This document discusses power supply filters and regulators. It covers topics like capacitor-input filters, ripple voltage, surge current, and voltage regulation. The key points are: 1) A power supply filter uses a capacitor to eliminate fluctuations in the rectified output voltage and produce a constant DC voltage, with any small remaining fluctuations called ripple voltage. 2) A capacitor-input filter charges the capacitor at the start of each cycle, and it discharges through the load resistance between cycles, causing ripple voltage. Larger capacitors and loads reduce ripple. 3) Voltage regulators maintain a constant output voltage despite input or load variations. They are used with filtered rectified outputs to produce stable power

1. ELECTRONIC DEVICES &
CIRCUITS (EE-122)
Ayla Safdar
Lecture-07
Power Supply

2. TOPICS COVERED IN PREVIOUS
LECTURE
 Full-Wave Rectifiers
Bridge Full-wave Rectifier
Diode Models
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3. TODAY’S AGENDA
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Power Supply Filters
Basic Power Supply

4. POWER SUPPLY FILTERS
A power supply filter ideally eliminates the fluctuations in
the output voltage of a half-wave or full-wave rectifier and
produces a constant-level dc voltage.
The small amount of fluctuation in the filter output voltage
is called ripple.
Filtering is necessary because electronic circuits require
a constant source of dc voltage and current to provide
power and biasing for proper operation.
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5. POWER SUPPLY FILTERS
Capacitor-Input Filter:
The filter is simply a capacitor connected from the rectifier
output to ground. RL represents the equivalent resistance
of a load.
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6. POWER SUPPLY FILTERS
Capacitor-Input Filter:
For Positive Cycle (First half quarter)
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7. POWER SUPPLY FILTERS
Capacitor-Input Filter:
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8. POWER SUPPLY FILTERS
Capacitor-Input Filter:
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9. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Ripple Voltage:
The capacitor quickly charges at the beginning of a cycle and
slowly discharges through RL after the positive peak of the
input voltage (when the diode is reverse-biased).
The variation in the capacitor voltage due to the charging and
discharging is called the ripple voltage.
Generally, ripple is undesirable; thus, the smaller the ripple, the
better the filtering action.
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10. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Ripple Voltage:
For a given input frequency, the output frequency of a full-
wave rectifier is twice that of a half-wave rectifier.
This makes a full-wave rectifier easier to filter because of
the shorter time between peaks.
When filtered, the full-wave rectified voltage has a smaller
ripple than does a half-wave voltage for the same load
resistance and capacitor values.
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11. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Ripple Voltage:
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12. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Ripple Factor:
The ripple factor (r) is an indication of the effectiveness of
the filter and is defined as
The lower the ripple factor, the better the filter.
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13. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Ripple Factor:
For a full-wave rectifier with a capacitor-input filter,
approximations for the peak-to peak ripple voltage, Vr(pp)
and the dc value of the filter output voltage, VDC, are
given in the following equations.
The variable Vp(rect) is the unfiltered peak rectified
voltage.
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14. POWER SUPPLY FILTERS
Capacitor-Input Filter:
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15. POWER SUPPLY FILTERS
Capacitor-Input Filter:
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16. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Surge Current:
Before the switch is closed, the filter capacitor is uncharged.
At the instant the switch is closed, voltage is connected to the bridge
and the uncharged capacitor appears as a short.
 This produces an initial surge of current, Isurge, through the two
forward-biased diodes D1 and D2.
The worst-case situation occurs when the switch is closed at a peak
of the secondary voltage and a maximum surge current, Isurge(max).
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17. POWER SUPPLY FILTERS
Capacitor-Input Filter:
Surge Current:
In dc power supplies, a fuse is always placed in the
primary circuit of the transformer.
The fuse rating is determined by calculating the power in
the power supply load, which is the output power.
Since Pin = Pout in an ideal transformer, the primary
current can be calculated as
The fuse rating should be at least 20% larger than the
calculated value of Ipri.
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18. POWER SUPPLY REGULATORS
Voltage Regulation:
A voltage regulator is connected to the output of a filtered
rectifier and maintains a constant output voltage (or
current) despite changes in the input, the load current, or
the temperature.
The capacitor-input filter reduces the input ripple to the
regulator to an acceptable level.
The combination of a large capacitor and a voltage
regulator helps produce an excellent power supply.
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19. POWER SUPPLY REGULATORS
Voltage Regulation:
Most regulators are integrated circuits and have three
terminals—an input terminal, an output terminal, and a
reference (or adjust) terminal.
 The input to the regulator is first filtered with a capacitor
to reduce the ripple to <10%. The regulator reduces the
ripple to a negligible amount.
Three-terminal regulators designed for fixed output
voltages require only external capacitors to complete the
regulation portion of the power supply.
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20. POWER SUPPLY REGULATORS
Voltage Regulation:
Filtering is accomplished by a large-value capacitor
between the input voltage and ground.
An output capacitor (typically 0.1µF to 1.0µF) is
connected from the output to ground to improve the
transient response.
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21. POWER SUPPLY REGULATORS
Percent Regulation
Line Regulation:
The line regulation specifies how much change occurs in the
output voltage for a given change in the input voltage.
Load Regulation:
The load regulation specifies how much change occurs in the
output voltage over a certain range of load current values,
usually from minimum current (no load, NL) to maximum
current (full load, FL).
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22. POWER SUPPLY REGULATORS
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23. BASIC POWER SUPPLY
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