EE 305 Power Electronics Dr Unnikrishnan P C

EE 305 Power Electronics Dr. Unnikrishnan P. C. Professor, EEE

Classification of Converters

Phase Controlled Rectifiers (Converters) • Uncontrolled Rectifier : Uses only Diodes • Half Controlled Rectifier: Uses Thyristors and Diodes • Fully Controlled Rectifier : Uses only Thyristors

Single Phase Fully Controlled Bridge Rectifier with R-Load

Single Phase Fully Controlled Bridge Rectifier with R-Load

Resistive load • During the positive half cycle of the input voltage T 1 and T 2 forward biased. At ɷt=α, T 1 and T 2 turned ON. • At ɷt=π, T 1 and T 2 commutated. • During negative half cycle, T 3 and T 4 forward biased. At ɷt= π +α, T 3 and T 4 turned ON. • At ɷt=2π, T 3 and T 4 commutated.

Simulation

Single Phase Fully Controlled Bridge Rectifier with RL-Load (Highly Inductive Load)

Why the Average Output power negative for /2< < Firing angle greater than /2 • Prior to firing T 1 & T 2, Thyristors T 3 & T 4 are conducting with V 0 negative and I 0 is positive. So the power is negative. After firing T 1 & T 2, part of power becomes positive but since the negative power is larger, average power is negative.

Average & RMS Value of Output The average value of output voltage Therefore, Changing the firing angle from 0 to π/2, the output voltage varies from to 0. The rms value of output voltage

Single Phase Fully Controlled Bridge Rectifier with R-L-E Load (a) Circuit Diagram (b) Conduction Table

Continuous Conduction Mode (CCM) The angle θ is given by Continuous conduction mode is possible only if i 0 remains greater than zero till T 3 T 4 are fired at ωt = π + α DC Link Voltage Waveforms The input ac current waveform ii is obtained from i 0 by noting that whenever T 1 T 2 conducts ii = i 0 and ii = - i 0 whenever T 3 T 4 conducts.

CCM When the thyristor turns off at ωt = π + α a negative voltage is applied across it for a duration of π – α. The thyristor must turn off during this interval for successful operation of the converter.

CCM- Output Voltage Equations • The dc voltage waveform is periodic over half the input cycle. Therefore, it can be expressed in a Fourier series.

Discontinuous Conduction Mode (DCM) Boundary between CCM and DCM Discontinuous Conduction Mode

Free Wheeling Diode D 1 is a free-wheeling diode necessary to ensure that the circuit be able to turn off an inductive load. Without this diode, when the gate pulses are stopped, the current may never drop to zero and one thyristor may continue to conduct. The free-wheeling diode also relieves the thyristors from freewheeling duty, allowing the use of lower power thyristors.

• When the load is inductive, the output voltage can be negative for part of the cycle. This is because an inductor stores energy in its magnetic field which is later released. Current continues to flow, and the same thyristors continue to conduct, until all the stored energy is released. Since this occurs some time after the ac source voltage passes through zero, the output voltage becomes negative for part of cycle. • The negative part of the output voltage waveform reduces the average output voltage E 0. A free-wheeling diode can be placed in the circuit to prevent the output voltage from going negative. When the output voltage begins to go negative, the free-wheeling diode conducts. This maintains the output voltage at approximately zero while the energy stored in the inductor is released. The output voltage waveform is the same as for a purely resistive load, and the average output voltage is therefore greater than it would be without the free-wheeling diode. Freewheeling diode makes the output current waveform smoother.