Diode Rectifiers EE 442-642 Fall 2012 5-1 Half-Bridge Rectifier Circuit: R and R-L Load Current continues to flow for a while even after the input voltage has gone negative. 5-2 Half Bridge Rectifier Circuit: Load with dc back-emf • Current begins to flow when the input voltage exceeds the dc back-emf. • Current continues to flows for a while even after the input voltage has gone below the dc back-emf. 5-3 Full Bridge Rectifier – Simple R Load Average value of output voltage Vdo (2 2 /)Vs 0.9Vs where Vs is the RMS value of input voltage. 5-4 Full Bridge Rectifier – Simple Constant Load Current I I RSM value of source current s d RMS value of fundamental current I s1 (2 2 / )Id 0.9Id RMS value of harmonic current I sh I s1 / h, h 3,5,7,... Current THD THD 100[ ( 2 /8) 1] 48.43% Displacement Power Factor DPF 1 Power Factor PF 0.9 5-5 Diode-Rectifier Bridge with AC-Side Inductance 2L I Commutation angle: cos 1 s d 2Vs 2L I Average of DC-side voltage: V 0.9V s d d s 5-6 Full Bridge Rectifier with dc-side Voltage Average value of DC-side current (obtained numerically): Zero current corresponds to dc voltage equal to the peak of the input ac voltage 5-7 Diode-Rectifier with a Capacitor Filter 5-8 Voltage Distortion at PCC • PCC is the point of common coupling • Distorted current flow results in distorted voltage 5-9 Dual Voltage Rectifier • In 115-V position, one capacitor at-a-time is charged from the input. 5-10 Three-Phase, Four-Wire System • A common neutral wire is assumed • The current in the neutral wire is composed mainly of the third harmonic and can be higher than the phase currents 5-11 Three-Phase, Full-Bridge Rectifier Average value of DC-side voltage: 3 Vdo 2VLL 1.35VLL where VLL is the rms value of the Line voltage 5-12 Three-Phase, Full-Bridge Rectifier I 2 / 3I 0.816I RSM value of source current s d d RMS value of fundamental current I s1 ( 6 / )Id 0.78Id RMS value of harmonic current I sh I s1 / h, h 3,5,7,... 2 Current THD THD 100[ ( / 9) 1] 31% Displacement Power Factor DPF 1 3 Power Factor PF 0.955 5-13 3-Phase, Full-Bridge Rectifier with ac-side inductance 2Ls Id Commutation angle: cos 1 2VLL 3Ls Id Average of DC-side voltage: V 1.35V d LL 5-14 3-Phase Rectifier with DC Source 5-15 Three-Phase Rectifier with Capacitor Filter • PSpice-based analysis 5-16 Inrush Current and Over-voltage at Turn ON • Theroretical maximum voltage across the capacitor occurs when the capacitor is initially discharged and the AC input is switched at its peak: – twice the peak phase voltage (in 1-phase ckts), – twice the peak line voltage (in 3-phase ckts). • Overvoltages and inrush currents that follow may be damaging to capacitors and diodes. • Possible solution: Use current limiting resistor on the DC side between the rectifier output and filter capacitor. The resistor should be shorted out after few cycles to limit losses. 5-17 .