Problem
The magnitude of the overshoot is the vector sum of two orthogonal voltages, the output voltage before the load is removed and the current through the inductor times the characteristic impedance of the output filter, Zo = (L/C)^1/2. This can be derived from conservation of energy considerations. The initial energy, Ei, is: Ei = 1/2*(L*Ii^2 + C*Vi^2)
The final energy, Ef, is: Ef = 1/2*(L*If^2 = C*Vf^2)
The two energies are equal when the load is removed, since the load is no longer taking energy from the system. Equating the two energies, substituting zero current for the final inductor current, then the solution for the final voltage Vf is: Vf = (Vi^2 + (Ii*Zo)^2)^1/2
This is the orthogonal vector sum of the output voltage and the load current times the characteristic impedance and is illustrated in Figure 1. 
Figure 1: Overshoot Voltage as Vector Sum
The problem becomes worse if the current in the inductor is established by a short circuit on the output and the short circuit clears. In this case, the initial voltage is zero (short circuit) and the overshoot is I*Zo, where I can be very large, resulting in a ruinous overshoot. Do not use this information for design without independent verification of the information. Editor: Jerrold Foutz |
