NPMC Series
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5
Detailed Operating Description
The TSPD or Thyristor Surge Protection Device are
specialized silicon based overvoltage protectors, used to
protect sensitive electronic circuits from damaging
overvoltage transient surges caused by induced lightning
and powercross conditions.
The TSPD protects by switching to a low on state voltage
when the specified protection voltage is exceeded. This is
known as a “crowbar” effect. When an overvoltage occurs,
the crowbar device changes from a high−impedance to a
low−impedance state. This low−impedance state then offers
a path to ground, shunting unwanted surges away from the
sensitive circuits.
This crowbar action defines the TSPD’s two states of
functionality: Open Circuit and Short Circuit.
Open Circuit
– The TSPD must remain transparent during
normal circuit operation. The device looks like an open
across the two wire line.
Short Circuit
– When a transient surge fault exceeds the
TSPD protection voltage threshold, the devices switches on,
and shorts the transient to ground, safely protecting the
circuit.
Figure 5. Normal and Fault Conditions
Protected
Equipment
+
−
+
−
V
(OP)
I
(OP)
TSPD
Normal Circuit Operation
Protected
Equipment
+
−
+
−
V
(Fault)
I
(Fault)
TSPD
Operation during a Fault
I
(Fault)
•TSPD looks like an open
•Circuit operates normally
•Fault voltage greater than V
bo
occurs
•TSPD shorts fault to ground
•After short duration events the O/V
switches back to an open condition
•Worst case (Fail/Safe)
•O/V permanent short
•Equipment protected
The electrical characteristics of the TSPD help the user to
define the protection threshold for the circuit. During the
open circuit condition the device must remain transparent;
this is defined by the I
DRM
. The I
DRM
should be as low as
possible. The typical value is less than 5 mA.
The circuit operating voltage and protection voltage must
be understood and considered during circuit design. The
V
(BO)
is the guaranteed maximum voltage that the protected
circuit will see, this is also known as the protection voltage.
The V
DRM
is the guaranteed maximum voltage that will
keep the TSPD in its normal open circuit state. The TSPD
V
(BO)
is typically a 20−30% higher than the V
DRM
. Based
on these characteristics it is critical to choose devices which
have a V
DRM
higher than the normal circuit operating
voltage, and a V
(BO)
which is less than the failure threshold
of the protected equipment circuit. A low on−state voltage
V
t
allows the TSPD to conduct large amounts of surge
current (500 A) in a small package size.
Once a transient surge has passed and the operating
voltage and currents have dropped to their normal level the
TSPD changes back to its open circuit state.
Normal System
Operating Voltage
Equipment Failure Threshold
Time
Transient Surge
TSPD Protection
(short)
Figure 6. Protection During a Transient Surge
TSPD Transparent
(open)
TSPD Transparent
(open)
Volts
TSPD Protection Voltage
Upper Limit
TSPD’s are useful in helping designers meet safety and
regulatory standards in Telecom equipment including
GR−1089−CORE, ITU−K.20, ITU−K.21, ITU−K.45, FCC
Part 68, UL1950, and EN 60950.
ON Semiconductor offers a full range of these products in
the NP series product line.
DEVICE SELECTION
When selecting a TSPD use the following key selection
parameters.
Off−State Voltage V
DRM
Choose a TSPD that has an Off−State Voltage greater than
the normal system operating voltage. The protector should
not operate under these conditions:
Example:
Vbat = 48 Vmax
Vring = 150 Vrms = 150*1.414 = 212 V peak
V
DRM
should be greater than the peak value of these two
components:
V
DRM
> 212 + 48 = 260 V
DRM
Breakover Voltage V
(BO)
Verify that the TSPD Breakover Voltage is a value less
than the peak voltage rating of the circuit it is protecting.
Example: Relay breakdown voltage, SLIC maximum
voltage, or coupling capacitor maximum rated voltage.
Peak Pulse Current Ipps
Choose a Peak Pulse current value which will exceed the
anticipated surge currents in testing. In some cases the 100 A
“C” series device may be needed when little or no series
resistance is used. When a series current limiter is used in the
circuit a lower current level of “A” or “B” may be used. To
determine the peak current divide the maximum surge
current by the series resistance.
Hold Current (I
H
)
The Hold Current must be greater than the maximum
system generated current. If it is not then the TSPD will
remain in a shorted condition, even after a transient event
has passed.
