CPC1580 Datasheet, PDF (8/11 Pages) Clare, Inc. – Optically Isolated Gate Drive Circuit

CPC1580
4. CPC1580 Over-Voltage Protection
Over-voltage protection is generally required for the
CPC1580 because of parasitic inductance in the load,
wires, board traces, and axial leads of protectors.
Purely resistive loads or loads with low voltage
switching may be able to rely on the transistor to
handle any parasitic energy and thereby not require
protection for the CPC1580. For very low inductance
loads and traces, over-voltage suppression may be
handled with a simple R-C filter consisting of R_OVP
and C_OVP, or by use of a free-wheeling diode (see
Figure 2). For more moderate load inductance, or
remote switching of a load (i.e. through a long cable) a
voltage suppressor can be used. For heavily inductive
loads only a free-wheeling diode, D_OVP, connected
across the load element is recommended, see
Figure 2.
The energy not consumed in switching losses must be
absorbed by the over-voltage protection element. Most
protective devices are designed to withstand certain
peak power, in the case of a Transient Voltage
Suppressor (TVS); or maximum avalanche energy, in
the case of a MOSFET. Understanding the switching
losses and load dynamics is absolutely essential.
One simple way to reduce the amount of stored
inductive energy is to increase the energy dissipated
in the switch. This can be accomplished by adding a
larger capacitor in parallel with the gate-drain
connection of the MOSFET, however care must be
taken so that the rise time and peak current do not
exceed the Safe Operating Area (SOA) rating of the
transistor.
The consequence of increasing the gate-drain
effective capacitance is reduced dV/dt tolerance.
When used in a circuit with an inductive load,
precautions must be taken to prevent damage to the
circuit from inductively generated voltage spikes. The
circuit shown in Figure 2 includes such protection
across the inductive load.
4.1 Other Protection Techniques
Switching loads with higher inductance characteristics
requires consideration of other circuit protection
techniques, device ratings, or protector types. Of
paramount importance is that the designer know the
characteristics of the load being switched.
Figure 2. CPC1580 Over-Voltage Protection for Inductive Loads
CPC1580 _{8 VCAP}
C_ST
D_OVP
1 _NC
4
NC
7
R_OVP
V
Z_LOAD
V+
D
C_OVP
V_IN+
R_LED
2
LED +
5
V
G
Q1
V_IN-
3
LED -
6
V-
V
S
8
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