CPC1580
5.2 Inductive/Resistive Loads
5. Application Switching Losses
During the transition intervals, the application and load
components change energy states and, in the
process, incur switching losses. The switching losses
are manifested as heat in the application circuit and
must be addressed by the designer to ensure that no
one component exceeds its power rating. The
designer must understand the details of the load
behavior in order to adequately size and protect the
application circuit. There are three general cases to
observe: (1) purely resistive loads,
If the load is resistive and inductive, and the
inductance doesn't saturate, the load current during
turn off, t
, in Amps is:
RISE
2
-R
L
LOAD
•
t
L
V
I
R
L
LOAD
LOAD
LOAD
G_SINK
LOAD
I
(t) =
-
•
•
t - 1 + e
•
LOAD
R
R
LOAD
L
• C
LOAD
LOAD
LOAD
RSS
and the MOSFET drain voltage during turn off, t
in Volts is:
,
RISE
(2) inductive/resistive loads, and (3) loads with
significant capacitance. Inductors and capacitors are
energy storage elements that require special
consideration for switching.
IG_SINK
VDRAIN(t) =
t
•
CRSS
The instantaneous power in the MOSFET will be the
product of the two equations and the energy will be the
integral of the power over time.
During the switching periods, energy is conserved.
Inductors turning off transfer their stored energy to
MOSFET switching losses, to the capacitance of the
load and application circuit, and to the protector.
5.3 Capacitive Loads
During the turn-on interval, the inductor energy is zero,
and so the capacitive energy in the load and parasitic
elements of the switching application must be
dissipated by the MOSFET, in order for the load to
change state.
The energy absorbed by the MOSFET for loads that
are more capacitive in nature occurs during the
MOSFET turn-on as opposed to the turn-off. The
energy absorbed by the MOSFET will be a function of
the load, the TVS (or other protector), and the
MOSFET drain capacitance. The MOSFET energy,
To calculate the stored inductive energy in Joules:
1
E
, in Joules is:
2
FALL
EL =
• L • ILOAD
2
1
2
EFALL
=
• (CTVS + COSS + CLOAD) •
VLOAD
2
5.1 Resistive Load Losses: The Ideal Case
C
is the MOSFET output capacitance found in the
OSS
data sheet. As mentioned earlier, the MOSFET
switching losses occur at different times, either rising
or falling, so loads with a combination of inductance
and capacitance can also be calculated by the energy
equations described above.
For purely resistive loads, the energy dissipated by
changing states occurs primarily in the MOSFET.
The equation describing MOSFET energy dissipation
during rise time, in Joules, is:
P
C
I
LOAD
2
LOAD
RSS
t
E
> V
LOAD
RISE
RISE
=
•
•
•
5.4 dV/dt Characteristics
6
I
6
G_SINK
The application circuit shown in Figure 1 dissipates
significant energy caused by large dV/dt events. Fault
voltages across the MOSFET will turn it on for the
same reason the part turns off slowly. For dV/dt events
The average power of the MOSFET for any load type
in Watts is:
2
PAVG
=
• RDSAT • D + fSWITCH • (ERISE + EFALL
)
ILOAD
> I
/C
(from Equation 2) the application
G_SINK RSS
circuit will dissipate energy proportional to the C
Where f
is the application switching frequency;
RSS
SWITCH
and g (forward conductance) of the selected
R
is the MOSFET’s on-resistance; D is the
FS
DSAT
transistor. C
is a function of the transistor's
switch's operational duty cycle: D = t /(t +t
);
RSS
ON ON OFF
on-resistance and current/power capability, so higher
load designs are more sensitive.
and E
time, in Joules.
is MOSFET energy dissipation during fall
FALL
The CPC1580 provides an internal clamp to protect
the gate of the MOSFET from damage in such an
event. The part can withstand 100mA for short
periods, like dV/dt transients.
R00G
www.clare.com
9
CLARE [ CLARE ]
