Digital Electronic Systems - DC Power Supply Overview

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Questions and Answers

What is the purpose of the bridge rectifier in the AC rectifier and filter process?

To convert high voltage DC into low voltage DC

To filter noise from the AC signal

To rectify mains AC into high voltage DC (correct)

To step down the voltage from the transformer

What component is typically used as a switch in the chopper process?

Diode

MOSFET (correct)

Bipolar Junction Transistor

Operational Amplifier

How does increasing the frequency of the switching signal affect the transformer size?

It has no effect on the transformer size

It makes the transformer less efficient

It decreases the transformer size (correct)

It increases the transformer size for efficiency

What is the function of the filtering capacitors in the pulsed DC rectifier and filter stage?

To produce non-pulsed DC from stepped down pulsed DC Signup and view all the answers

What is controlled by the duty cycle of the switching signal in a switcher?

The output voltage Signup and view all the answers

What occurs during the positive first quarter-cycle when the capacitor is charging in a half-wave rectifier?

The diode is forward-biased. Signup and view all the answers

What determines the rate at which the capacitor discharges in a half-wave rectifier?

The load resistance and capacitance values. Signup and view all the answers

What is the Peak Inverse Voltage (PIV) in a half-wave rectifier configuration?

2$V_p$ Signup and view all the answers

What is the effect of a larger time constant in the capacitor-input filter?

The capacitor retains its charge longer. Signup and view all the answers

How does the ripple voltage behave in a half-wave rectifier compared to a full-wave rectifier?

It is smaller for a full-wave rectifier. Signup and view all the answers

What is the primary reason a full-wave rectifier is easier to filter than a half-wave rectifier?

It produces a higher output frequency. Signup and view all the answers

What denotes the variation in output voltage due to the charging and discharging of the capacitor?

Ripple voltage. Signup and view all the answers

During which cycle does the diode become reverse-biased in a half-wave rectifier?

When the input voltage drops below capacitor voltage. Signup and view all the answers

What happens to the ripple voltage when the capacitor value increases?

The ripple voltage decreases. Signup and view all the answers

What is the formula for the ripple factor?

r = Vr / VDC * 100% Signup and view all the answers

What is a good guideline for the relationship between load resistance, capacitance, and the period of rectified voltage?

RL C ≥ 10T Signup and view all the answers

What is the primary function of an integrated circuit voltage regulator?

To maintain a constant output voltage. Signup and view all the answers

What is the purpose of combining a capacitor-input filter with an IC regulator?

To produce a low-cost power supply. Signup and view all the answers

Which of the following statements about ripple factor is accurate?

A lower ripple factor implies better filter performance. Signup and view all the answers

Why is it important for the ripple voltage to be less than 10% before entering the regulator?

To ensure accurate voltage regulation. Signup and view all the answers

Which components are typically found in most integrated circuit regulators?

Voltage reference, short-circuit protection, and thermal shutdown circuitry. Signup and view all the answers

What is the primary disadvantage of linear power supplies mentioned?

Large size and weight Signup and view all the answers

Which component is typically used as a switch in the operation of a Switching Mode Power Supply (SMPS)?

MOSFET Signup and view all the answers

What happens to transformer size as frequency increases?

Transformer size decreases Signup and view all the answers

How is the output voltage regulated in a Switching Mode Power Supply?

By varying the duty cycle of the switch Signup and view all the answers

Which of the following is NOT an advantage of linear power supplies?

Flexibility in operating voltages Signup and view all the answers

What is the first step in the operation of a Switching Mode Power Supply?

High-voltage AC rectification Signup and view all the answers

What is the function of the transformer in a SMPS?

To convert high frequency pulsed signal to lower voltage Signup and view all the answers

Which supply configuration allows for a reduction in transformer size due to fixed AC mains frequency?

Switching Mode Power Supplies Signup and view all the answers

What is the primary function of a voltage regulator?

To maintain a constant output voltage despite changes in load impedance. Signup and view all the answers

In the context of a voltage regulator, what happens if the output voltage is too high?

The duty cycle of the pulsed gate signal is decreased. Signup and view all the answers

Which type of DC-DC converter increases the input voltage?

Boost converter. Signup and view all the answers

What is one disadvantage of switching mode power supplies (SMPS)?

They are relatively complicated in design. Signup and view all the answers

What role does feedback play in a voltage regulator?

It monitors the output voltage and adjusts the duty cycle accordingly. Signup and view all the answers

Which of the following statements is true regarding SMPS?

They can convert AC to DC and between DC voltages. Signup and view all the answers

What is one way a SMPS can reduce the size of components?

By operating at high frequency to minimize required transformer size. Signup and view all the answers

In an application using a buck-boost converter, what can be expected?

Both an increase and a decrease in output voltage can happen. Signup and view all the answers

What is the minimum peak secondary voltage required for a 7805 regulator to function properly?

