ANALOG CIRCUIT AND DEVICES

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ANALOG CIRCUIT AND DEVICES. Course Code: EEE 3123. Semester I 2013/2014. TOPIC: VOLTAGE REGULATORS. Power Supply Regulation. An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions.

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ANALOG CIRCUIT AND DEVICES

Course Code:

EEE 3123

Semester I

2013/2014

Power Supply Regulation

An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions.

The output voltage of a real power supply changes under load as shown in the second plot. The output is also sensitive to input voltage changes.

VNL

VNL

VFL

Ideal power supply

Real power supply

Example;

A line regulation of 0.05 %/V means that the output voltage changes 0.05 percent when the input voltage increases or decreases by one volt.

Line Regulation

Line regulation is a measure of how well a power supply is able to maintain the dc output voltage for a change in the ac input line voltage. The formula for line regulation is

Line regulation can also be expressed in terms of percent change in VOUT per volt change on the VIN (%/V).

Example 17.1
• Page 853

Example;

A load regulation of 0.01 %/mA means that the output voltage changes 0.01 percent when the load current increases or decreases by 1 mA.

Load regulation is a measure of how well a power supply is able to maintain the dc output voltage between no load and full load with the input voltage constant. It can be expressed as a percentage change in load voltage:

Load regulation can also be expressed in terms of percent change in the output per mA change in load current (%/mA).

Sometimes a maximum error voltage is given in the specification as illustrated in the next slide for a commercial power supply.

Example 17.2
• Page 854

Commercial power supplies, such as you have in lab, have excellent line and load regulation specifications.

The BKPrecision 1651A is an example of a triple output supply (two 0-24 V outputs and a fixed 5 V output). Voltage regulation specifications for this power supply are:

Line regulation: ≤0.01% +3 mV (Main supply)

≤5 mV (Fixed 5 V supply)

Load regulation: ≤0.01%+3 mV (Main supply)

≤5 mV (Fixed 5 V supply)

Refer Page 854-855 for derivation.

Sometimes the power supply manufactures use the equivalent Thevenin resistance of a supply in place of a load regulation specification.

In this case, VOUT can be found by applying the voltage divider rule:

Power Supply

In terms of resistances, load regulation can be expressed as:

Example:

A power supply has an output resistance of 25 mW and a full load current of 0.50 A to a 10.0 W load.

• What is the load regulation?
• What is the no load output voltage?

Solution:

= 0.25%

(a)

(b)

By Ohm’s law, VOUT = 5.0 V.

= 5.013 V

Series Regulators

A simple representation of a series type of linear regulator:

Control element: a pass transistor in series with the load between the input and output.

Sample circuit: senses a change in the output voltage.

Error detector: compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.

Refer Page 856 for full explanation.

Series Regulator block diagram:

Basic series regulator circuit:

The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor.

Series Regulators

The output voltage for the series regulator circuit is:

Example:

(a) What is the output voltage for the series regulator?

(b) If the load current is 200 mA, what is the power dissipated by Q1?

Solution:

(a)

18 V

4.7 kW

= 12.2 V

3.9 V

100 kW

(b)

P = VI

= (18 V – 12.2 V)(0.2 A)

47 kW

= 1.16 W

Series Regulators

Current limiting prevents excessive load current. Q2 will conduct when the current through R4 develops 0.7 V across Q2’s VBE. This reduces base current to Q1, limiting the load current.

The current limit is:

For example, a 1.4 W resistor, limits current to about 0.5 A.

Example 17.4
• Page 859

Series Regulators

Fold-back current limiting drops the load current well below the peak during overload conditions. Q2 conducts when VR5 +VBE = VR4 and begins current limiting. VR5 is found by applying the voltage-divider rule:

An overload causes VR5 to drop because VOUT drops. This means that less current is needed to maintain conduction in Q2 and the load current drops.

Shunt Regulators

A simple representation of a shunt type of linear regulator;

Shunt Regulator block diagram:

Basic shunt regulator circuit:

The control element maintains a constant output voltage by varying the collector current in the transistor.

Q1 in parallel with the load.

R1 in series with the load.

Regulation is achieved by controlling the current through the parallel transistor Q1.

Shunt Regulators

Shunt regulators use a parallel transistor for the control element. If the output voltage changes, the op-amp senses the change and corrects the bias on Q1 to follow. For example, a decrease in output voltage causes a decrease in VB and an increase in VC.

Although it is less efficient than the series regulator, the shunt regulator has inherent short-circuit protection. The maximum current when the output is shorted is VIN/R1.

