Rectifier smoothing circuits 1

1
Rectifier smoothing circuits (1)

• Direct output from a rectifier (controlled or
  uncontrolled) is often not smooth enough for
  the load requirements and some sort of smoothing
  circuit is employed
• 2 Basic types of smoothing Inductive smoothing
  and Capacitive smoothing
• Inductive smoothing attempt to smooth rectifier
  output current

• Used for diode and thyristor circuits
• Often LDC is partially (or possibly completely)
  supplied by the load itself (ie DC Motor)

2
Rectifier smoothing circuits (2)

• Capacitive smoothing attempt to smooth
  rectifier output voltage

• Only used with diode circuits
• Extensively used as the front end to provide
  the DC link (DC supply) needed for PWM inverter
  variable speed drive systems (see later in
  module), often LC version (next page) is used at
  higher powers

3
Rectifier smoothing circuits (3)

• A combination of Capacitive and Inductive (LC)
  smoothing is sometimes used for diode circuits
• Operates like inductive smoothing if rectifier
  output current is continuous
• Operates like capacitive smoothing if rectifier
  output current is discontinuous (ie has intervals
  where it 0)

• Analysis of all smoothing circuits depends on
  whether rectifier output current is continuous or
  discontinuous
• Discontinuous capacitive (usually), Inductive
  at very low output currents, L-C at low output
  currents
• Continuous inductive at normal output currents,
  LC at high output currents

4
Inductive Smoothing- continuous current

• Waveforms see handout
• Analysis method to find ?I for a given L or vice
  versa
• Assume VL is smooth
• Use mean value of ID to determine VXY waveform
  (ie ? and ?)
• VL mean value of VXY (mean voltage across
  inductor 0)
• Draw VXY on template and superimpose VL
• Determine whether VL crosses VXY above or below
  the overlap part of the waveform
• Calculate the easier of the two areas (above or
  below VL) and use ?I voltage-time area/L
• If VL happens to cross through the overlap part
  of VXY then neglect the smaller of the two little
  triangular areas formed
• If the inductor has a resistance R then use the
  approximation VL Id(mean) R mean of VXY
• See worked example (to be handed out)

5
Inductive Smoothing- discontinuous current

• If the mean value of Id is progressively reduced
  (due to changing load for example), the ripple in
  Id means that it will eventually touch 0 (before
  the mean value gets to zero) and all the devices
  in the rectifier will turn off
• The current will stay at zero until another pair
  of devices is forward biassed (diode circuit) or
  another pair of devices is fired (thyristor
  circuit)
• This is discontinuous current operation see
  handout for waveforms
• Things to note
• There is no overlap
• Each thyristor must be fired twice (once at the
  normal place and again 60O later) to get proper
  operation if the current is discontinuous
  otherwise we will never get a current path
• The expression for VXY(mean) derived before no
  longer holds VXY(mean) is bigger than this
• VXY(mean) depends strongly on the load current
  rectifier has considerable regulation in this
  mode can cause control problems see H5CEDR
• Can be analysed like capacitive smoothing (see
  later) for the diode case more difficult for
  thyristor case in general

6
Capacitive Smoothing

• See handout for waveforms
• Current normally discontinuous unless extra AC
  side inductance is added and current is high we
  will assume discontinuous current
• For analysis assume capacitor voltage is smooth
  see handout
• Things to note
• There is no overlap
• Supply current has very high harmonic distortion
  distortion factor is poor (displacement factor
  is OK) hence power factor is poor
• Power factor can be improved by adding AC side
  inductance to make pulses of current broader and
  lower
• Poor power quality aspects of this circuit are
  making it increasingly unacceptable but it is
  still used in very large numbers

7
LC Smoothing

• If current is continuous analyse exactly as
  inductive smoothing
• If current is discontinuous analyse as
  capacitive smoothing
• Remember to add the DC side inductance in the
  equation for calculating the peak current