NP−SCMC Series

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5

and shorts the transient to ground, safely protecting the

circuit.

Figure 4. Normal and Fault Conditions

Protected

Equipment

+

−

+

−

TSPD

Normal Circuit Operation

Protected

Equipment

+

−

+

−

TSPD

Operation during a Fault

•TSPD looks like an open

•Circuit operates normally

•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;

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

circuit will see, this is also known as the protection voltage.

keep the TSPD in its normal open circuit state. The TSPD

on these characteristics it is critical to choose devices which

of the protected equipment circuit. A low on−state voltage

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 5. Protection During a Transient Surge

TSPD Transparent

(open)

TSPD Transparent

(open)

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.

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

components:

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.

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.