1SMC5.0AT3 Series

http://onsemi.com

5

APPLICATION NOTES

RESPONSE TIME

In most applications, the transient suppressor device is

placed in parallel with the equipment or component to be

protected. In this situation, there is a time delay associated

with the capacitance of the device and an overshoot

condition associated with the inductance of the device and

the inductance of the connection method. The capacitive

effect is of minor importance in the parallel protection

scheme because it only produces a time delay in the

transition from the operating voltage to the clamp voltage as

shown in Figure 5.

The inductive effects in the device are due to actual

turn-on time (time required for the device to go from zero

current to full current) and lead inductance. This inductive

effect produces an overshoot in the voltage across the

equipment or component being protected as shown in

Figure 6. Minimizing this overshoot is very important in the

application, since the main purpose for adding a transient

suppressor is to clamp voltage spikes. The SMC series have

a very good response time, typically < 1 ns and negligible

inductance. However, external inductive effects could

produce unacceptable overshoot. Proper circuit layout,

minimum lead lengths and placing the suppressor device as

close as possible to the equipment or components to be

protected will minimize this overshoot.

prevent overstress of the protection device. This impedance

should be as high as possible, without restricting the circuit

operation.

DUTY CYCLE DERATING

The data of Figure 1 applies for non-repetitive conditions

and at a lead temperature of 25

°C. If the duty cycle increases,

the peak power must be reduced as indicated by the curves

of Figure 7. Average power must be derated as the lead or

ambient temperature rises above 25

°C. The average power

derating curve normally given on data sheets may be

normalized and used for this purpose.

At first glance the derating curves of Figure 7 appear to be

in error as the 10 ms pulse has a higher derating factor than

the 10

µs pulse. However, when the derating factor for a

given pulse of Figure 7 is multiplied by the peak power value

of Figure 1 for the same pulse, the results follow the

expected trend.