 Hello. Myself, Sunil Kalshati, Ascend Professor, Department of Electronics Engineering, Walshan Institute of Technology, SolarPur. Today, I am going to explain the step down chopper, learning.com. At the end of this session, students can analyze step down chopper, choppers. A DC to DC converter is very much needed nowadays as many industrial applications are dependent upon DC voltage source. The performance of these applications will be improved if we use a variable DC supply. It will help to improve controllability of the equipment also. Chopper is a basically static power electronics device which converts fixed DC voltage or power to variable DC voltage or power. It is nothing but a high speed switch which connects and disconnects the load from a source at a high rate to get variable or chopped voltage at the output. Chopper can increase or decrease the DC voltage level at its opposite side. So chopper serves the same purpose in the DC circuit transfers in case of AC circuits. So it is also known as a DC transformer. Devices used in choppers for low power application, GTO, gate turn off, thyristor, IGBT, power BZT, power MOSFET. Devices used in chopper for high power application, thyristor. A chopper is also known as a DC to DC converter. Choppers are widely used in trolley cars, battery operated vehicles, traction motor control, control of large number of DC motors. They are also used in regenerative braking of DC motors to return energy back to supply and also as DC voltage regulators. It offers greater efficiency, faster response, lower maintenance, smaller size and smooth control. The DC choppers can be classified according to the following manner. Choppers are classified according to the input and output voltage level and according to the channel of voltage and current. First type, choppers are classified according to the input and output voltage level. Step down chopper and step-up chopper. In the step down chopper output voltage is less than input voltage. And in the step-up chopper output voltage is greater than input voltage and in the In the second type, choppers are classified according to the direction of output voltage and current. Class A, Class B, Class C, Class D and Class C. Class A and Class B are single quadrant chopper. Class C, Class D are two quadrant chopper and Class C four quadrant chopper, principle of step down chopper. The figure shows the circuit diagram of step down chopper with purely resistive load. The dotted rectangle represents the chopper switch. It consists of power device with internal firing circuit and internal protection circuits and commutation circuits. The thyristor in the circuit acts as a switch. Now during on time, when the gate pulse is applied to the chopper switch, it conducts, it acts as short. Therefore, the current flows through the V chopper switch R and input is appears across the output. During T off, when thyristor is off, it acts as open, the voltage across load will be 0. Therefore, V0 is equal to 0. These are the waveforms of step down chopper. The waveform shows the output voltage and output current. During T on, during T on, the chopper switch is in conducting set, so input is appears across the output. So, during T on, V0 is equal to V and during T off, chopper switch is in on conducting set, so V0 is equal to 0, no? And it shows the average DC voltage. The dotted line shows the average DC voltage. And same during T on, the current I flows through the conducting switch and the value of current is V by R and during T off, the current is 0. Expression of step down choppers, first expression for average output voltage. Now, for the step down chopper, output is pulsating DC, so take the average value. So, V0 average is equal to 1 upon T, limits of integration 0 to T, V0 of T dT. Here the length of the base, the length of complete cycle is capital T, that is why the limits of integration 0 to T. During T on, V0 is equal to V in and during T off, V0 is equal to 0, therefore the equation becomes 1 upon T, limits of integration 0 to T on, V in into dT plus T on to T, 0 dT. After solving this, we obtain T on upon capital T into V in, therefore T on upon capital T is equal to K, is equal to duty cycle, therefore V0 average is equal to K into V in, where K is the duty cycle. Average output current, IDC is equal to VDC upon R, VDC is equal to K into V in, here V in is equal to V, therefore IDC is equal to V upon R into T on upon capital T, therefore IDC is equal to V upon R into K, output DC current is depends upon the duty cycle. Integration of RMS value of output voltage, root means square value, so V0 is equal to under root 1 upon T, limits of integration 0 to T on, V0 square dT. Now during T on, input is appears across the output, therefore V0 is equal to V, therefore RMS output voltage V0 is equal to under root 1 upon T, limits of integration 0 to T on, V square dT. Now after solving this we obtain V0 is equal to root K into V, this is the RMS value of output voltage, expression for output power, output power P0 is equal to V0 into I0, but I0 is equal to V0 upon R, therefore output power P0 is equal to V0 square upon R, but V0 P0 is equal to V0 square upon R, therefore P0 is equal to K V square upon R, because V0 is equal to root K into V, therefore V0 square is equal to K into V, therefore P0 is equal to K V square upon R, effective input resistance of chopper, R i is equal to V upon I dc, now I dc is equal to V ds upon R, and V dc is equal to K into V, therefore R i is equal to R upon K, the output voltage can be varied by varying the duty cycle, how to vary output voltage of step down chopper, as we know in the step down chopper, output voltage is equal to K into V in, when K is equal to 0, output voltage is equal to 0, and when K is equal to 1, output voltage is equal to V in, by varying K from 0 to 100 percent, the value of output voltage changes from 0 to V in, methods of control, the output dc voltage can be varied by the following methods, pulse width modulation control or constant frequency operation, and second methodology variable frequency control, pulse width modulation control, in converter switching frequency or time period T is kept constant, and on time is varied, the width of pulse is varied, and this type of control is known as a PWM control, variable frequency control, the chopping frequency is varied either on time or off time is kept constant, and this type of control is known as a variable frequency control, these are the references, thank you.