MMDF3N06HD Datasheet, PDF (4/12 Pages) Motorola, Inc – DUAL TMOS POWER MOSFET 60 VOLTS

MMDF3N06HD
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 (I_G(AV)) can be made from a
rudimentary analysis of the drive circuit so that
t = Q/I_G(AV)
During the rise and fall time interval when switching a
resistive load, V_GS remains virtually constant at a level
known as the plateau voltage, V_SGP. Therefore, rise and fall
times may be approximated by the following:
t_r = Q₂ x R_G/(V_GG − V_GSP)
t_f = Q₂ x R_G/V_GSP
where
V_GG = the gate drive voltage, which varies from zero to V_GG
R_G = the gate drive resistance
and Q₂ and V_GSP 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:
t_d(on) = R_G C_iss In [V_GG/(V_GG − V_GSP)]
t_d(off) = R_G C_iss In (V_GG/V_GSP)
The capacitance (C_iss) is read from the capacitance curve at
a voltage corresponding to the off−state condition when
calculating t_d(on) and is read at a voltage corresponding to the
on−state when calculating t_d(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, t_rr, due
to the storage of minority carrier charge, Q_RR, 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 t_rr and low Q_RR 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 t_a is directly
controlled by the device clearing the stored charge.
However, the positive di/dt during t_b is an uncontrollable
diode characteristic and is usually the culprit that induces
current ringing. Therefore, when comparing diodes, the
ratio of t_b/t_a 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 t_rr), 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.
http://onsemi.com
4