The NEBS standard requires passing 75V transient for 10ms to the system as illustrated in Figure 1-2 during which the output voltage needs to be clamped to 62.5V. In Figure 6-2, the RTN voltage is at 48V and then a transient of 75V is applied at the input for 10ms with a load of 10A.

As soon as the transient is applied, the device enters into over-current protection turns off the FET, and then immediately turns back ON in current limiting mode. During this time, the output voltage (V_OUT) slowly ramps up to RTN. Once V_OUT reaches 62.5V, the output of the op-amp feedback circuit reaches over-voltage protection threshold of 1V, the TPS23521 turns the FET off. Then, the C_OUT discharges, and as soon as the V_OUT goes below 62.5V, the TPS23521 comes out of OVP mode and switches ON the FET again. This cycle continues, operating the FET in hysteretic mode to clamp the output voltage around 62.5V. Figure 6-2 and Figure 6-3 show the over-voltage response of the proposed design. Due to the fast response of the TPS23521 device and the feedback circuit, the ripple on the output voltage is less than 10%.

Figure 6-2 Test Waveforms During 75V/10ms Over-voltage Event

Figure 6-3 Test Waveforms of Input Current During 75V/10ms Over-voltage Event