOURCE VOLTAGE (VOLTS)

Figure 7a. Capacitance Variation

9000

8000

7000

6000

5000

3000

1000

0

Figure 7b. High Voltage Capacitance

Variation

10

100

1000

10000

1000

100

10

15

010

4000

2000