Example 17.5
• Page 862

Switching Regulators

All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy.

An increase in the duty cycle increases the output voltage.

A decrease in the duty cycle decreases the output voltage.

on/off control

VOUT

Switching Regulators

A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement.

C charges

L reverses polarity

Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter.

on

off

+-

- +

Switching Regulators

In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage.

C charges

Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage.

L field collapses

L field builds

on

C discharges

off

+-

- +

on

off

on

off

-

+

C charges

L field collapses

off

C discharges

on

L field builds

Switching Regulators

In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source.

Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input.

+

-

IC Voltage Regulators

Integrated circuit voltage regulators are available as series regulators or as switching regulators. The popular three-terminal regulators are often used on separate pc boards within a system because they are inexpensive and avoid problems associated with large power distribution systems (such as noise pickup).

78XX Regulators

The 78XX series is a fixed positive output regulator available in various packages and with standard voltage outputs.

TO-220 case

D-PAK

IC Voltage Regulators

The onlyexternal components required with the 78XX series are input and output capacitors and some form of heat sink. These IC’s include thermal shutdown protection and internal current limiting.

Heat-sink surface

The 78XX series are primarily used for fixed output voltages, but with additional components, they can be set up for variable voltages or currents.

IC Voltage Regulators

The 79XX series is the negative output counterpart to the 78XX series, however the pin assignments are different on this series. Other specifications are basically the same.

IC Voltage Regulators

The LM317 is a adjustablepositive output IC regulator. There is a fixed reference voltage of +1.25 V between the output andadjustment terminals. There is no ground pin.

The output voltage is calculated by:

Example:

What is VOUT?

+20 V

+16.8 V

Solution:

150 W

2 kW

= 16.8 V

IC Voltage Regulators

IC regulators are limited to a maximum allowable current before shutting down. The circuit shown is uses an external pass transistor to increase the maximum available load current.

Rext sets the point where Qext begins to conduct:

For example, if Imax is 1.0 A, Rext =

0.7 W

Question:

What minimum power rating is required for Rext?

P = I2R = (1 A)2(0.7 W) = 0.7 W

IC Voltage Regulators

The 78S40 is an IC containing all of the elements needed to configure a switching regulator, using a few external parts.

It is a universal switching regulator subsystem because it can be configured as a step-down, step-up, or inverting regulator by the user. The data sheet shows typical circuits for these configurations.

Here is the step-down configuration.

Selected Key Terms

Regulator

Line regulation

Linear regulator

Switching regulator

A electronic circuit that maintains an essentially constant output voltage with changing input voltage or load current.

The percentage change in output voltage for a given change in input (line) voltage.

The percentage change in output voltage for a given change in load current.

A voltage regulator in which the control element operates in the linear region.

A voltage regulator in which the control element operates as a switch.

Quiz

1. The load regulation of an ideal power supply is

a. 0%

b. 25%

c. 50%

d. 100%

Quiz

2. A correct formula for load regulation is

a.

b.

c.

d.

Quiz

3. An alternate way to express load regulation is in terms of the

a. output resistance and the full-load resistance

b. output resistance and the shorted-load resistance

c. input resistance and the full-load resistance

d. input resistance and the shorted-load resistance

Quiz

4. In the circuit shown, R4 = 0.7 W. The output current will be limited to

a. 0.5 A

b. 0.7 A

c. 1.0 A

d. 1.4 A

0.7 W

Quiz

5. The block diagram for a series voltage regulator is shown. The yellow box represents a

a. control element

b. sample circuit

c. error detector

d. reference voltage

Quiz

6. The block diagram for a shunt voltage regulator is shown. The yellow box represents a

a. control element

b. sample circuit

c. error detector

d. reference voltage

Quiz

7. The circuit in the blue shaded area is a

a. high speed switching circuit

b. fold-back current limiter

c. reference source

d. shunt regulator

Quiz

8. A major advantage of all switching regulators is

a. low noise

b. high output impedance

c. high efficiency

d. all of the above

on

off

-

+

C charges

L field collapses

off

C discharges

on

L field builds

Quiz

9. The type of regulator circuit shown is a

a. series linear

b. series switching

c. shunt switching

d. none of the above

+

-

Quiz

10. The output voltage from a 7912 is a regulated

a. +5 V

b. +12 V

c. -5 V

d. -12 V

References

Thomas L. Floyd, “Electronic Devices Conventional Current Version, Ninth Edition (International Edition)”, Pearson Prentice Hall, 2012