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

2L I Commutation angle: cos  1 s d 2Vs 2L 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

2Ls Id Commutation angle: cos  1 2VLL

3Ls 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.

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