Power Supply

1
Power Supply

• You are required to design a power supply circuit
  that supplies power to electronic equipment. The
  power equipment operate on a dc voltage in the
  range of 5 to 20 V. Your power supply is fed from
  the normal ac line (220 V, 50 Hz)
• Describe the change of signal shape required

2
pulsating DC
AC
3

• The block diagram for a complete power supply is
  shown below with the signals shapes at different
  locations

4
TYPES OF RECTIFIERS
Center-tapped full-wave rec.
Full-wave Bridge rec.
5
Diodes in AC Circuits
6
Diodes in AC Circuits
7
The DC Voltage out of the diode VDC 0.318Vm
where Vm the peak voltage
8
Half-Wave Rectification
This circuit is called half-wave rectifier, which
generate waveform vo that will have an average
value of particular use in the ac-to-dc
conversion process. The diode only conducts
when it is in forward bias, therefore only half
of the AC cycle passes through the diode. The DC
Voltage out of the diode Vdc 0.318Vm
where Vm the peak voltage
9
Peak Inverse Voltage (PIV)

• Because the diode is only forward biased for
  one-half of the AC cycle, it is then also off for
  one-half of the AC cycle. It is important that
  the reverse breakdown voltage rating of the diode
  be high enough to withstand the peak AC voltage.
• PIV (PRV) gt Vm
• PIV Peak Inverse Voltage PRV Peak Reverse
  Voltage
• Vm Peak AC Voltage

10

• To get the d.c current, i.e the average value of
  a complete period (as measured across the load R,
  then we use the following

11

• But
• If diode is is ideal and/or RgtgtRf then

12

• To get the effective current that is measured by
  using a.c (r.m.s) ammeter we use

13
The effect of using a silicon diode with VT0.7
is demonstrated
14
Example

• (a) Sketch the output voltage and determine the
  dc level of the output for the network shown
• (b) Repeat part (a) if the ideal diode is
  replaced by a silicon diode.
• (c) Repeat parts (a) and (b) if Vi is increased
  to 200 V

15
Solution

• (a) Vdc -0.318Vm -0.318(20 V)
  -6.36 V
• The negative sign indicates that the polarity of
  the output is opposite to the defined positive
  polarity

16

• (b) using a silicon diode
• Vdc -0.318(Vm -0.7) -6.14 V
• (c) Vdc -0.318(200) -63.6 V
• Vdc -0.318(200-0.7) -63.38 V (the difference
  can be ignored in this case)

17
Half-Wave Rectifier with Transformer-Coupled
Input Voltage

• A transformer is often used to couple the ac
  input voltages from the source to the rectifier
  circuit. Transformer coupling provides two
  advantages.
• First, it allows the source voltage to be stepped
  up or stepped down as needed.
• Second, the ac power source is electrically
  isolated from the rectifier circuit, thus
  reducing the shock hazard.
• The primary and secondary voltages are related as
  follows

18
Example

• Determine the peak value of the output voltage
  for the following figure

19
Solution

• Vsec 55 V
• Vout 55-0.7 54.3 V