9 V Signup and view all the answers

How is line regulation defined?

Change in output voltage for a change in input voltage Signup and view all the answers

What is the formula for calculating load regulation?

(VNL - VFL) / VFL x 100% Signup and view all the answers

For a voltage regulator, why is heat sinking typically necessary?

To prevent overheating of the regulator Signup and view all the answers

If a 7805 regulator has a no-load output voltage of 5.18 V and a full-load output voltage of 5.15 V, what is the load regulation percentage?

0.58% Signup and view all the answers

What does load regulation specifically measure?

Output voltage change over a range of load currents Signup and view all the answers

What is indicated by a high percentage of line regulation?

High stability of output voltage Signup and view all the answers

Which of the following statements is true regarding the use of a transformer with a 7805 regulator?

A transformer with a peak secondary voltage below 9 V is acceptable. Signup and view all the answers

Study Notes

Digital Electronic Systems (1) - Lecture 1

This lecture covers power supplies, specifically the basic DC power supply
A DC power supply converts standard 120 V, 60 Hz AC from wall outlets to a constant DC voltage
This constant DC voltage powers various electronic circuits, including television receivers, stereo systems, VCRs, computers, and laboratory equipment
Power supplies are essential components in electronic systems, ranging from simple to complex circuits
A basic power supply comprises a rectifier, a filter, and a regulator
The rectifier converts AC input voltage into a pulsating DC voltage (half-wave or full-wave rectified)
A capacitor filter smooths out fluctuations in the rectified voltage, producing a relatively stable DC voltage
A regulator circuit maintains a constant DC voltage despite variations in input line voltage or load
Regulators vary in complexity, from simple components to complex integrated circuits
Half-wave rectifiers and full-wave rectifiers are discussed, along with their respective waveforms

Capacitor-Input Filter

A half-wave rectifier with a capacitor-input filter is a common circuit configuration
During the positive first quarter-cycle of the input, the diode is forward-biased, charging the capacitor
The capacitor retains its charge as the input voltage decreases, and the diode becomes reverse-biased
The capacitor discharges only through the load resistance, determined by the time constant R₁C
The larger the time constant, the less the capacitor discharges
The peak inverse voltage (PIV) of the diode in this application is PIV = 2Vp(in)
During the next cycle, the diode again becomes forward-biased when the input voltage exceeds the capacitor voltage, recharging the capacitor
The variation in the output voltage due to the charging and discharging is known as ripple voltage. Lower ripple is better filtering.
For a given input frequency, the output frequency of a full-wave rectifier is twice that of a half-wave rectifier. Therefore, full-wave rectifiers are easier to filter compared to half-wave rectifiers
A good rule of thumb for effective filtering is to make R₁C ≥ 10T, where T is the period of the rectified voltage
The ripple factor (r) is an indication of filter effectiveness, calculated as r = (Vr/VDC) * 100%

IC Regulated Power Supplies

The most effective power supply filter combines a capacitor-input filter with an integrated circuit (IC) voltage regulator
The regulator maintains a constant output voltage despite variations in input, load current, and temperature
The capacitor-input filter reduces the input ripple to the regulator, enabling stable operation
IC regulators are inexpensive, supporting high current output with minimal ripple rejection
Three-terminal regulators need only a few external capacitors for a complete power supply, commonly featuring large-value input capacitors and, sometimes, a second, smaller input capacitor for stability
Typical output capacitors, around 0.1µF to 1µF, are connected in parallel to improve transient response.
Examples of three-terminal regulators include the 78XX series (positive outputs) and 79XX series (negative outputs)
The last two digits of the 78XX/79XX number indicate the output voltage
For example, a 7812 regulator provides a +12V output and a 7912 provides a -12V output
Heat sinking is often required to prevent overheating

Switching Mode Power Supplies (SMPS)

Mains AC is rectified and filtered
High-voltage DC is pulsed into a high-frequency signal
Transformers convert high voltage, high-frequency signal to lower voltage
Lower amplitude pulsed DC is filtered into a constant DC output voltage
Output voltage regulation is achieved by altering the switching duty cycle of the power switch
SMPS offers a significant size and weight advantage over linear power supplies

Percent Regulation

Regulation, as a figure of merit, specifies the stability of a voltage regulator in response to input (line) or load changes.
Line regulation expresses the percentage change in output voltage for a given input voltage change.
Load regulation assesses the impact of load changes on output voltage.

Advantages and Disadvantages of SMPS

Advantages: Small size/weight, high efficiency, flexible line voltage/frequency
Disadvantages: Complicated design, higher cost, increased conducted and radiated emissions.

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Description

This lecture introduces the fundamentals of DC power supplies, including the conversion of AC to DC voltage. It covers essential components such as rectifiers, filters, and regulators, which ensure stable power for various electronic circuits. Understanding these concepts is crucial for anyone working with electronic systems.