I
NTEGRATED
C
IRCUITS
D
IVISION
CPC1580
6 www.ixysic.com R01
2. Introduction
The CPC1580 Isolated Gate Driver uses an efficient
optocoupler design to provide remote gate drive
current to a MOSFET, while providing 3750V
rms
of
isolation between the LED control current input and
the MOSFET gate drive output. The CPC1580 is used
in conjunction with a MOSFET transistor for remote
switching of DC loads (Figure 1) and two MOSFETS
and a diode at low-frequency AC rates (Figure 2)
where isolated power is unavailable.
By selecting a few external components, the charge
capacitor and resistors, the designer has control over
the operating parameters of the CPC1580 circuit, and
can customize the circuit to accommodate the
requirements of a wide selection of MOSFETs.
The designer just needs to know the MOSFET total
gate charge (Q
G
), and with this information a capacitor
can be chosen. The capacitance of the storage
capacitor, C
ST
, should be greater than or equal to Q
G
.
3. Functional Description
The CPC1580 is operational when sufficient input
control current is flowing, the LED is turned on, and
the gate current is flowing. The LED illuminates the
photovoltaics (converts light into electrical power, or
photocurrent), which provides current to turn on the
NPN bipolar transistor. The NPN transistor then allows
for charge to go to the gate of the MOSFET. When an
external storage capacitor is added to the CPC1580,
the photocurrent that is produced turns on the NPN
bipolar transistor and provides the charge (I x t = Q)
plus the charge of the capacitor to turn on the
MOSFET rapidly. If sufficient input control current is
not flowing, the LED is turned off, and gate current is
not flowing. The LED is off due to the V
F
<< the
minimum forward voltage required and not enough
current being applied. This turns on the PNP bipolar
transistor, which provides a path for Q
G
to discharge to
V
S
.
When V
L
is first applied, the external storage capacitor
begins to charge. The value of the storage capacitor
should be equal to or greater than the MOSFET gate
capacitance: this will ensure proper operation. The
charge is sent through a bootstrap diode to prevent
the charge from escaping and discharging through a
turned-on MOSFET. The input control current is
applied, then the charge is transferred from the
storage capacitor through the NPN bipolar transistor,
along with the charge from the photovoltaics, to the
MOSFET gate to accomplish a rapid turn-on. After the
capacitor has discharged and the MOSFET has
turned on, the photocurrent from the input optocoupler
continues to flow into the gate to keep the MOSFET
turned on.
When the input control current is removed, the gate
current stops flowing and the PNP bipolar transistor is
on and is discharging the MOSFET gate. The
MOSFET is now off. At this point, the capacitor begins
to recharge for the next turn-on cycle.
4. Device Configuration
4.1 LED Resistor
LED resistor selection should comply with the
recommended operating conditions. This will provide
reliability to the design, and should help with
temperature. The CPC1580 is capable of being
operated at up to the maximum ratings, but this is not
recommended. It will shorten the life-span of the
device and could cause temperature problems that will
produce inaccuracies. The reason for using a higher I
F
current is to provide for faster turn-on. Proper design
will have to be used to decide on the needs of the
application. The equation used to calculate the
resistor value:
• I
F
= Input Control Current
• V
IN
= Input Power Source
• V
F
= Forward Voltage Drop of LED
• R
LED
= Input Resistor connected to LED
4.2 Storage Capacitor Selection
The storage capacitor (C
ST
) enables the part to turn
on quickly by holding a reservoir of charge to be
transferred to the gate of the MOSFET. The turn-off
cycle does not depend on the storage capacitor.
The equation used to calculate the value of the charge
storage capacitor is:
C
ST
>
Q
G
V
LOAD
- V
CAP
(FARADS)
