CPC1560 Datasheet, PDF (9/14 Pages) Clare, Inc. – Solid State Relay with Integrated Current Limit

4.2 Storage Capacitor
The CPC1560 requires the use of an external
capacitor (C_EXT) to meet the device’s specifications.
This external storage capacitor enables the relay to
turn on quickly by holding a reservoir of charge to be
transferred to the gates of the MOSFET pair. The
capacitor must have a minimum working voltage
greater than the load voltage, and must be connected
from pin 8 (C+), the capacitor’s positive voltage
terminal, to pin 5 (C-), the capacitor’s negative voltage
terminal.
Proper selection of the external capacitor begins with
the recommended range provided in the
“Recommended Operating Conditions” on page 4, and the
maximum voltage at the CPC1560 outputs, including
transients and faults. The nominal value of the
capacitor needs to be chosen so that when the effects
of tolerance, temperature coefficient, and (for some
types of capacitor) derating due to bias voltage are
accounted for, the capacitor’s value remains within the
recommended range over the operational conditions
of the end product.
5. Operational Behavior
5.1 Operating Frequency
5.1.1 Duty Cycle/Power Dissipation
Equation 1 shows the relationship between power
dissipation, operating frequency, and duty cycle for the
CPC1560 device. From this equation, it can be seen
that both switching frequency (f_switch) and duty cycle
(D) contribute to power dissipation. The first one by
generating switching losses, and the second one by
generating ON losses. Switching losses are those
caused by changes in the energy state of the load
components when the device is switching on and off
(i.e. E_RISE and E_FALL), and ON losses are those
caused by the flow of current (I_L) through the part’s
on-resistance (R_ON) when it is switched on.
(1) P_avg = I_L2 • R_ON • D + f_switch • (E_RISE + E_FALL)
Because a higher operating frequency translates into
higher power consumed by the part, care must be
taken to limit its value in order to protect the device
from exceeding its maximum power rating. When
doing this, both the maximum allowed power
dissipation in the part and the ON duty cycle,
D=t_ON / (t_ON+t_OFF), must be taken into consideration.
CPC1560
5.1.2 Temperature Effects
When setting the operating frequency of the
CPC1560, the user must also take into account power
dissipation over temperature.
5.1.3 Elements of Operating Frequency
In addition to ambient temperature, the maximum
frequency of the CPC1560 is also determined by the
MOSFET’s turn-on and turn-off times and the load
voltage rise and fall times as follows:
(2)
f_MAX =
1
3
(t_ON + t_OFF)^-1
Where 1/3 is a multiplication factor for temperature
and process variations.
5.2 Switching Losses
During the transition intervals of the switching process,
the load components change energy states, which
results in switching losses as the energy passes
through the MOSFETs. This energy transfer is
manifested in the form of heat dissipation and must be
taken into consideration.
Energy is transferred during the turn-off intervals. This
energy, called E_rise, will be absorbed by the MOSFET
output switches, and if present parasitic load
capacitance and the protection device.
Energy is also transferred during the turn-on intervals
and is called E_fall. This energy will be absorbed by the
MOSFET output switches, which is why this energy
should be limited to the “peak turn-on energy” values
specified in the Absolute Maximum Ratings Table of
this datasheet.
The user of the CPC1560 device must understand the
details of the load behavior and keep in mind the
device’s recommended operating conditions in order
to adequately size the load components and protect
the application circuit.
The average power of the CPC1560 output MOSFET
for any specific application and for any load type given
by Equation 1 and repeated here is:
(3) P_avg = I_L2 • R_ON • D + f_switch • (E_RISE + E_FALL)
From this equation we can see how the switching
losses (E_RISE and E_FALL), together with the “on
losses,” contribute to the CPC1560’s output power
dissipation.
The user must also know that the recommended
operating conditions for I_L, f_SWITCH, load capacitance
R00F
PRELIMINARY
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