**Subject :**Power Electronics

**Unit :**DC to DC Converters

## Maximum and Minimum Values of the Load Current

**Maximum and Minimum Values of the Load Current:**

The procedure for finding the maximum and minimum of the load current, assuming continuous conduction, is described. The operation of the chopper circuit has been discussed in the earlier lesson. There are two modes of operation. Mode 1 starts at 0=*t*, when the thyristor is turned ON, with the diode, D_{F }being OFF at that time, and continues till t = T_{ON}. During this time period, the load current increases. The induced emf in the load inductance L, is positive, i.e., having the same polarity as that of the input voltage, opposing it. Mode 2 starts at t = T_{ON}, when the thyristor is turned OFF by auxiliary circuit , and the diode, D_{F }turns ON at that time, as the load current starts decreasing, and the induced emf in the load inductance, L changes polarity, with the voltage across the diode now being positive. This continues till t = T, end of the time period. Then the cycle repeats.

**Mode 1: **

The equation for the load (output) current in the circuit during this time interval, 0 ≤ t ≤ T_{ON} is,

The current is the load current, same as the source current during this time interval. The values of the load current (i_{o}) at t = 0 and t = T_{ON}, are I_{min} and I_{max} resp. The expression for the load current is **, **where A and B are constants and time constant is **.**

**Mode 2: **T

he equation for the load (output) current in the circuit during this time interval, 0 ≤ t ≤ T_{OFF} is

The current is the load current, and the current through the diode, D_{F} during this time interval. The values of the load current (i_{0}) at t = 0 and t = T_{OFF}, are I_{min }and I_{max} resp. The expression for the load current is **.**

**Ripple content in the Load Current:**

The load (output) current varies between the maximum and minimum values (I_{max} and I_{min}). Therefore, the ripple content of the current is,