During the adapter hot plug-in, the ACDRV has not been enabled. The AC switch is off and the simplified equivalent circuit of the input is shown in Figure 7-1.

Figure 7-1 Simplified Equivalent Circuit During Adapter Insertion

The voltage on the input capacitor(s) is given by:

Equation 6.

in which,

Equation 7.

Equation 8.

The damping conditions is:

Equation 9.

Figure 7-2 (a) demonstrates a higher Ci helps dampen the voltage spike. Figure 7-2 (b) demonstrates the effect of the input stray inductance Li upon the input voltage spike. Figure 7-2 (c) shows how increased resistance helps to suppress the input voltage spike.

Figure 7-2 Parametric Study Of The Input Voltage

As shown in Figure 7-2, minimizing the input stray inductance, increasing the input capacitance, and adding resistance (including using higher ESR capacitors) helps suppress the input voltage spike. However, a user often cannot control input stray inductance and increasing capacitance can increase costs. Therefore, the most efficient and cost-effective approach is to add an external resistor.

Figure 7-3 depicts the recommended input filter design. The measured input voltage and current waveforms are shown in Figure 7-4. The input voltage spike has been well damped by adding a 2Ω resistor, while keeping the capacitance low.

Figure 7-3 Recommended Input Filter Design

Figure 7-4 Adapter DC Side Hot Plug-in Test Waveforms