MMDF3N03HD

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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.

DRAIN–TO–SOURCE DIODE CHARACTERISTICS

The switching characteristics of a MOSFET body diode

are very important in systems using it as a freewheeling or

commutating diode. Of particular interest are the reverse

recovery characteristics which play a major role in

determining switching losses, radiated noise, EMI and RFI.

System switching losses are largely due to the nature of

the body diode itself. The body diode is a minority carrier

device, therefore it has a finite reverse recovery time, trr, due

to the storage of minority carrier charge, QRR, as shown in

the typical reverse recovery wave form of Figure 11. It is this

stored charge that, when cleared from the diode, passes

through a potential and defines an energy loss. Obviously,

repeatedly forcing the diode through reverse recovery

further increases switching losses. Therefore, one would

like a diode with short trr and low QRR specifications to

minimize these losses.

The abruptness of diode reverse recovery effects the

amount of radiated noise, voltage spikes, and current

ringing. The mechanisms at work are finite irremovable

circuit parasitic inductances and capacitances acted upon by

high di/dts. The diode’s negative di/dt during ta is directly

controlled by the device clearing the stored charge.

However, the positive di/dt during tb is an uncontrollable

diode characteristic and is usually the culprit that induces

current ringing. Therefore, when comparing diodes, the

ratio of tb/ta serves as a good indicator of recovery

abruptness and thus gives a comparative estimate of

probable noise generated. A ratio of 1 is considered ideal and

values less than 0.5 are considered snappy.

Compared to ON Semiconductor standard cell density

low voltage MOSFETs, high cell density MOSFET diodes

are faster (shorter trr), have less stored charge and a softer

reverse recovery characteristic. The softness advantage of

the high cell density diode means they can be forced through

reverse recovery at a higher di/dt than a standard cell

MOSFET diode without increasing the current ringing or the

noise generated. In addition, power dissipation incurred

from switching the diode will be less due to the shorter

recovery time and lower switching losses.