I
NTEGRATED
C
IRCUITS
D
IVISION
CPC1580
10 www.ixysic.com R01
dissipated by the MOSFET, in order for the load to
change state.
To calculate the stored inductive energy in Joules:
6.1 Resistive Load Losses: The Ideal Case
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:
The average power of the MOSFET for any load type
in Watts is:
Where f
SWITCH
is the application switching frequency;
R
DSAT
is the MOSFET’s on-resistance; D is the
switch's operational duty cycle: D = t
on
/(t
on
+t
off
); and
E
FALL
is MOSFET energy dissipation during fall time,
in Joules.
6.2 Inductive/Resistive Loads
If the load is resistive and inductive, and the
inductance doesn't saturate, the load current during
turn off, t
RISE
, in Amps is:
and the MOSFET drain voltage during turn off, t
RISE
,
in Volts is:
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.
6.3 Capacitive Loads
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,
E
FALL
, in Joules is:
C
OSS
is the MOSFET output capacitance found in the
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.
6.4 dV/dt Characteristics
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
> I
G_SINK
/C
RSS
(from Equation 2) the application
circuit will dissipate energy proportional to the C
RSS
and g
FS
(forward conductance) of the selected
transistor. C
RSS
is a function of the transistor's
on-resistance and current/power capability, so higher
load designs are more sensitive.
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.
7. Design Switching Frequency
The maximum switching frequency is the last design
value to be calculated, because the over-voltage
protection and the storage capacitor play a significant
role in determining the result. Inasmuch as those
factors are already determined, the following gives a
good approximation for the maximum switching
frequency. The maximum switching frequency is a
function of the gate charge of the MOSFET, the
storage capacitor (C
ST
), and R
OVP
. The maximum
switching frequency relationship in Hz is:
Where:
E
RISE
>
V
LOAD
2
I
G_SINK
C
RSS
I
LOAD
6
•
=
P
LOAD
6
•
t
RISE
•
P
AVG
=
I
LOAD
2
•• •R
DSAT
D + f
SWITCH
(E
RISE
+ E
FALL
)
I
LOAD
(t) =
V
LOAD
R
LOAD
I
G_SINK
L
LOAD
• C
RSS
-
()
•
R
LOAD
L
LOAD
•
t - 1 + e
-R
LOAD
L
LOAD
•t
2
R
LOAD
L
LOAD
][
•
V
DRAIN
(t) =
I
G_SINK
C
RSS
•
t
E
FALL
=
1
2
•(C
TVS
+ C
OSS
+ C
LOAD
)•
V
LOAD
2
