P6SMB11CAT3 Series

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4

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 4.

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 5. Minimizing this

overshoot is very important in the application, since the main

purpose for adding a transient suppressor is to clamp voltage

spikes. The SMB 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 6. 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 6 appear to be

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

the 10

ms pulse. However, when the derating factor for a

given pulse of Figure 6 is multiplied by the peak power value

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

expected trend.