MTP29N15E

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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

average input current (IG(AV)) can be made from a

rudimentary analysis of the drive circuit so that

t = Q/IG(AV)

During the rise and fall time interval when switching a

resistive load, VGS remains virtually constant at a level

known as the plateau voltage, VSGP. Therefore, rise and fall

times may be approximated by the following:

tr = Q2 x RG/(VGG – VGSP)

tf = Q2 x RG/VGSP

where

VGG = the gate drive voltage, which varies from zero to VGG

RG = the gate drive resistance

and Q2 and VGSP are read from the gate charge curve.

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:

td(on) = RG Ciss In [VGG/(VGG – VGSP)]

td(off) = RG Ciss In (VGG/VGSP)

The capacitance (Ciss) is read from the capacitance curve at

a voltage corresponding to the off–state condition when

calculating td(on) and is read at a voltage corresponding to the

on–state when calculating td(off).

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.

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

1000

7500

Figure 7. Capacitance Variation

-10

0

5

10

-5

15

20

25

0

2000

6000

VDS

VGS

3000

5000

7000

Ciss

Coss

Ciss

Crss

Crss

TJ = 25°C

VDS = 0 V VGS = 0 V

4000

1500

2500

6500

3500

5500

4500

500