CPC1580 Datasheet, PDF (9/11 Pages) Clare, Inc. – Optically Isolated Gate Drive Circuit

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,
(2) inductive/resistive loads, and (3) loads with
significant capacitance. Inductors and capacitors are
energy storage elements that require special
consideration for switching.
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.
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.
To calculate the stored inductive energy in Joules:
E_L =
1
2
• L • I_LOAD2
5.1 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:
E_RISE > V_{LOAD2 •}
C_RSS
I_LOAD
•
=
P_{LOAD • tRISE}
I_{G_SINK}
6
6
The average power of the MOSFET for any load type
in Watts is:
P_AVG = I_LOAD2 • R_DSAT • D + f_SWITCH • (E_RISE + E_FALL)
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.
CPC1580
5.2 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:
( ) [ ] ^VLOAD
I_{G_SINK}
I_LOAD(t) =
R_LOAD
-
•
L_LOAD • C_RSS
2
L_LOAD
•
R_LOAD
R_LOAD
•
-R _{LOAD • t}
L_LOAD
t-1+e
L_LOAD
and the MOSFET drain voltage during turn off, t_RISE,
in Volts is:
V_DRAIN(t)
=
I_{G_SINK}
C_RSS
•
t
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.
5.3 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:
1
E_FALL = ₂ • (C_TVS + C_OSS + C_LOAD) • V_LOAD2
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.
5.4 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.
R00G
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